{"kind": "plan", "major": "18", "item": {"slug": "seq-scan", "name": "Seq Scan", "name_zh": "SeqScan", "category": "\u626b\u63cf", "summary": "\u901a\u8fc7\u987a\u5e8f\u626b\u63cf\u8bfb\u53d6\u5173\u7cfb\uff0c\u5e76\u5e94\u7528\u626b\u63cf\u6761\u4ef6\u3002", "aliases": ["Seq Scan", "SeqScan", "T_SeqScan"], "content_hash": "221148297d52560aea9d41e95090788d2bb4e3d8abedcfb69ec0a0d03bcf44b4", "versions": {"10": {"facts": [{"label": "\u6838\u5fc3\u8282\u70b9\u6807\u7b7e", "value": "T_SeqScan"}, {"label": "\u7ed3\u6784\u5316 EXPLAIN \u8282\u70b9\u7c7b\u578b", "value": "Seq Scan"}, {"label": "\u8f93\u5165", "value": "\u5173\u7cfb"}, {"label": "\u8f93\u51fa", "value": "\u6ee1\u8db3\u6761\u4ef6\u7684\u5173\u7cfb\u5143\u7ec4"}, {"label": "\u6267\u884c\u5668\u521d\u59cb\u5316\u51fd\u6570", "value": "ExecInitSeqScan"}, {"label": "\u5185\u5b58\u673a\u5236", "value": "unclassified"}], "memory": {"evidence": [{"url": "https://ftp.postgresql.org/pub/source/v10.23/postgresql-10.23.tar.bz2", "path": "src/backend/executor/nodeSeqscan.c", "label": "src/backend/executor/nodeSeqscan.c", "sha256": "607f24cec96999e344a7e512dc8bc14d3ec51663878d05c4194a3f70bfab615c", "archive_sha256": "94a4b2528372458e5662c18d406629266667c437198160a18cdfd2c4a4d6eee9"}], "mechanism": "unclassified", "description": "\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002", "source_notes": ["save the tuple and the buffer returned to us by the access methods in our scan tuple slot and return the slot. Note: we pass 'false' because tuples returned by heap_getnext() are pointers onto disk pages and were not created with palloc() and so should not be pfree()'d. Note also that ExecStoreTuple will increment the refcount of the buffer; the refcount will not be dropped until the tuple table slot is cleared."]}, "tables": [{"key": "explain-labels", "rows": [{"label": "Seq Scan", "identity": "Seq Scan"}], "title": "\u672c\u6784\u5efa\u4e2d\u7684 EXPLAIN \u6807\u7b7e", "columns": [{"key": "label", "label": "\u6587\u672c\u683c\u5f0f\u6807\u7b7e"}, {"key": "identity", "label": "\u7ed3\u6784\u5316\u8282\u70b9\u6807\u8bc6"}]}], "related": [{"url": "/wiki/sql/explain/?v=10", "label": "EXPLAIN"}, {"url": "/docs/10/using-explain.html", "label": "\u4f7f\u7528 EXPLAIN"}, {"url": "/docs/10/parallel-plans.html", "label": "\u5e76\u884c\u8ba1\u5212"}, {"url": "/wiki/guc/enable_seqscan/?v=10", "label": "enable_seqscan"}], "release": {"ref": "PostgreSQL 10.23 source archive", "label": "10.23", "major": "10", "channel": "historical", "revision": "94a4b2528372458e5662c18d406629266667c437198160a18cdfd2c4a4d6eee9", "source_url": "https://ftp.postgresql.org/pub/source/v10.23/postgresql-10.23.tar.bz2", "source_snapshot_utc": ""}, "sources": [{"url": "https://ftp.postgresql.org/pub/source/v10.23/postgresql-10.23.tar.bz2", "line": 932, "path": "src/backend/commands/explain.c", "label": "src/backend/commands/explain.c:932", "sha256": "a785298532047cfeda969e78c3597a343dc1c56d61ba85830b0f16a02a14b5a1", "archive_sha256": "94a4b2528372458e5662c18d406629266667c437198160a18cdfd2c4a4d6eee9"}, {"url": "https://ftp.postgresql.org/pub/source/v10.23/postgresql-10.23.tar.bz2", "line": 207, "path": "src/backend/executor/execProcnode.c", "label": "src/backend/executor/execProcnode.c:207", "sha256": "cea76648bb38ae55f18f989768bee1a4ee025691ceea0f86bccb29dcdc166acc", "archive_sha256": "94a4b2528372458e5662c18d406629266667c437198160a18cdfd2c4a4d6eee9"}, {"url": "https://ftp.postgresql.org/pub/source/v10.23/postgresql-10.23.tar.bz2", "path": "src/backend/executor/nodeSeqscan.c", "label": "src/backend/executor/nodeSeqscan.c", "sha256": "607f24cec96999e344a7e512dc8bc14d3ec51663878d05c4194a3f70bfab615c", "archive_sha256": "94a4b2528372458e5662c18d406629266667c437198160a18cdfd2c4a4d6eee9"}, {"url": "https://ftp.postgresql.org/pub/source/v10.23/postgresql-10.23.tar.bz2", "path": "src/include/nodes/plannodes.h", "label": "src/include/nodes/plannodes.h", "sha256": "d562c321108844798cd234303fffb618f13d4ee3f3a5ac79bfd963b077e47c22", "archive_sha256": "94a4b2528372458e5662c18d406629266667c437198160a18cdfd2c4a4d6eee9"}, {"url": "https://pg.center/docs/10/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 10.23 \u00b7 using-explain", "sha256": "a4b4304aedb0a2da0145cc7c35b03b319a5c7ee3df35a8ef84ab6d3a610f98f3", "language": "en", "original_url": "/docs/10/using-explain.html#USING-EXPLAIN-BASICS"}], "node_tag": "T_SeqScan", "sections": [{"title": "EXPLAIN \u540d\u79f0\u4e0e\u5c5e\u6027", "paragraphs": ["\u7ed3\u6784\u5316\u683c\u5f0f\u4f7f\u7528\u4e0a\u8ff0 Node Type\u3002\u6587\u672c\u683c\u5f0f\u540d\u79f0\u8fd8\u53ef\u80fd\u5305\u542b\u64cd\u4f5c\u3001\u7b56\u7565\u3001\u8fde\u63a5\u7c7b\u578b\u3001\u626b\u63cf\u65b9\u5411\u6216\u805a\u5408\u9636\u6bb5\u5c5e\u6027\u3002", "\u6b64\u6e90\u7801\u8bb0\u5f55\u7684\u6587\u672c\u540d\u79f0\uff1aSeq Scan.", "\u5e76\u884c\u611f\u77e5\u4e0e\u5e76\u884c\u5b89\u5168\u662f\u4e0d\u540c\u7684\u8ba1\u5212\u5c5e\u6027\u3002\u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u5185\u8fd0\u884c\u7684\u8282\u70b9\u4e0d\u4e00\u5b9a\u662f\u5e76\u884c\u611f\u77e5\u8282\u70b9\u3002"]}, {"title": "\u5185\u5b58\u4e0e\u4e34\u65f6\u5b58\u50a8", "paragraphs": ["\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002", "\u5c06\u8bbf\u95ee\u65b9\u6cd5\u8fd4\u56de\u7684\u5143\u7ec4\u548c\u7f13\u51b2\u533a\u4fdd\u5b58\u5728\u626b\u63cf\u5143\u7ec4\u69fd\u4e2d\uff0c\u7136\u540e\u8fd4\u56de\u8be5\u69fd\u3002\u6ce8\u610f\u4f20\u5165 false\uff0c\u56e0\u4e3a heap_getnext() \u8fd4\u56de\u7684\u5143\u7ec4\u6307\u5411\u78c1\u76d8\u9875\uff0c\u5e76\u975e\u7531 palloc() \u521b\u5efa\uff0c\u4e0d\u80fd\u5bf9\u5176\u8c03\u7528 pfree()\u3002ExecStoreTuple \u4f1a\u589e\u52a0\u7f13\u51b2\u533a\u7684\u5f15\u7528\u8ba1\u6570\uff0c\u76f4\u5230\u5143\u7ec4\u8868\u69fd\u6e05\u7a7a\u65f6\u624d\u51cf\u5c11\u3002"]}, {"title": "\u5e76\u884c\u6267\u884c\u4e0e\u8fd0\u884c\u4fe1\u606f\u91c7\u96c6", "paragraphs": ["\u4ee5\u4e0b\u6e90\u7801\u56de\u8c03\u53ef\u4ee5\u534f\u8c03\u6267\u884c\u6216\u6536\u96c6\u5de5\u4f5c\u8fdb\u7a0b\u7684\u6d4b\u91cf\u6570\u636e\u3002\u56de\u8c03\u5b58\u5728\u4e0d\u4ee3\u8868\u8be5\u8282\u70b9\u666e\u904d\u652f\u6301\u5171\u4eab\u5e76\u884c\u626b\u63cf\u6216\u5171\u4eab\u72b6\u6001\u3002", "\u6b64\u6784\u5efa\u7684\u56de\u8c03\uff1aExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM."]}, {"title": "\u540c\u7248\u672c\u624b\u518c\u8bf4\u660e", "paragraphs": ["EXPLAIN \u5c06 WHERE \u5b50\u53e5\u663e\u793a\u4e3a Seq Scan \u8282\u70b9\u4e0a\u7684 filter \u6761\u4ef6\u3002\u8fd9\u8868\u793a\u8282\u70b9\u5bf9\u626b\u63cf\u7684\u6bcf\u4e00\u884c\u68c0\u67e5\u6761\u4ef6\uff0c\u4ec5\u8f93\u51fa\u901a\u8fc7\u6761\u4ef6\u7684\u884c\u3002WHERE \u964d\u4f4e\u4e86\u9884\u8ba1\u8f93\u51fa\u884c\u6570\uff0c\u4f46\u626b\u63cf\u4ecd\u9700\u8bbf\u95ee\u5168\u90e8 10000 \u884c\uff0c\u6240\u4ee5\u6210\u672c\u6ca1\u6709\u964d\u4f4e\uff1b\u76f8\u53cd\u8fd8\u7565\u6709\u589e\u52a0\uff08\u51c6\u786e\u8bf4\u589e\u52a0 10000 * cpu_operator_cost\uff09\uff0c\u4ee5\u8ba1\u5165\u68c0\u67e5 WHERE \u6761\u4ef6\u6d88\u8017\u7684\u989d\u5916 CPU \u65f6\u95f4\u3002"]}, {"title": "\u672c\u7248\u624b\u518c\u4e2d\u7684\u793a\u4f8b", "blocks": [{"code": "EXPLAIN SELECT * FROM tenk1;\n\n                         QUERY PLAN\n-------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..458.00 rows=10000 width=244)", "source": {"url": "/docs/10/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 10.23 \u00b7 using-explain", "sha256": "a4b4304aedb0a2da0145cc7c35b03b319a5c7ee3df35a8ef84ab6d3a610f98f3"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 10.23 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u4e0b\u9762\u662f\u4e00\u4e2a\u7b80\u5355\u793a\u4f8b\uff0c\u4ec5\u7528\u6765\u5c55\u793a\u8f93\u51fa\u5f62\u5f0f\uff1a"]}, {"code": "EXPLAIN SELECT * FROM tenk1 WHERE unique1 < 7000;\n\n                         QUERY PLAN\n------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..483.00 rows=7001 width=244)\n   Filter: (unique1 < 7000)", "source": {"url": "/docs/10/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 10.23 \u00b7 using-explain", "sha256": "a4b4304aedb0a2da0145cc7c35b03b319a5c7ee3df35a8ef84ab6d3a610f98f3"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 10.23 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u73b0\u5728\u4fee\u6539\u67e5\u8be2\uff0c\u6dfb\u52a0 WHERE \u6761\u4ef6\uff1a"]}]}, {"title": "\u6267\u884c\u5668\u5b9e\u73b0\u8bf4\u660e", "paragraphs": ["nodeSeqscan.c\uff1a\u652f\u6301\u5173\u7cfb\u987a\u5e8f\u626b\u63cf\u7684\u4f8b\u7a0b\u3002", "ExecSeqScanEstimate \u4f30\u7b97\u5e76\u884c\u626b\u63cf\u6240\u9700\u7684 DSM \u7a7a\u95f4\uff1bExecSeqScanInitializeDSM \u521d\u59cb\u5316\u5e76\u884c\u626b\u63cf\u7684 DSM\uff1bExecSeqScanReInitializeDSM \u4e3a\u65b0\u4e00\u8f6e\u5e76\u884c\u626b\u63cf\u91cd\u65b0\u521d\u59cb\u5316 DSM\uff1bExecSeqScanInitializeWorker \u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u4e2d\u9644\u52a0\u5230 DSM \u4fe1\u606f\u3002", "\u626b\u63cf\u4e0d\u5e76\u884c\u6267\u884c\uff0c\u6216\u539f\u8ba1\u5212\u5e76\u884c\u7684\u626b\u63cf\u4ee5\u4e32\u884c\u65b9\u5f0f\u6267\u884c\u65f6\uff0c\u4f1a\u5230\u8fbe\u6b64\u5904\u3002", "\u5c06\u8bbf\u95ee\u65b9\u6cd5\u8fd4\u56de\u7684\u5143\u7ec4\u548c\u7f13\u51b2\u533a\u4fdd\u5b58\u5728\u626b\u63cf\u5143\u7ec4\u69fd\u4e2d\uff0c\u7136\u540e\u8fd4\u56de\u8be5\u69fd\u3002\u6ce8\u610f\u4f20\u5165 false\uff0c\u56e0\u4e3a heap_getnext() \u8fd4\u56de\u7684\u5143\u7ec4\u6307\u5411\u78c1\u76d8\u9875\uff0c\u5e76\u975e\u7531 palloc() \u521b\u5efa\uff0c\u4e0d\u80fd\u5bf9\u5176\u8c03\u7528 pfree()\u3002ExecStoreTuple \u4f1a\u589e\u52a0\u7f13\u51b2\u533a\u7684\u5f15\u7528\u8ba1\u6570\uff0c\u76f4\u5230\u5143\u7ec4\u8868\u69fd\u6e05\u7a7a\u65f6\u624d\u51cf\u5c11\u3002", "SeqRecheck\uff1a\u5728 EvalPlanQual \u4e2d\u91cd\u65b0\u68c0\u67e5\u5143\u7ec4\u7684\u8bbf\u95ee\u65b9\u6cd5\u4f8b\u7a0b\u3002"]}, {"code": "case T_SeqScan:\n\t\t\tpname = sname = \"Seq Scan\";\n\t\t\tbreak;", "title": "\u6838\u5fc3\u6e90\u7801\u4e2d\u7684 EXPLAIN \u6807\u8bc6"}], "strategies": [], "description": ["\u901a\u8fc7\u987a\u5e8f\u626b\u63cf\u8bfb\u53d6\u5173\u7cfb\uff0c\u5e76\u5e94\u7528\u626b\u63cf\u6761\u4ef6\u3002"], "localization": {"status": "complete", "sources": [], "language": "zh", "original_text": {"/versions/10/description/0": "Reads a relation through a sequential scan and applies its scan qualifications.", "/versions/10/facts/0/label": "Core node tag", "/versions/10/facts/1/label": "Structured EXPLAIN Node Type", "/versions/10/facts/2/label": "Inputs", "/versions/10/facts/2/value": "A relation", "/versions/10/facts/3/label": "Output", "/versions/10/facts/3/value": "Qualified relation tuples", "/versions/10/facts/4/label": "Executor initializer", "/versions/10/facts/5/label": "Memory mechanism", "/versions/10/tables/0/title": "EXPLAIN labels in this source build", "/versions/10/related/1/label": "Using EXPLAIN", "/versions/10/related/2/label": "Parallel plans", "/versions/10/sections/0/title": "EXPLAIN names and attributes", "/versions/10/sections/1/title": "Memory and temporary storage", "/versions/10/sections/2/title": "Parallel execution and instrumentation", "/versions/10/sections/3/title": "Same-version manual discussion", "/versions/10/sections/4/title": "Examples from this manual build", "/versions/10/sections/5/title": "Executor implementation notes", "/versions/10/sections/6/title": "EXPLAIN identity in core source", "/versions/10/memory/description": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/10/sections/0/paragraphs/0": "Structured formats use the Node Type above. Text-format spellings can also include operation, strategy, join type, scan direction or aggregation-stage attributes.", "/versions/10/sections/0/paragraphs/1": "Text names recorded by this source: Seq Scan.", "/versions/10/sections/0/paragraphs/2": "Parallel-aware and parallel-safe are different plan properties. A node running inside a parallel worker is not necessarily a parallel-aware node.", "/versions/10/sections/1/paragraphs/0": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/10/sections/1/paragraphs/1": "save the tuple and the buffer returned to us by the access methods in our scan tuple slot and return the slot. Note: we pass 'false' because tuples returned by heap_getnext() are pointers onto disk pages and were not created with palloc() and so should not be pfree()'d. Note also that ExecStoreTuple will increment the refcount of the buffer; the refcount will not be dropped until the tuple table slot is cleared.", "/versions/10/sections/2/paragraphs/0": "The source callbacks below can coordinate execution or collect worker instrumentation. Their presence is not a blanket claim that this node supports a shared parallel scan or shared state.", "/versions/10/sections/2/paragraphs/1": "Callbacks in this build: ExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM.", "/versions/10/sections/3/paragraphs/0": "Notice that the EXPLAIN output shows the WHERE clause being applied as a \u201c filter \u201d condition attached to the Seq Scan plan node. This means that the plan node checks the condition for each row it scans, and outputs only the ones that pass the condition. The estimate of output rows has been reduced because of the WHERE clause. However, the scan will still have to visit all 10000 rows, so the cost hasn't decreased; in fact it has gone up a bit (by 10000 * cpu_operator_cost , to be exact) to reflect the extra CPU time spent checking the WHERE condition.", "/versions/10/sections/5/paragraphs/0": "nodeSeqscan.c Support routines for sequential scans of relations.", "/versions/10/sections/5/paragraphs/1": "ExecSeqScanEstimate estimates DSM space needed for parallel scan ExecSeqScanInitializeDSM initialize DSM for parallel scan ExecSeqScanReInitializeDSM reinitialize DSM for fresh parallel scan ExecSeqScanInitializeWorker attach to DSM info in parallel worker", "/versions/10/sections/5/paragraphs/2": "We reach here if the scan is not parallel, or if we're serially executing a scan that was planned to be parallel.", "/versions/10/sections/5/paragraphs/3": "save the tuple and the buffer returned to us by the access methods in our scan tuple slot and return the slot. Note: we pass 'false' because tuples returned by heap_getnext() are pointers onto disk pages and were not created with palloc() and so should not be pfree()'d. Note also that ExecStoreTuple will increment the refcount of the buffer; the refcount will not be dropped until the tuple table slot is cleared.", "/versions/10/sections/5/paragraphs/4": "SeqRecheck -- access method routine to recheck a tuple in EvalPlanQual", "/versions/10/tables/0/columns/0/label": "Text-format label", "/versions/10/tables/0/columns/1/label": "Structured node identity", "/versions/10/sections/4/blocks/0/paragraphs/0": "Example copied from the PostgreSQL 10.23 manual; it was not executed for this collection.", "/versions/10/sections/4/blocks/0/paragraphs/1": "Here is a trivial example, just to show what the output looks like:", "/versions/10/sections/4/blocks/1/paragraphs/0": "Example copied from the PostgreSQL 10.23 manual; it was not executed for this collection.", "/versions/10/sections/4/blocks/1/paragraphs/1": "Now let's modify the query to add a WHERE condition:"}, "fallback_fields": [], "source_language": "en", "original_snapshot_sha256": "2b576fa4773449389304f5b420756dd3bb2dbab75ad08eef77b55c15da2c4096"}, "evidence_kind": "source and documentation", "explain_names": ["Seq Scan"], "partial_modes": [], "comparison_data": {"node_tag": "T_SeqScan", "strategies": [], "text_names": ["Seq Scan"], "initializer": "ExecInitSeqScan", "partial_modes": [], "memory_mechanism": "unclassified", "parallel_callbacks": ["ExecSeqScanEstimate", "ExecSeqScanInitializeDSM", "ExecSeqScanInitializeWorker", "ExecSeqScanReInitializeDSM"]}, "comparison_hash": "5631c0db91382334dc4ca321ae0335013d10ae1c0c24f3523fcca0184bbbc81b", "explain_prefixes": ["Parallel"], "runtime_verified": false, "source_inventory": {"explain": "src/backend/commands/explain.c", "executor": "src/backend/executor/execProcnode.c", "implementation": "src/backend/executor/nodeSeqscan.c"}, "parallel_callbacks": ["ExecSeqScanEstimate", "ExecSeqScanInitializeDSM", "ExecSeqScanInitializeWorker", "ExecSeqScanReInitializeDSM"]}, "11": {"facts": [{"label": "\u6838\u5fc3\u8282\u70b9\u6807\u7b7e", "value": "T_SeqScan"}, {"label": "\u7ed3\u6784\u5316 EXPLAIN \u8282\u70b9\u7c7b\u578b", "value": "Seq Scan"}, {"label": "\u8f93\u5165", "value": "\u5173\u7cfb"}, {"label": "\u8f93\u51fa", "value": "\u6ee1\u8db3\u6761\u4ef6\u7684\u5173\u7cfb\u5143\u7ec4"}, {"label": "\u6267\u884c\u5668\u521d\u59cb\u5316\u51fd\u6570", "value": "ExecInitSeqScan"}, {"label": "\u5185\u5b58\u673a\u5236", "value": "unclassified"}], "memory": {"evidence": [{"url": "https://ftp.postgresql.org/pub/source/v11.22/postgresql-11.22.tar.bz2", "path": "src/backend/executor/nodeSeqscan.c", "label": "src/backend/executor/nodeSeqscan.c", "sha256": "d94f121505b666ab98f675e9821a81cea42eddd565399d2388a52ef3b0c3f77d", "archive_sha256": "2cb7c97d7a0d7278851bbc9c61f467b69c094c72b81740b751108e7892ebe1f0"}], "mechanism": "unclassified", "description": "\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002", "source_notes": ["save the tuple and the buffer returned to us by the access methods in our scan tuple slot and return the slot. Note: we pass 'false' because tuples returned by heap_getnext() are pointers onto disk pages and were not created with palloc() and so should not be pfree()'d. Note also that ExecStoreTuple will increment the refcount of the buffer; the refcount will not be dropped until the tuple table slot is cleared."]}, "tables": [{"key": "explain-labels", "rows": [{"label": "Seq Scan", "identity": "Seq Scan"}], "title": "\u672c\u6784\u5efa\u4e2d\u7684 EXPLAIN \u6807\u7b7e", "columns": [{"key": "label", "label": "\u6587\u672c\u683c\u5f0f\u6807\u7b7e"}, {"key": "identity", "label": "\u7ed3\u6784\u5316\u8282\u70b9\u6807\u8bc6"}]}], "related": [{"url": "/wiki/sql/explain/?v=11", "label": "EXPLAIN"}, {"url": "/docs/11/using-explain.html", "label": "\u4f7f\u7528 EXPLAIN"}, {"url": "/docs/11/parallel-plans.html", "label": "\u5e76\u884c\u8ba1\u5212"}, {"url": "/wiki/guc/enable_seqscan/?v=11", "label": "enable_seqscan"}], "release": {"ref": "PostgreSQL 11.22 source archive", "label": "11.22", "major": "11", "channel": "historical", "revision": "2cb7c97d7a0d7278851bbc9c61f467b69c094c72b81740b751108e7892ebe1f0", "source_url": 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"sha256": "d94f121505b666ab98f675e9821a81cea42eddd565399d2388a52ef3b0c3f77d", "archive_sha256": "2cb7c97d7a0d7278851bbc9c61f467b69c094c72b81740b751108e7892ebe1f0"}, {"url": "https://ftp.postgresql.org/pub/source/v11.22/postgresql-11.22.tar.bz2", "path": "src/include/nodes/plannodes.h", "label": "src/include/nodes/plannodes.h", "sha256": "5e0511194183800e8d6eb293fd4b40639c7d3118e2d199c8e7865ba4fa4cf67f", "archive_sha256": "2cb7c97d7a0d7278851bbc9c61f467b69c094c72b81740b751108e7892ebe1f0"}, {"url": "https://pg.center/docs/11/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 11.22 \u00b7 using-explain", "sha256": "8411bc78085d4737e32d7cca103d859da6c6539e33ec5e2fba46e74c6f199d8a", "language": "en", "original_url": "/docs/11/using-explain.html#USING-EXPLAIN-BASICS"}], "node_tag": "T_SeqScan", "sections": [{"title": "EXPLAIN \u540d\u79f0\u4e0e\u5c5e\u6027", "paragraphs": ["\u7ed3\u6784\u5316\u683c\u5f0f\u4f7f\u7528\u4e0a\u8ff0 Node Type\u3002\u6587\u672c\u683c\u5f0f\u540d\u79f0\u8fd8\u53ef\u80fd\u5305\u542b\u64cd\u4f5c\u3001\u7b56\u7565\u3001\u8fde\u63a5\u7c7b\u578b\u3001\u626b\u63cf\u65b9\u5411\u6216\u805a\u5408\u9636\u6bb5\u5c5e\u6027\u3002", "\u6b64\u6e90\u7801\u8bb0\u5f55\u7684\u6587\u672c\u540d\u79f0\uff1aSeq Scan.", "\u5e76\u884c\u611f\u77e5\u4e0e\u5e76\u884c\u5b89\u5168\u662f\u4e0d\u540c\u7684\u8ba1\u5212\u5c5e\u6027\u3002\u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u5185\u8fd0\u884c\u7684\u8282\u70b9\u4e0d\u4e00\u5b9a\u662f\u5e76\u884c\u611f\u77e5\u8282\u70b9\u3002"]}, {"title": "\u5185\u5b58\u4e0e\u4e34\u65f6\u5b58\u50a8", "paragraphs": ["\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002", "\u5c06\u8bbf\u95ee\u65b9\u6cd5\u8fd4\u56de\u7684\u5143\u7ec4\u548c\u7f13\u51b2\u533a\u4fdd\u5b58\u5728\u626b\u63cf\u5143\u7ec4\u69fd\u4e2d\uff0c\u7136\u540e\u8fd4\u56de\u8be5\u69fd\u3002\u6ce8\u610f\u4f20\u5165 false\uff0c\u56e0\u4e3a heap_getnext() \u8fd4\u56de\u7684\u5143\u7ec4\u6307\u5411\u78c1\u76d8\u9875\uff0c\u5e76\u975e\u7531 palloc() \u521b\u5efa\uff0c\u4e0d\u80fd\u5bf9\u5176\u8c03\u7528 pfree()\u3002ExecStoreTuple \u4f1a\u589e\u52a0\u7f13\u51b2\u533a\u7684\u5f15\u7528\u8ba1\u6570\uff0c\u76f4\u5230\u5143\u7ec4\u8868\u69fd\u6e05\u7a7a\u65f6\u624d\u51cf\u5c11\u3002"]}, {"title": "\u5e76\u884c\u6267\u884c\u4e0e\u8fd0\u884c\u4fe1\u606f\u91c7\u96c6", "paragraphs": ["\u4ee5\u4e0b\u6e90\u7801\u56de\u8c03\u53ef\u4ee5\u534f\u8c03\u6267\u884c\u6216\u6536\u96c6\u5de5\u4f5c\u8fdb\u7a0b\u7684\u6d4b\u91cf\u6570\u636e\u3002\u56de\u8c03\u5b58\u5728\u4e0d\u4ee3\u8868\u8be5\u8282\u70b9\u666e\u904d\u652f\u6301\u5171\u4eab\u5e76\u884c\u626b\u63cf\u6216\u5171\u4eab\u72b6\u6001\u3002", "\u6b64\u6784\u5efa\u7684\u56de\u8c03\uff1aExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM."]}, {"title": "\u540c\u7248\u672c\u624b\u518c\u8bf4\u660e", "paragraphs": ["EXPLAIN \u5c06 WHERE \u5b50\u53e5\u663e\u793a\u4e3a Seq Scan \u8282\u70b9\u4e0a\u7684 filter \u6761\u4ef6\u3002\u8fd9\u8868\u793a\u8282\u70b9\u5bf9\u626b\u63cf\u7684\u6bcf\u4e00\u884c\u68c0\u67e5\u6761\u4ef6\uff0c\u4ec5\u8f93\u51fa\u901a\u8fc7\u6761\u4ef6\u7684\u884c\u3002WHERE \u964d\u4f4e\u4e86\u9884\u8ba1\u8f93\u51fa\u884c\u6570\uff0c\u4f46\u626b\u63cf\u4ecd\u9700\u8bbf\u95ee\u5168\u90e8 10000 \u884c\uff0c\u6240\u4ee5\u6210\u672c\u6ca1\u6709\u964d\u4f4e\uff1b\u76f8\u53cd\u8fd8\u7565\u6709\u589e\u52a0\uff08\u51c6\u786e\u8bf4\u589e\u52a0 10000 * cpu_operator_cost\uff09\uff0c\u4ee5\u8ba1\u5165\u68c0\u67e5 WHERE \u6761\u4ef6\u6d88\u8017\u7684\u989d\u5916 CPU \u65f6\u95f4\u3002"]}, {"title": "\u672c\u7248\u624b\u518c\u4e2d\u7684\u793a\u4f8b", "blocks": [{"code": "EXPLAIN SELECT * FROM tenk1;\n\n                         QUERY PLAN\n-------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..458.00 rows=10000 width=244)", "source": {"url": "/docs/11/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 11.22 \u00b7 using-explain", "sha256": "8411bc78085d4737e32d7cca103d859da6c6539e33ec5e2fba46e74c6f199d8a"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 11.22 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u4e0b\u9762\u662f\u4e00\u4e2a\u7b80\u5355\u793a\u4f8b\uff0c\u4ec5\u7528\u6765\u5c55\u793a\u8f93\u51fa\u5f62\u5f0f\uff1a"]}, {"code": "EXPLAIN SELECT * FROM tenk1 WHERE unique1 < 7000;\n\n                         QUERY PLAN\n------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..483.00 rows=7001 width=244)\n   Filter: (unique1 < 7000)", "source": {"url": "/docs/11/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 11.22 \u00b7 using-explain", "sha256": "8411bc78085d4737e32d7cca103d859da6c6539e33ec5e2fba46e74c6f199d8a"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 11.22 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u73b0\u5728\u4fee\u6539\u67e5\u8be2\uff0c\u6dfb\u52a0 WHERE \u6761\u4ef6\uff1a"]}]}, {"title": "\u6267\u884c\u5668\u5b9e\u73b0\u8bf4\u660e", "paragraphs": ["nodeSeqscan.c\uff1a\u652f\u6301\u5173\u7cfb\u987a\u5e8f\u626b\u63cf\u7684\u4f8b\u7a0b\u3002", "ExecSeqScanEstimate \u4f30\u7b97\u5e76\u884c\u626b\u63cf\u6240\u9700\u7684 DSM \u7a7a\u95f4\uff1bExecSeqScanInitializeDSM \u521d\u59cb\u5316\u5e76\u884c\u626b\u63cf\u7684 DSM\uff1bExecSeqScanReInitializeDSM \u4e3a\u65b0\u4e00\u8f6e\u5e76\u884c\u626b\u63cf\u91cd\u65b0\u521d\u59cb\u5316 DSM\uff1bExecSeqScanInitializeWorker \u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u4e2d\u9644\u52a0\u5230 DSM \u4fe1\u606f\u3002", "\u626b\u63cf\u4e0d\u5e76\u884c\u6267\u884c\uff0c\u6216\u539f\u8ba1\u5212\u5e76\u884c\u7684\u626b\u63cf\u4ee5\u4e32\u884c\u65b9\u5f0f\u6267\u884c\u65f6\uff0c\u4f1a\u5230\u8fbe\u6b64\u5904\u3002", "\u5c06\u8bbf\u95ee\u65b9\u6cd5\u8fd4\u56de\u7684\u5143\u7ec4\u548c\u7f13\u51b2\u533a\u4fdd\u5b58\u5728\u626b\u63cf\u5143\u7ec4\u69fd\u4e2d\uff0c\u7136\u540e\u8fd4\u56de\u8be5\u69fd\u3002\u6ce8\u610f\u4f20\u5165 false\uff0c\u56e0\u4e3a heap_getnext() \u8fd4\u56de\u7684\u5143\u7ec4\u6307\u5411\u78c1\u76d8\u9875\uff0c\u5e76\u975e\u7531 palloc() \u521b\u5efa\uff0c\u4e0d\u80fd\u5bf9\u5176\u8c03\u7528 pfree()\u3002ExecStoreTuple \u4f1a\u589e\u52a0\u7f13\u51b2\u533a\u7684\u5f15\u7528\u8ba1\u6570\uff0c\u76f4\u5230\u5143\u7ec4\u8868\u69fd\u6e05\u7a7a\u65f6\u624d\u51cf\u5c11\u3002", "SeqRecheck\uff1a\u5728 EvalPlanQual \u4e2d\u91cd\u65b0\u68c0\u67e5\u5143\u7ec4\u7684\u8bbf\u95ee\u65b9\u6cd5\u4f8b\u7a0b\u3002"]}, {"code": "case T_SeqScan:\n\t\t\tpname = sname = \"Seq Scan\";\n\t\t\tbreak;", "title": "\u6838\u5fc3\u6e90\u7801\u4e2d\u7684 EXPLAIN \u6807\u8bc6"}], "strategies": [], "description": ["\u901a\u8fc7\u987a\u5e8f\u626b\u63cf\u8bfb\u53d6\u5173\u7cfb\uff0c\u5e76\u5e94\u7528\u626b\u63cf\u6761\u4ef6\u3002"], "localization": {"status": "complete", "sources": [], "language": "zh", "original_text": {"/versions/11/description/0": "Reads a relation through a sequential scan and applies its scan qualifications.", "/versions/11/facts/0/label": "Core node tag", "/versions/11/facts/1/label": "Structured EXPLAIN Node Type", "/versions/11/facts/2/label": "Inputs", "/versions/11/facts/2/value": "A relation", "/versions/11/facts/3/label": "Output", "/versions/11/facts/3/value": "Qualified relation tuples", "/versions/11/facts/4/label": "Executor initializer", "/versions/11/facts/5/label": "Memory mechanism", "/versions/11/tables/0/title": "EXPLAIN labels in this source build", "/versions/11/related/1/label": "Using EXPLAIN", "/versions/11/related/2/label": "Parallel plans", "/versions/11/sections/0/title": "EXPLAIN names and attributes", "/versions/11/sections/1/title": "Memory and temporary storage", "/versions/11/sections/2/title": "Parallel execution and instrumentation", "/versions/11/sections/3/title": "Same-version manual discussion", "/versions/11/sections/4/title": "Examples from this manual build", "/versions/11/sections/5/title": "Executor implementation notes", "/versions/11/sections/6/title": "EXPLAIN identity in core source", "/versions/11/memory/description": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/11/sections/0/paragraphs/0": "Structured formats use the Node Type above. Text-format spellings can also include operation, strategy, join type, scan direction or aggregation-stage attributes.", "/versions/11/sections/0/paragraphs/1": "Text names recorded by this source: Seq Scan.", "/versions/11/sections/0/paragraphs/2": "Parallel-aware and parallel-safe are different plan properties. A node running inside a parallel worker is not necessarily a parallel-aware node.", "/versions/11/sections/1/paragraphs/0": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/11/sections/1/paragraphs/1": "save the tuple and the buffer returned to us by the access methods in our scan tuple slot and return the slot. Note: we pass 'false' because tuples returned by heap_getnext() are pointers onto disk pages and were not created with palloc() and so should not be pfree()'d. Note also that ExecStoreTuple will increment the refcount of the buffer; the refcount will not be dropped until the tuple table slot is cleared.", "/versions/11/sections/2/paragraphs/0": "The source callbacks below can coordinate execution or collect worker instrumentation. Their presence is not a blanket claim that this node supports a shared parallel scan or shared state.", "/versions/11/sections/2/paragraphs/1": "Callbacks in this build: ExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM.", "/versions/11/sections/3/paragraphs/0": "Notice that the EXPLAIN output shows the WHERE clause being applied as a \u201c filter \u201d condition attached to the Seq Scan plan node. This means that the plan node checks the condition for each row it scans, and outputs only the ones that pass the condition. The estimate of output rows has been reduced because of the WHERE clause. However, the scan will still have to visit all 10000 rows, so the cost hasn't decreased; in fact it has gone up a bit (by 10000 * cpu_operator_cost , to be exact) to reflect the extra CPU time spent checking the WHERE condition.", "/versions/11/sections/5/paragraphs/0": "nodeSeqscan.c Support routines for sequential scans of relations.", "/versions/11/sections/5/paragraphs/1": "ExecSeqScanEstimate estimates DSM space needed for parallel scan ExecSeqScanInitializeDSM initialize DSM for parallel scan ExecSeqScanReInitializeDSM reinitialize DSM for fresh parallel scan ExecSeqScanInitializeWorker attach to DSM info in parallel worker", "/versions/11/sections/5/paragraphs/2": "We reach here if the scan is not parallel, or if we're serially executing a scan that was planned to be parallel.", "/versions/11/sections/5/paragraphs/3": "save the tuple and the buffer returned to us by the access methods in our scan tuple slot and return the slot. Note: we pass 'false' because tuples returned by heap_getnext() are pointers onto disk pages and were not created with palloc() and so should not be pfree()'d. Note also that ExecStoreTuple will increment the refcount of the buffer; the refcount will not be dropped until the tuple table slot is cleared.", "/versions/11/sections/5/paragraphs/4": "SeqRecheck -- access method routine to recheck a tuple in EvalPlanQual", "/versions/11/tables/0/columns/0/label": "Text-format label", "/versions/11/tables/0/columns/1/label": "Structured node identity", "/versions/11/sections/4/blocks/0/paragraphs/0": "Example copied from the PostgreSQL 11.22 manual; it was not executed for this collection.", "/versions/11/sections/4/blocks/0/paragraphs/1": "Here is a trivial example, just to show what the output looks like:", "/versions/11/sections/4/blocks/1/paragraphs/0": "Example copied from the PostgreSQL 11.22 manual; it was not executed for this collection.", "/versions/11/sections/4/blocks/1/paragraphs/1": "Now let's modify the query to add a WHERE condition:"}, "fallback_fields": [], "source_language": "en", "original_snapshot_sha256": 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["\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002"]}, {"title": "\u5e76\u884c\u6267\u884c\u4e0e\u8fd0\u884c\u4fe1\u606f\u91c7\u96c6", "paragraphs": ["\u4ee5\u4e0b\u6e90\u7801\u56de\u8c03\u53ef\u4ee5\u534f\u8c03\u6267\u884c\u6216\u6536\u96c6\u5de5\u4f5c\u8fdb\u7a0b\u7684\u6d4b\u91cf\u6570\u636e\u3002\u56de\u8c03\u5b58\u5728\u4e0d\u4ee3\u8868\u8be5\u8282\u70b9\u666e\u904d\u652f\u6301\u5171\u4eab\u5e76\u884c\u626b\u63cf\u6216\u5171\u4eab\u72b6\u6001\u3002", "\u6b64\u6784\u5efa\u7684\u56de\u8c03\uff1aExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM."]}, {"title": "\u540c\u7248\u672c\u624b\u518c\u8bf4\u660e", "paragraphs": ["EXPLAIN \u5c06 WHERE \u5b50\u53e5\u663e\u793a\u4e3a Seq Scan \u8282\u70b9\u4e0a\u7684 filter \u6761\u4ef6\u3002\u8fd9\u8868\u793a\u8282\u70b9\u5bf9\u626b\u63cf\u7684\u6bcf\u4e00\u884c\u68c0\u67e5\u6761\u4ef6\uff0c\u4ec5\u8f93\u51fa\u901a\u8fc7\u6761\u4ef6\u7684\u884c\u3002WHERE \u964d\u4f4e\u4e86\u9884\u8ba1\u8f93\u51fa\u884c\u6570\uff0c\u4f46\u626b\u63cf\u4ecd\u9700\u8bbf\u95ee\u5168\u90e8 10000 \u884c\uff0c\u6240\u4ee5\u6210\u672c\u6ca1\u6709\u964d\u4f4e\uff1b\u76f8\u53cd\u8fd8\u7565\u6709\u589e\u52a0\uff08\u51c6\u786e\u8bf4\u589e\u52a0 10000 * cpu_operator_cost\uff09\uff0c\u4ee5\u8ba1\u5165\u68c0\u67e5 WHERE \u6761\u4ef6\u6d88\u8017\u7684\u989d\u5916 CPU \u65f6\u95f4\u3002"]}, {"title": "\u672c\u7248\u624b\u518c\u4e2d\u7684\u793a\u4f8b", "blocks": [{"code": "EXPLAIN SELECT * FROM tenk1;\n\n                         QUERY PLAN\n-------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..458.00 rows=10000 width=244)", "source": {"url": "/docs/12/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 12.22 \u00b7 using-explain", "sha256": "06297525f2180b07e752837a3351be9c871b56559f535bcd0a67dec9baac10c0"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 12.22 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u4e0b\u9762\u662f\u4e00\u4e2a\u7b80\u5355\u793a\u4f8b\uff0c\u4ec5\u7528\u6765\u5c55\u793a\u8f93\u51fa\u5f62\u5f0f\uff1a"]}, {"code": "EXPLAIN SELECT * FROM tenk1 WHERE unique1 < 7000;\n\n                         QUERY PLAN\n------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..483.00 rows=7001 width=244)\n   Filter: (unique1 < 7000)", "source": {"url": "/docs/12/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 12.22 \u00b7 using-explain", "sha256": "06297525f2180b07e752837a3351be9c871b56559f535bcd0a67dec9baac10c0"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 12.22 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u73b0\u5728\u4fee\u6539\u67e5\u8be2\uff0c\u6dfb\u52a0 WHERE \u6761\u4ef6\uff1a"]}]}, {"title": "\u6267\u884c\u5668\u5b9e\u73b0\u8bf4\u660e", "paragraphs": ["nodeSeqscan.c\uff1a\u652f\u6301\u5173\u7cfb\u987a\u5e8f\u626b\u63cf\u7684\u4f8b\u7a0b\u3002", "ExecSeqScanEstimate \u4f30\u7b97\u5e76\u884c\u626b\u63cf\u6240\u9700\u7684 DSM \u7a7a\u95f4\uff1bExecSeqScanInitializeDSM \u521d\u59cb\u5316\u5e76\u884c\u626b\u63cf\u7684 DSM\uff1bExecSeqScanReInitializeDSM \u4e3a\u65b0\u4e00\u8f6e\u5e76\u884c\u626b\u63cf\u91cd\u65b0\u521d\u59cb\u5316 DSM\uff1bExecSeqScanInitializeWorker \u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u4e2d\u9644\u52a0\u5230 DSM \u4fe1\u606f\u3002", "\u626b\u63cf\u4e0d\u5e76\u884c\u6267\u884c\uff0c\u6216\u539f\u8ba1\u5212\u5e76\u884c\u7684\u626b\u63cf\u4ee5\u4e32\u884c\u65b9\u5f0f\u6267\u884c\u65f6\uff0c\u4f1a\u5230\u8fbe\u6b64\u5904\u3002", "SeqRecheck\uff1a\u5728 EvalPlanQual \u4e2d\u91cd\u65b0\u68c0\u67e5\u5143\u7ec4\u7684\u8bbf\u95ee\u65b9\u6cd5\u4f8b\u7a0b\u3002", "\u4e0e IndexScan \u4e0d\u540c\uff0cSeqScan \u4ece\u4e0d\u5728 heap_beginscan \u4e2d\u4f7f\u7528\u952e\uff08\u8fd9\u5f88\u4e0d\u597d\uff09\uff0c\u56e0\u6b64\u8fd9\u91cc\u4e0d\u68c0\u67e5\u952e\u662f\u5426\u6ee1\u8db3\u6761\u4ef6\u3002"]}, {"code": "case T_SeqScan:\n\t\t\tpname = sname = \"Seq Scan\";\n\t\t\tbreak;", "title": "\u6838\u5fc3\u6e90\u7801\u4e2d\u7684 EXPLAIN \u6807\u8bc6"}], "strategies": [], "description": ["\u901a\u8fc7\u987a\u5e8f\u626b\u63cf\u8bfb\u53d6\u5173\u7cfb\uff0c\u5e76\u5e94\u7528\u626b\u63cf\u6761\u4ef6\u3002"], "localization": {"status": "complete", "sources": [], "language": "zh", "original_text": {"/versions/12/description/0": "Reads a relation through a sequential scan and applies its scan qualifications.", "/versions/12/facts/0/label": "Core node tag", "/versions/12/facts/1/label": "Structured EXPLAIN Node Type", "/versions/12/facts/2/label": "Inputs", "/versions/12/facts/2/value": "A relation", "/versions/12/facts/3/label": "Output", "/versions/12/facts/3/value": "Qualified relation tuples", "/versions/12/facts/4/label": "Executor initializer", "/versions/12/facts/5/label": "Memory mechanism", "/versions/12/tables/0/title": "EXPLAIN labels in this source build", "/versions/12/related/1/label": "Using EXPLAIN", "/versions/12/related/2/label": "Parallel plans", "/versions/12/sections/0/title": "EXPLAIN names and attributes", "/versions/12/sections/1/title": "Memory and temporary storage", "/versions/12/sections/2/title": "Parallel execution and instrumentation", "/versions/12/sections/3/title": "Same-version manual discussion", "/versions/12/sections/4/title": "Examples from this manual build", "/versions/12/sections/5/title": "Executor implementation notes", "/versions/12/sections/6/title": "EXPLAIN identity in core source", "/versions/12/memory/description": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/12/sections/0/paragraphs/0": "Structured formats use the Node Type above. Text-format spellings can also include operation, strategy, join type, scan direction or aggregation-stage attributes.", "/versions/12/sections/0/paragraphs/1": "Text names recorded by this source: Seq Scan.", "/versions/12/sections/0/paragraphs/2": "Parallel-aware and parallel-safe are different plan properties. A node running inside a parallel worker is not necessarily a parallel-aware node.", "/versions/12/sections/1/paragraphs/0": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/12/sections/2/paragraphs/0": "The source callbacks below can coordinate execution or collect worker instrumentation. Their presence is not a blanket claim that this node supports a shared parallel scan or shared state.", "/versions/12/sections/2/paragraphs/1": "Callbacks in this build: ExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM.", "/versions/12/sections/3/paragraphs/0": "Notice that the EXPLAIN output shows the WHERE clause being applied as a \u201c filter \u201d condition attached to the Seq Scan plan node. This means that the plan node checks the condition for each row it scans, and outputs only the ones that pass the condition. The estimate of output rows has been reduced because of the WHERE clause. However, the scan will still have to visit all 10000 rows, so the cost hasn't decreased; in fact it has gone up a bit (by 10000 * cpu_operator_cost , to be exact) to reflect the extra CPU time spent checking the WHERE condition.", "/versions/12/sections/5/paragraphs/0": "nodeSeqscan.c Support routines for sequential scans of relations.", "/versions/12/sections/5/paragraphs/1": "ExecSeqScanEstimate estimates DSM space needed for parallel scan ExecSeqScanInitializeDSM initialize DSM for parallel scan ExecSeqScanReInitializeDSM reinitialize DSM for fresh parallel scan ExecSeqScanInitializeWorker attach to DSM info in parallel worker", "/versions/12/sections/5/paragraphs/2": "We reach here if the scan is not parallel, or if we're serially executing a scan that was planned to be parallel.", "/versions/12/sections/5/paragraphs/3": "SeqRecheck -- access method routine to recheck a tuple in EvalPlanQual", "/versions/12/sections/5/paragraphs/4": "Note that unlike IndexScan, SeqScan never use keys in heap_beginscan (and this is very bad) - so, here we do not check are keys ok or not.", "/versions/12/tables/0/columns/0/label": "Text-format label", "/versions/12/tables/0/columns/1/label": "Structured node identity", "/versions/12/sections/4/blocks/0/paragraphs/0": "Example copied from the PostgreSQL 12.22 manual; it was not executed for this collection.", "/versions/12/sections/4/blocks/0/paragraphs/1": "Here is a trivial example, just to show what the output looks like:", "/versions/12/sections/4/blocks/1/paragraphs/0": "Example copied from the PostgreSQL 12.22 manual; it was not executed for this collection.", "/versions/12/sections/4/blocks/1/paragraphs/1": "Now let's modify the query to add a WHERE condition:"}, "fallback_fields": [], "source_language": "en", "original_snapshot_sha256": "c963ef82546769d044f4fc62dc8c7933228416ed7db353da1d537676cddfdce5"}, "evidence_kind": "source and documentation", "explain_names": ["Seq Scan"], "partial_modes": [], "comparison_data": 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"\u8f93\u51fa", "value": "\u6ee1\u8db3\u6761\u4ef6\u7684\u5173\u7cfb\u5143\u7ec4"}, {"label": "\u6267\u884c\u5668\u521d\u59cb\u5316\u51fd\u6570", "value": "ExecInitSeqScan"}, {"label": "\u5185\u5b58\u673a\u5236", "value": "unclassified"}], "memory": {"evidence": [{"url": "https://ftp.postgresql.org/pub/source/v13.23/postgresql-13.23.tar.bz2", "path": "src/backend/executor/nodeSeqscan.c", "label": "src/backend/executor/nodeSeqscan.c", "sha256": "bc261aa6ebbb4b49dfcd29da31eb09169502c4c38fa75af007522949ae713e28", "archive_sha256": "6ec3c82726af92b7dec873fa1cdf881eca92a4219787dfad05acb6b10e041fd6"}], "mechanism": "unclassified", "description": "\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002", "source_notes": []}, "tables": [{"key": "explain-labels", "rows": [{"label": 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Type\u3002\u6587\u672c\u683c\u5f0f\u540d\u79f0\u8fd8\u53ef\u80fd\u5305\u542b\u64cd\u4f5c\u3001\u7b56\u7565\u3001\u8fde\u63a5\u7c7b\u578b\u3001\u626b\u63cf\u65b9\u5411\u6216\u805a\u5408\u9636\u6bb5\u5c5e\u6027\u3002", "\u6b64\u6e90\u7801\u8bb0\u5f55\u7684\u6587\u672c\u540d\u79f0\uff1aSeq Scan.", "\u5e76\u884c\u611f\u77e5\u4e0e\u5e76\u884c\u5b89\u5168\u662f\u4e0d\u540c\u7684\u8ba1\u5212\u5c5e\u6027\u3002\u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u5185\u8fd0\u884c\u7684\u8282\u70b9\u4e0d\u4e00\u5b9a\u662f\u5e76\u884c\u611f\u77e5\u8282\u70b9\u3002"]}, {"title": "\u5185\u5b58\u4e0e\u4e34\u65f6\u5b58\u50a8", "paragraphs": ["\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002"]}, {"title": "\u5e76\u884c\u6267\u884c\u4e0e\u8fd0\u884c\u4fe1\u606f\u91c7\u96c6", "paragraphs": ["\u4ee5\u4e0b\u6e90\u7801\u56de\u8c03\u53ef\u4ee5\u534f\u8c03\u6267\u884c\u6216\u6536\u96c6\u5de5\u4f5c\u8fdb\u7a0b\u7684\u6d4b\u91cf\u6570\u636e\u3002\u56de\u8c03\u5b58\u5728\u4e0d\u4ee3\u8868\u8be5\u8282\u70b9\u666e\u904d\u652f\u6301\u5171\u4eab\u5e76\u884c\u626b\u63cf\u6216\u5171\u4eab\u72b6\u6001\u3002", "\u6b64\u6784\u5efa\u7684\u56de\u8c03\uff1aExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM."]}, {"title": "\u540c\u7248\u672c\u624b\u518c\u8bf4\u660e", "paragraphs": ["EXPLAIN \u5c06 WHERE \u5b50\u53e5\u663e\u793a\u4e3a Seq Scan \u8282\u70b9\u4e0a\u7684 filter \u6761\u4ef6\u3002\u8fd9\u8868\u793a\u8282\u70b9\u5bf9\u626b\u63cf\u7684\u6bcf\u4e00\u884c\u68c0\u67e5\u6761\u4ef6\uff0c\u4ec5\u8f93\u51fa\u901a\u8fc7\u6761\u4ef6\u7684\u884c\u3002WHERE \u964d\u4f4e\u4e86\u9884\u8ba1\u8f93\u51fa\u884c\u6570\uff0c\u4f46\u626b\u63cf\u4ecd\u9700\u8bbf\u95ee\u5168\u90e8 10000 \u884c\uff0c\u6240\u4ee5\u6210\u672c\u6ca1\u6709\u964d\u4f4e\uff1b\u76f8\u53cd\u8fd8\u7565\u6709\u589e\u52a0\uff08\u51c6\u786e\u8bf4\u589e\u52a0 10000 * cpu_operator_cost\uff09\uff0c\u4ee5\u8ba1\u5165\u68c0\u67e5 WHERE \u6761\u4ef6\u6d88\u8017\u7684\u989d\u5916 CPU \u65f6\u95f4\u3002"]}, {"title": "\u672c\u7248\u624b\u518c\u4e2d\u7684\u793a\u4f8b", "blocks": [{"code": "EXPLAIN SELECT * FROM tenk1;\n\n                         QUERY PLAN\n-------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..458.00 rows=10000 width=244)", "source": {"url": "/docs/13/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 13.23 \u00b7 using-explain", "sha256": "650fd8629382d5dc8f9f8412c50ec5f32442a9ad2f98ca88e5348a7c2bd0ac7a"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 13.23 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u4e0b\u9762\u662f\u4e00\u4e2a\u7b80\u5355\u793a\u4f8b\uff0c\u4ec5\u7528\u6765\u5c55\u793a\u8f93\u51fa\u5f62\u5f0f\uff1a"]}, {"code": "EXPLAIN SELECT * FROM tenk1 WHERE unique1 < 7000;\n\n                         QUERY PLAN\n------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..483.00 rows=7001 width=244)\n   Filter: (unique1 < 7000)", "source": {"url": "/docs/13/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 13.23 \u00b7 using-explain", "sha256": "650fd8629382d5dc8f9f8412c50ec5f32442a9ad2f98ca88e5348a7c2bd0ac7a"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 13.23 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u73b0\u5728\u4fee\u6539\u67e5\u8be2\uff0c\u6dfb\u52a0 WHERE \u6761\u4ef6\uff1a"]}]}, {"title": "\u6267\u884c\u5668\u5b9e\u73b0\u8bf4\u660e", "paragraphs": ["nodeSeqscan.c\uff1a\u652f\u6301\u5173\u7cfb\u987a\u5e8f\u626b\u63cf\u7684\u4f8b\u7a0b\u3002", "ExecSeqScanEstimate \u4f30\u7b97\u5e76\u884c\u626b\u63cf\u6240\u9700\u7684 DSM \u7a7a\u95f4\uff1bExecSeqScanInitializeDSM \u521d\u59cb\u5316\u5e76\u884c\u626b\u63cf\u7684 DSM\uff1bExecSeqScanReInitializeDSM \u4e3a\u65b0\u4e00\u8f6e\u5e76\u884c\u626b\u63cf\u91cd\u65b0\u521d\u59cb\u5316 DSM\uff1bExecSeqScanInitializeWorker \u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u4e2d\u9644\u52a0\u5230 DSM \u4fe1\u606f\u3002", "\u626b\u63cf\u4e0d\u5e76\u884c\u6267\u884c\uff0c\u6216\u539f\u8ba1\u5212\u5e76\u884c\u7684\u626b\u63cf\u4ee5\u4e32\u884c\u65b9\u5f0f\u6267\u884c\u65f6\uff0c\u4f1a\u5230\u8fbe\u6b64\u5904\u3002", "SeqRecheck\uff1a\u5728 EvalPlanQual \u4e2d\u91cd\u65b0\u68c0\u67e5\u5143\u7ec4\u7684\u8bbf\u95ee\u65b9\u6cd5\u4f8b\u7a0b\u3002", "\u4e0e IndexScan \u4e0d\u540c\uff0cSeqScan \u4ece\u4e0d\u5728 heap_beginscan \u4e2d\u4f7f\u7528\u952e\uff08\u8fd9\u5f88\u4e0d\u597d\uff09\uff0c\u56e0\u6b64\u8fd9\u91cc\u4e0d\u68c0\u67e5\u952e\u662f\u5426\u6ee1\u8db3\u6761\u4ef6\u3002"]}, {"code": "case T_SeqScan:\n\t\t\tpname = sname = \"Seq Scan\";\n\t\t\tbreak;", "title": "\u6838\u5fc3\u6e90\u7801\u4e2d\u7684 EXPLAIN \u6807\u8bc6"}], "strategies": [], "description": ["\u901a\u8fc7\u987a\u5e8f\u626b\u63cf\u8bfb\u53d6\u5173\u7cfb\uff0c\u5e76\u5e94\u7528\u626b\u63cf\u6761\u4ef6\u3002"], "localization": {"status": "complete", "sources": [], "language": "zh", "original_text": {"/versions/13/description/0": "Reads a relation through a sequential scan and applies its scan qualifications.", "/versions/13/facts/0/label": "Core node tag", "/versions/13/facts/1/label": "Structured EXPLAIN Node Type", "/versions/13/facts/2/label": "Inputs", "/versions/13/facts/2/value": "A relation", "/versions/13/facts/3/label": "Output", "/versions/13/facts/3/value": "Qualified relation tuples", "/versions/13/facts/4/label": "Executor initializer", "/versions/13/facts/5/label": "Memory mechanism", "/versions/13/tables/0/title": "EXPLAIN labels in this source build", "/versions/13/related/1/label": "Using EXPLAIN", "/versions/13/related/2/label": "Parallel plans", "/versions/13/sections/0/title": "EXPLAIN names and attributes", "/versions/13/sections/1/title": "Memory and temporary storage", "/versions/13/sections/2/title": "Parallel execution and instrumentation", "/versions/13/sections/3/title": "Same-version manual discussion", "/versions/13/sections/4/title": "Examples from this manual build", "/versions/13/sections/5/title": "Executor implementation notes", "/versions/13/sections/6/title": "EXPLAIN identity in core source", "/versions/13/memory/description": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/13/sections/0/paragraphs/0": "Structured formats use the Node Type above. Text-format spellings can also include operation, strategy, join type, scan direction or aggregation-stage attributes.", "/versions/13/sections/0/paragraphs/1": "Text names recorded by this source: Seq Scan.", "/versions/13/sections/0/paragraphs/2": "Parallel-aware and parallel-safe are different plan properties. A node running inside a parallel worker is not necessarily a parallel-aware node.", "/versions/13/sections/1/paragraphs/0": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/13/sections/2/paragraphs/0": "The source callbacks below can coordinate execution or collect worker instrumentation. Their presence is not a blanket claim that this node supports a shared parallel scan or shared state.", "/versions/13/sections/2/paragraphs/1": "Callbacks in this build: ExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM.", "/versions/13/sections/3/paragraphs/0": "Notice that the EXPLAIN output shows the WHERE clause being applied as a \u201c filter \u201d condition attached to the Seq Scan plan node. This means that the plan node checks the condition for each row it scans, and outputs only the ones that pass the condition. The estimate of output rows has been reduced because of the WHERE clause. However, the scan will still have to visit all 10000 rows, so the cost hasn't decreased; in fact it has gone up a bit (by 10000 * cpu_operator_cost , to be exact) to reflect the extra CPU time spent checking the WHERE condition.", "/versions/13/sections/5/paragraphs/0": "nodeSeqscan.c Support routines for sequential scans of relations.", "/versions/13/sections/5/paragraphs/1": "ExecSeqScanEstimate estimates DSM space needed for parallel scan ExecSeqScanInitializeDSM initialize DSM for parallel scan ExecSeqScanReInitializeDSM reinitialize DSM for fresh parallel scan ExecSeqScanInitializeWorker attach to DSM info in parallel worker", "/versions/13/sections/5/paragraphs/2": "We reach here if the scan is not parallel, or if we're serially executing a scan that was planned to be parallel.", "/versions/13/sections/5/paragraphs/3": "SeqRecheck -- access method routine to recheck a tuple in EvalPlanQual", "/versions/13/sections/5/paragraphs/4": "Note that unlike IndexScan, SeqScan never use keys in heap_beginscan (and this is very bad) - so, here we do not check are keys ok or not.", "/versions/13/tables/0/columns/0/label": "Text-format label", "/versions/13/tables/0/columns/1/label": "Structured node identity", "/versions/13/sections/4/blocks/0/paragraphs/0": "Example copied from the PostgreSQL 13.23 manual; it was not executed for this collection.", "/versions/13/sections/4/blocks/0/paragraphs/1": "Here is a trivial example, just to show what the output looks like:", "/versions/13/sections/4/blocks/1/paragraphs/0": "Example copied from the PostgreSQL 13.23 manual; it was not executed for this collection.", "/versions/13/sections/4/blocks/1/paragraphs/1": "Now let's modify the query to add a WHERE condition:"}, "fallback_fields": [], "source_language": "en", "original_snapshot_sha256": "6b6b55ca548da68d0c48cbb463cff35aaa5a65ea988a53ee1dc9664000a57c75"}, "evidence_kind": "source and documentation", "explain_names": ["Seq Scan"], "partial_modes": [], "comparison_data": {"node_tag": "T_SeqScan", "strategies": [], "text_names": ["Seq Scan"], "initializer": "ExecInitSeqScan", "partial_modes": [], "memory_mechanism": "unclassified", "parallel_callbacks": ["ExecSeqScanEstimate", "ExecSeqScanInitializeDSM", "ExecSeqScanInitializeWorker", "ExecSeqScanReInitializeDSM"]}, "comparison_hash": "5631c0db91382334dc4ca321ae0335013d10ae1c0c24f3523fcca0184bbbc81b", "explain_prefixes": ["Parallel"], "runtime_verified": false, "source_inventory": {"explain": "src/backend/commands/explain.c", "executor": "src/backend/executor/execProcnode.c", "implementation": "src/backend/executor/nodeSeqscan.c"}, "parallel_callbacks": ["ExecSeqScanEstimate", "ExecSeqScanInitializeDSM", "ExecSeqScanInitializeWorker", "ExecSeqScanReInitializeDSM"]}, "14": {"facts": [{"label": "\u6838\u5fc3\u8282\u70b9\u6807\u7b7e", "value": "T_SeqScan"}, {"label": "\u7ed3\u6784\u5316 EXPLAIN \u8282\u70b9\u7c7b\u578b", "value": "Seq Scan"}, {"label": "\u8f93\u5165", "value": "\u5173\u7cfb"}, {"label": "\u8f93\u51fa", "value": "\u6ee1\u8db3\u6761\u4ef6\u7684\u5173\u7cfb\u5143\u7ec4"}, {"label": "\u6267\u884c\u5668\u521d\u59cb\u5316\u51fd\u6570", "value": "ExecInitSeqScan"}, {"label": "\u5185\u5b58\u673a\u5236", "value": "unclassified"}], "memory": {"evidence": [{"url": "https://ftp.postgresql.org/pub/source/v14.24/postgresql-14.24.tar.bz2", "path": "src/backend/executor/nodeSeqscan.c", "label": "src/backend/executor/nodeSeqscan.c", "sha256": "d00560f3c733326028e0b3dba13e7cd28265beaea958ca48ecac5c78eafe8256", "archive_sha256": "a7fa7ed3d558172355f51406097a7bd4f6b473be80f311ef7cda96bf383d8897"}], "mechanism": "unclassified", "description": "\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002", "source_notes": []}, "tables": [{"key": "explain-labels", "rows": [{"label": "Seq Scan", "identity": "Seq Scan"}], "title": "\u672c\u6784\u5efa\u4e2d\u7684 EXPLAIN \u6807\u7b7e", "columns": [{"key": "label", "label": "\u6587\u672c\u683c\u5f0f\u6807\u7b7e"}, {"key": "identity", "label": "\u7ed3\u6784\u5316\u8282\u70b9\u6807\u8bc6"}]}], "related": [{"url": "/wiki/sql/explain/?v=14", "label": "EXPLAIN"}, {"url": "/docs/14/using-explain.html", "label": "\u4f7f\u7528 EXPLAIN"}, {"url": "/docs/14/parallel-plans.html", "label": "\u5e76\u884c\u8ba1\u5212"}, {"url": "/wiki/guc/enable_seqscan/?v=14", "label": "enable_seqscan"}], "release": {"ref": "PostgreSQL 14.24 source archive", "label": "14.24", "major": "14", "channel": "stable", "revision": "a7fa7ed3d558172355f51406097a7bd4f6b473be80f311ef7cda96bf383d8897", "source_url": "https://ftp.postgresql.org/pub/source/v14.24/postgresql-14.24.tar.bz2", "source_snapshot_utc": ""}, "sources": [{"url": "https://ftp.postgresql.org/pub/source/v14.24/postgresql-14.24.tar.bz2", "line": 1222, "path": 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"https://ftp.postgresql.org/pub/source/v14.24/postgresql-14.24.tar.bz2", "path": "src/include/nodes/plannodes.h", "label": "src/include/nodes/plannodes.h", "sha256": "302f51a16b570dba7ec4e7bc045f7df5800d21630280354d1a24025f3baec75d", "archive_sha256": "a7fa7ed3d558172355f51406097a7bd4f6b473be80f311ef7cda96bf383d8897"}, {"url": "https://pg.center/docs/14/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 14.24 \u00b7 using-explain", "sha256": "7f5ab59cb21a035ada45ea3426c5d1cca3f781273483677f73fdd76753555206", "language": "en", "original_url": "/docs/14/using-explain.html#USING-EXPLAIN-BASICS"}], "node_tag": "T_SeqScan", "sections": [{"title": "EXPLAIN \u540d\u79f0\u4e0e\u5c5e\u6027", "paragraphs": ["\u7ed3\u6784\u5316\u683c\u5f0f\u4f7f\u7528\u4e0a\u8ff0 Node Type\u3002\u6587\u672c\u683c\u5f0f\u540d\u79f0\u8fd8\u53ef\u80fd\u5305\u542b\u64cd\u4f5c\u3001\u7b56\u7565\u3001\u8fde\u63a5\u7c7b\u578b\u3001\u626b\u63cf\u65b9\u5411\u6216\u805a\u5408\u9636\u6bb5\u5c5e\u6027\u3002", "\u6b64\u6e90\u7801\u8bb0\u5f55\u7684\u6587\u672c\u540d\u79f0\uff1aSeq Scan.", "\u5e76\u884c\u611f\u77e5\u4e0e\u5e76\u884c\u5b89\u5168\u662f\u4e0d\u540c\u7684\u8ba1\u5212\u5c5e\u6027\u3002\u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u5185\u8fd0\u884c\u7684\u8282\u70b9\u4e0d\u4e00\u5b9a\u662f\u5e76\u884c\u611f\u77e5\u8282\u70b9\u3002"]}, {"title": "\u5185\u5b58\u4e0e\u4e34\u65f6\u5b58\u50a8", "paragraphs": ["\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002"]}, {"title": "\u5e76\u884c\u6267\u884c\u4e0e\u8fd0\u884c\u4fe1\u606f\u91c7\u96c6", "paragraphs": ["\u4ee5\u4e0b\u6e90\u7801\u56de\u8c03\u53ef\u4ee5\u534f\u8c03\u6267\u884c\u6216\u6536\u96c6\u5de5\u4f5c\u8fdb\u7a0b\u7684\u6d4b\u91cf\u6570\u636e\u3002\u56de\u8c03\u5b58\u5728\u4e0d\u4ee3\u8868\u8be5\u8282\u70b9\u666e\u904d\u652f\u6301\u5171\u4eab\u5e76\u884c\u626b\u63cf\u6216\u5171\u4eab\u72b6\u6001\u3002", "\u6b64\u6784\u5efa\u7684\u56de\u8c03\uff1aExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM."]}, {"title": "\u540c\u7248\u672c\u624b\u518c\u8bf4\u660e", "paragraphs": ["EXPLAIN \u5c06 WHERE \u5b50\u53e5\u663e\u793a\u4e3a Seq Scan \u8282\u70b9\u4e0a\u7684 filter \u6761\u4ef6\u3002\u8fd9\u8868\u793a\u8282\u70b9\u5bf9\u626b\u63cf\u7684\u6bcf\u4e00\u884c\u68c0\u67e5\u6761\u4ef6\uff0c\u4ec5\u8f93\u51fa\u901a\u8fc7\u6761\u4ef6\u7684\u884c\u3002WHERE \u964d\u4f4e\u4e86\u9884\u8ba1\u8f93\u51fa\u884c\u6570\uff0c\u4f46\u626b\u63cf\u4ecd\u9700\u8bbf\u95ee\u5168\u90e8 10000 \u884c\uff0c\u6240\u4ee5\u6210\u672c\u6ca1\u6709\u964d\u4f4e\uff1b\u76f8\u53cd\u8fd8\u7565\u6709\u589e\u52a0\uff08\u51c6\u786e\u8bf4\u589e\u52a0 10000 * cpu_operator_cost\uff09\uff0c\u4ee5\u8ba1\u5165\u68c0\u67e5 WHERE \u6761\u4ef6\u6d88\u8017\u7684\u989d\u5916 CPU \u65f6\u95f4\u3002"]}, {"title": "\u672c\u7248\u624b\u518c\u4e2d\u7684\u793a\u4f8b", "blocks": [{"code": "EXPLAIN SELECT * FROM tenk1;\n\n                         QUERY PLAN\n-------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..458.00 rows=10000 width=244)", "source": {"url": "/docs/14/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 14.24 \u00b7 using-explain", "sha256": "7f5ab59cb21a035ada45ea3426c5d1cca3f781273483677f73fdd76753555206"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 14.24 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u4e0b\u9762\u662f\u4e00\u4e2a\u7b80\u5355\u793a\u4f8b\uff0c\u4ec5\u7528\u6765\u5c55\u793a\u8f93\u51fa\u5f62\u5f0f\uff1a"]}, {"code": "EXPLAIN SELECT * FROM tenk1 WHERE unique1 < 7000;\n\n                         QUERY PLAN\n------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..483.00 rows=7001 width=244)\n   Filter: (unique1 < 7000)", "source": {"url": "/docs/14/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 14.24 \u00b7 using-explain", "sha256": "7f5ab59cb21a035ada45ea3426c5d1cca3f781273483677f73fdd76753555206"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 14.24 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u73b0\u5728\u4fee\u6539\u67e5\u8be2\uff0c\u6dfb\u52a0 WHERE \u6761\u4ef6\uff1a"]}]}, {"title": "\u6267\u884c\u5668\u5b9e\u73b0\u8bf4\u660e", "paragraphs": ["nodeSeqscan.c\uff1a\u652f\u6301\u5173\u7cfb\u987a\u5e8f\u626b\u63cf\u7684\u4f8b\u7a0b\u3002", "ExecSeqScanEstimate \u4f30\u7b97\u5e76\u884c\u626b\u63cf\u6240\u9700\u7684 DSM \u7a7a\u95f4\uff1bExecSeqScanInitializeDSM \u521d\u59cb\u5316\u5e76\u884c\u626b\u63cf\u7684 DSM\uff1bExecSeqScanReInitializeDSM \u4e3a\u65b0\u4e00\u8f6e\u5e76\u884c\u626b\u63cf\u91cd\u65b0\u521d\u59cb\u5316 DSM\uff1bExecSeqScanInitializeWorker \u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u4e2d\u9644\u52a0\u5230 DSM \u4fe1\u606f\u3002", "\u626b\u63cf\u4e0d\u5e76\u884c\u6267\u884c\uff0c\u6216\u539f\u8ba1\u5212\u5e76\u884c\u7684\u626b\u63cf\u4ee5\u4e32\u884c\u65b9\u5f0f\u6267\u884c\u65f6\uff0c\u4f1a\u5230\u8fbe\u6b64\u5904\u3002", "SeqRecheck\uff1a\u5728 EvalPlanQual \u4e2d\u91cd\u65b0\u68c0\u67e5\u5143\u7ec4\u7684\u8bbf\u95ee\u65b9\u6cd5\u4f8b\u7a0b\u3002", "\u4e0e IndexScan \u4e0d\u540c\uff0cSeqScan \u4ece\u4e0d\u5728 heap_beginscan \u4e2d\u4f7f\u7528\u952e\uff08\u8fd9\u5f88\u4e0d\u597d\uff09\uff0c\u56e0\u6b64\u8fd9\u91cc\u4e0d\u68c0\u67e5\u952e\u662f\u5426\u6ee1\u8db3\u6761\u4ef6\u3002"]}, {"code": "case T_SeqScan:\n\t\t\tpname = sname = \"Seq Scan\";\n\t\t\tbreak;", "title": "\u6838\u5fc3\u6e90\u7801\u4e2d\u7684 EXPLAIN \u6807\u8bc6"}], "strategies": [], "description": ["\u901a\u8fc7\u987a\u5e8f\u626b\u63cf\u8bfb\u53d6\u5173\u7cfb\uff0c\u5e76\u5e94\u7528\u626b\u63cf\u6761\u4ef6\u3002"], "localization": {"status": "complete", "sources": [], "language": "zh", "original_text": {"/versions/14/description/0": "Reads a relation through a sequential scan and applies its scan qualifications.", "/versions/14/facts/0/label": "Core node tag", "/versions/14/facts/1/label": "Structured EXPLAIN Node Type", "/versions/14/facts/2/label": "Inputs", "/versions/14/facts/2/value": "A relation", "/versions/14/facts/3/label": "Output", "/versions/14/facts/3/value": "Qualified relation tuples", "/versions/14/facts/4/label": "Executor initializer", "/versions/14/facts/5/label": "Memory mechanism", "/versions/14/tables/0/title": "EXPLAIN labels in this source build", "/versions/14/related/1/label": "Using EXPLAIN", "/versions/14/related/2/label": "Parallel plans", "/versions/14/sections/0/title": "EXPLAIN names and attributes", "/versions/14/sections/1/title": "Memory and temporary storage", "/versions/14/sections/2/title": "Parallel execution and instrumentation", "/versions/14/sections/3/title": "Same-version manual discussion", "/versions/14/sections/4/title": "Examples from this manual build", "/versions/14/sections/5/title": "Executor implementation notes", "/versions/14/sections/6/title": "EXPLAIN identity in core source", "/versions/14/memory/description": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/14/sections/0/paragraphs/0": "Structured formats use the Node Type above. Text-format spellings can also include operation, strategy, join type, scan direction or aggregation-stage attributes.", "/versions/14/sections/0/paragraphs/1": "Text names recorded by this source: Seq Scan.", "/versions/14/sections/0/paragraphs/2": "Parallel-aware and parallel-safe are different plan properties. A node running inside a parallel worker is not necessarily a parallel-aware node.", "/versions/14/sections/1/paragraphs/0": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/14/sections/2/paragraphs/0": "The source callbacks below can coordinate execution or collect worker instrumentation. Their presence is not a blanket claim that this node supports a shared parallel scan or shared state.", "/versions/14/sections/2/paragraphs/1": "Callbacks in this build: ExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM.", "/versions/14/sections/3/paragraphs/0": "Notice that the EXPLAIN output shows the WHERE clause being applied as a \u201c filter \u201d condition attached to the Seq Scan plan node. This means that the plan node checks the condition for each row it scans, and outputs only the ones that pass the condition. The estimate of output rows has been reduced because of the WHERE clause. However, the scan will still have to visit all 10000 rows, so the cost hasn't decreased; in fact it has gone up a bit (by 10000 * cpu_operator_cost , to be exact) to reflect the extra CPU time spent checking the WHERE condition.", "/versions/14/sections/5/paragraphs/0": "nodeSeqscan.c Support routines for sequential scans of relations.", "/versions/14/sections/5/paragraphs/1": "ExecSeqScanEstimate estimates DSM space needed for parallel scan ExecSeqScanInitializeDSM initialize DSM for parallel scan ExecSeqScanReInitializeDSM reinitialize DSM for fresh parallel scan ExecSeqScanInitializeWorker attach to DSM info in parallel worker", "/versions/14/sections/5/paragraphs/2": "We reach here if the scan is not parallel, or if we're serially executing a scan that was planned to be parallel.", "/versions/14/sections/5/paragraphs/3": "SeqRecheck -- access method routine to recheck a tuple in EvalPlanQual", "/versions/14/sections/5/paragraphs/4": "Note that unlike IndexScan, SeqScan never use keys in heap_beginscan (and this is very bad) - so, here we do not check are keys ok or not.", "/versions/14/tables/0/columns/0/label": "Text-format label", "/versions/14/tables/0/columns/1/label": "Structured node identity", "/versions/14/sections/4/blocks/0/paragraphs/0": "Example copied from the PostgreSQL 14.24 manual; it was not executed for this collection.", "/versions/14/sections/4/blocks/0/paragraphs/1": "Here is a trivial example, just to show what the output looks like:", "/versions/14/sections/4/blocks/1/paragraphs/0": "Example copied from the PostgreSQL 14.24 manual; it was not executed for this collection.", "/versions/14/sections/4/blocks/1/paragraphs/1": "Now let's modify the query to add a WHERE condition:"}, "fallback_fields": [], "source_language": "en", "original_snapshot_sha256": "6d30e9676a7ddbbd06cae569cef24476d35f587d31ab2c2fa020602e397c1210"}, "evidence_kind": "source and documentation", "explain_names": ["Seq Scan"], "partial_modes": [], "comparison_data": {"node_tag": "T_SeqScan", "strategies": [], "text_names": ["Seq Scan"], "initializer": "ExecInitSeqScan", "partial_modes": [], "memory_mechanism": "unclassified", "parallel_callbacks": ["ExecSeqScanEstimate", "ExecSeqScanInitializeDSM", "ExecSeqScanInitializeWorker", "ExecSeqScanReInitializeDSM"]}, "comparison_hash": "5631c0db91382334dc4ca321ae0335013d10ae1c0c24f3523fcca0184bbbc81b", "explain_prefixes": ["Parallel", "Async"], "runtime_verified": false, "source_inventory": {"explain": "src/backend/commands/explain.c", "executor": "src/backend/executor/execProcnode.c", "implementation": "src/backend/executor/nodeSeqscan.c"}, "parallel_callbacks": ["ExecSeqScanEstimate", "ExecSeqScanInitializeDSM", "ExecSeqScanInitializeWorker", "ExecSeqScanReInitializeDSM"]}, "15": {"facts": [{"label": "\u6838\u5fc3\u8282\u70b9\u6807\u7b7e", "value": "T_SeqScan"}, {"label": "\u7ed3\u6784\u5316 EXPLAIN \u8282\u70b9\u7c7b\u578b", "value": "Seq Scan"}, {"label": "\u8f93\u5165", "value": "\u5173\u7cfb"}, {"label": "\u8f93\u51fa", "value": "\u6ee1\u8db3\u6761\u4ef6\u7684\u5173\u7cfb\u5143\u7ec4"}, {"label": "\u6267\u884c\u5668\u521d\u59cb\u5316\u51fd\u6570", "value": "ExecInitSeqScan"}, {"label": "\u5185\u5b58\u673a\u5236", "value": "unclassified"}], "memory": {"evidence": [{"url": "https://ftp.postgresql.org/pub/source/v15.19/postgresql-15.19.tar.bz2", "path": "src/backend/executor/nodeSeqscan.c", "label": "src/backend/executor/nodeSeqscan.c", "sha256": "b3482a0b98d5decf764f08ac286df61e6374a5d4d6349e9ae9007b086099e9eb", "archive_sha256": "e1a64a87a46b825b88c082e4518161a47aab53c45694964f8ba1df28f7859f89"}], "mechanism": "unclassified", "description": "\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002", "source_notes": []}, "tables": [{"key": "explain-labels", "rows": [{"label": "Seq Scan", "identity": "Seq Scan"}], "title": "\u672c\u6784\u5efa\u4e2d\u7684 EXPLAIN \u6807\u7b7e", "columns": [{"key": "label", "label": "\u6587\u672c\u683c\u5f0f\u6807\u7b7e"}, {"key": "identity", "label": "\u7ed3\u6784\u5316\u8282\u70b9\u6807\u8bc6"}]}], "related": [{"url": "/wiki/sql/explain/?v=15", "label": "EXPLAIN"}, {"url": "/docs/15/using-explain.html", "label": "\u4f7f\u7528 EXPLAIN"}, {"url": "/docs/15/parallel-plans.html", "label": "\u5e76\u884c\u8ba1\u5212"}, {"url": "/wiki/guc/enable_seqscan/?v=15", "label": "enable_seqscan"}], "release": {"ref": "PostgreSQL 15.19 source archive", "label": "15.19", "major": "15", "channel": "stable", "revision": "e1a64a87a46b825b88c082e4518161a47aab53c45694964f8ba1df28f7859f89", "source_url": "https://ftp.postgresql.org/pub/source/v15.19/postgresql-15.19.tar.bz2", "source_snapshot_utc": ""}, "sources": [{"url": "https://ftp.postgresql.org/pub/source/v15.19/postgresql-15.19.tar.bz2", "line": 1225, "path": 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"https://ftp.postgresql.org/pub/source/v15.19/postgresql-15.19.tar.bz2", "path": "src/include/nodes/plannodes.h", "label": "src/include/nodes/plannodes.h", "sha256": "fb4a4c8165495299131173680bc02a950d88e1ff610231fd97997bc0c9afc1d7", "archive_sha256": "e1a64a87a46b825b88c082e4518161a47aab53c45694964f8ba1df28f7859f89"}, {"url": "https://pg.center/docs/15/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 15.19 \u00b7 using-explain", "sha256": "d1f509457c647da453d2575c772d91022a0f115dd84f9a3b20f5ff25a243648f", "language": "en", "original_url": "/docs/15/using-explain.html#USING-EXPLAIN-BASICS"}], "node_tag": "T_SeqScan", "sections": [{"title": "EXPLAIN \u540d\u79f0\u4e0e\u5c5e\u6027", "paragraphs": ["\u7ed3\u6784\u5316\u683c\u5f0f\u4f7f\u7528\u4e0a\u8ff0 Node Type\u3002\u6587\u672c\u683c\u5f0f\u540d\u79f0\u8fd8\u53ef\u80fd\u5305\u542b\u64cd\u4f5c\u3001\u7b56\u7565\u3001\u8fde\u63a5\u7c7b\u578b\u3001\u626b\u63cf\u65b9\u5411\u6216\u805a\u5408\u9636\u6bb5\u5c5e\u6027\u3002", "\u6b64\u6e90\u7801\u8bb0\u5f55\u7684\u6587\u672c\u540d\u79f0\uff1aSeq Scan.", "\u5e76\u884c\u611f\u77e5\u4e0e\u5e76\u884c\u5b89\u5168\u662f\u4e0d\u540c\u7684\u8ba1\u5212\u5c5e\u6027\u3002\u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u5185\u8fd0\u884c\u7684\u8282\u70b9\u4e0d\u4e00\u5b9a\u662f\u5e76\u884c\u611f\u77e5\u8282\u70b9\u3002"]}, {"title": "\u5185\u5b58\u4e0e\u4e34\u65f6\u5b58\u50a8", "paragraphs": ["\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002"]}, {"title": "\u5e76\u884c\u6267\u884c\u4e0e\u8fd0\u884c\u4fe1\u606f\u91c7\u96c6", "paragraphs": ["\u4ee5\u4e0b\u6e90\u7801\u56de\u8c03\u53ef\u4ee5\u534f\u8c03\u6267\u884c\u6216\u6536\u96c6\u5de5\u4f5c\u8fdb\u7a0b\u7684\u6d4b\u91cf\u6570\u636e\u3002\u56de\u8c03\u5b58\u5728\u4e0d\u4ee3\u8868\u8be5\u8282\u70b9\u666e\u904d\u652f\u6301\u5171\u4eab\u5e76\u884c\u626b\u63cf\u6216\u5171\u4eab\u72b6\u6001\u3002", "\u6b64\u6784\u5efa\u7684\u56de\u8c03\uff1aExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM."]}, {"title": "\u540c\u7248\u672c\u624b\u518c\u8bf4\u660e", "paragraphs": ["EXPLAIN \u5c06 WHERE \u5b50\u53e5\u663e\u793a\u4e3a Seq Scan \u8282\u70b9\u4e0a\u7684 filter \u6761\u4ef6\u3002\u8fd9\u8868\u793a\u8282\u70b9\u5bf9\u626b\u63cf\u7684\u6bcf\u4e00\u884c\u68c0\u67e5\u6761\u4ef6\uff0c\u4ec5\u8f93\u51fa\u901a\u8fc7\u6761\u4ef6\u7684\u884c\u3002WHERE \u964d\u4f4e\u4e86\u9884\u8ba1\u8f93\u51fa\u884c\u6570\uff0c\u4f46\u626b\u63cf\u4ecd\u9700\u8bbf\u95ee\u5168\u90e8 10000 \u884c\uff0c\u6240\u4ee5\u6210\u672c\u6ca1\u6709\u964d\u4f4e\uff1b\u76f8\u53cd\u8fd8\u7565\u6709\u589e\u52a0\uff08\u51c6\u786e\u8bf4\u589e\u52a0 10000 * cpu_operator_cost\uff09\uff0c\u4ee5\u8ba1\u5165\u68c0\u67e5 WHERE \u6761\u4ef6\u6d88\u8017\u7684\u989d\u5916 CPU \u65f6\u95f4\u3002"]}, {"title": "\u672c\u7248\u624b\u518c\u4e2d\u7684\u793a\u4f8b", "blocks": [{"code": "EXPLAIN SELECT * FROM tenk1;\n\n                         QUERY PLAN\n-------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..458.00 rows=10000 width=244)", "source": {"url": "/docs/15/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 15.19 \u00b7 using-explain", "sha256": "d1f509457c647da453d2575c772d91022a0f115dd84f9a3b20f5ff25a243648f"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 15.19 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u4e0b\u9762\u662f\u4e00\u4e2a\u7b80\u5355\u793a\u4f8b\uff0c\u4ec5\u7528\u6765\u5c55\u793a\u8f93\u51fa\u5f62\u5f0f\uff1a"]}, {"code": "EXPLAIN SELECT * FROM tenk1 WHERE unique1 < 7000;\n\n                         QUERY PLAN\n------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..483.00 rows=7001 width=244)\n   Filter: (unique1 < 7000)", "source": {"url": "/docs/15/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 15.19 \u00b7 using-explain", "sha256": "d1f509457c647da453d2575c772d91022a0f115dd84f9a3b20f5ff25a243648f"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 15.19 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u73b0\u5728\u4fee\u6539\u67e5\u8be2\uff0c\u6dfb\u52a0 WHERE \u6761\u4ef6\uff1a"]}]}, {"title": "\u6267\u884c\u5668\u5b9e\u73b0\u8bf4\u660e", "paragraphs": ["nodeSeqscan.c\uff1a\u652f\u6301\u5173\u7cfb\u987a\u5e8f\u626b\u63cf\u7684\u4f8b\u7a0b\u3002", "ExecSeqScanEstimate \u4f30\u7b97\u5e76\u884c\u626b\u63cf\u6240\u9700\u7684 DSM \u7a7a\u95f4\uff1bExecSeqScanInitializeDSM \u521d\u59cb\u5316\u5e76\u884c\u626b\u63cf\u7684 DSM\uff1bExecSeqScanReInitializeDSM \u4e3a\u65b0\u4e00\u8f6e\u5e76\u884c\u626b\u63cf\u91cd\u65b0\u521d\u59cb\u5316 DSM\uff1bExecSeqScanInitializeWorker \u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u4e2d\u9644\u52a0\u5230 DSM \u4fe1\u606f\u3002", "\u626b\u63cf\u4e0d\u5e76\u884c\u6267\u884c\uff0c\u6216\u539f\u8ba1\u5212\u5e76\u884c\u7684\u626b\u63cf\u4ee5\u4e32\u884c\u65b9\u5f0f\u6267\u884c\u65f6\uff0c\u4f1a\u5230\u8fbe\u6b64\u5904\u3002", "SeqRecheck\uff1a\u5728 EvalPlanQual \u4e2d\u91cd\u65b0\u68c0\u67e5\u5143\u7ec4\u7684\u8bbf\u95ee\u65b9\u6cd5\u4f8b\u7a0b\u3002", "\u4e0e IndexScan \u4e0d\u540c\uff0cSeqScan \u4ece\u4e0d\u5728 heap_beginscan \u4e2d\u4f7f\u7528\u952e\uff08\u8fd9\u5f88\u4e0d\u597d\uff09\uff0c\u56e0\u6b64\u8fd9\u91cc\u4e0d\u68c0\u67e5\u952e\u662f\u5426\u6ee1\u8db3\u6761\u4ef6\u3002"]}, {"code": "case T_SeqScan:\n\t\t\tpname = sname = \"Seq Scan\";\n\t\t\tbreak;", "title": "\u6838\u5fc3\u6e90\u7801\u4e2d\u7684 EXPLAIN \u6807\u8bc6"}], "strategies": [], "description": ["\u901a\u8fc7\u987a\u5e8f\u626b\u63cf\u8bfb\u53d6\u5173\u7cfb\uff0c\u5e76\u5e94\u7528\u626b\u63cf\u6761\u4ef6\u3002"], "localization": {"status": "complete", "sources": [], "language": "zh", "original_text": {"/versions/15/description/0": "Reads a relation through a sequential scan and applies its scan qualifications.", "/versions/15/facts/0/label": "Core node tag", "/versions/15/facts/1/label": "Structured EXPLAIN Node Type", "/versions/15/facts/2/label": "Inputs", "/versions/15/facts/2/value": "A relation", "/versions/15/facts/3/label": "Output", "/versions/15/facts/3/value": "Qualified relation tuples", "/versions/15/facts/4/label": "Executor initializer", "/versions/15/facts/5/label": "Memory mechanism", "/versions/15/tables/0/title": "EXPLAIN labels in this source build", "/versions/15/related/1/label": "Using EXPLAIN", "/versions/15/related/2/label": "Parallel plans", "/versions/15/sections/0/title": "EXPLAIN names and attributes", "/versions/15/sections/1/title": "Memory and temporary storage", "/versions/15/sections/2/title": "Parallel execution and instrumentation", "/versions/15/sections/3/title": "Same-version manual discussion", "/versions/15/sections/4/title": "Examples from this manual build", "/versions/15/sections/5/title": "Executor implementation notes", "/versions/15/sections/6/title": "EXPLAIN identity in core source", "/versions/15/memory/description": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/15/sections/0/paragraphs/0": "Structured formats use the Node Type above. Text-format spellings can also include operation, strategy, join type, scan direction or aggregation-stage attributes.", "/versions/15/sections/0/paragraphs/1": "Text names recorded by this source: Seq Scan.", "/versions/15/sections/0/paragraphs/2": "Parallel-aware and parallel-safe are different plan properties. A node running inside a parallel worker is not necessarily a parallel-aware node.", "/versions/15/sections/1/paragraphs/0": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/15/sections/2/paragraphs/0": "The source callbacks below can coordinate execution or collect worker instrumentation. Their presence is not a blanket claim that this node supports a shared parallel scan or shared state.", "/versions/15/sections/2/paragraphs/1": "Callbacks in this build: ExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM.", "/versions/15/sections/3/paragraphs/0": "Notice that the EXPLAIN output shows the WHERE clause being applied as a \u201c filter \u201d condition attached to the Seq Scan plan node. This means that the plan node checks the condition for each row it scans, and outputs only the ones that pass the condition. The estimate of output rows has been reduced because of the WHERE clause. However, the scan will still have to visit all 10000 rows, so the cost hasn't decreased; in fact it has gone up a bit (by 10000 * cpu_operator_cost , to be exact) to reflect the extra CPU time spent checking the WHERE condition.", "/versions/15/sections/5/paragraphs/0": "nodeSeqscan.c Support routines for sequential scans of relations.", "/versions/15/sections/5/paragraphs/1": "ExecSeqScanEstimate estimates DSM space needed for parallel scan ExecSeqScanInitializeDSM initialize DSM for parallel scan ExecSeqScanReInitializeDSM reinitialize DSM for fresh parallel scan ExecSeqScanInitializeWorker attach to DSM info in parallel worker", "/versions/15/sections/5/paragraphs/2": "We reach here if the scan is not parallel, or if we're serially executing a scan that was planned to be parallel.", "/versions/15/sections/5/paragraphs/3": "SeqRecheck -- access method routine to recheck a tuple in EvalPlanQual", "/versions/15/sections/5/paragraphs/4": "Note that unlike IndexScan, SeqScan never use keys in heap_beginscan (and this is very bad) - so, here we do not check are keys ok or not.", "/versions/15/tables/0/columns/0/label": "Text-format label", "/versions/15/tables/0/columns/1/label": "Structured node identity", "/versions/15/sections/4/blocks/0/paragraphs/0": "Example copied from the PostgreSQL 15.19 manual; it was not executed for this collection.", "/versions/15/sections/4/blocks/0/paragraphs/1": "Here is a trivial example, just to show what the output looks like:", "/versions/15/sections/4/blocks/1/paragraphs/0": "Example copied from the PostgreSQL 15.19 manual; it was not executed for this collection.", "/versions/15/sections/4/blocks/1/paragraphs/1": "Now let's modify the query to add a WHERE condition:"}, "fallback_fields": [], "source_language": "en", "original_snapshot_sha256": "c34a6d6f20e84d011d72f1ef69c4df12e860d677c3977737562c9c0fb5549dc9"}, "evidence_kind": "source and documentation", "explain_names": ["Seq Scan"], "partial_modes": [], "comparison_data": {"node_tag": "T_SeqScan", "strategies": [], "text_names": ["Seq Scan"], "initializer": "ExecInitSeqScan", "partial_modes": [], "memory_mechanism": "unclassified", "parallel_callbacks": ["ExecSeqScanEstimate", "ExecSeqScanInitializeDSM", "ExecSeqScanInitializeWorker", "ExecSeqScanReInitializeDSM"]}, "comparison_hash": "5631c0db91382334dc4ca321ae0335013d10ae1c0c24f3523fcca0184bbbc81b", "explain_prefixes": ["Parallel", "Async"], "runtime_verified": false, "source_inventory": {"explain": "src/backend/commands/explain.c", "executor": "src/backend/executor/execProcnode.c", "implementation": "src/backend/executor/nodeSeqscan.c"}, "parallel_callbacks": ["ExecSeqScanEstimate", "ExecSeqScanInitializeDSM", "ExecSeqScanInitializeWorker", "ExecSeqScanReInitializeDSM"]}, "16": {"facts": [{"label": "\u6838\u5fc3\u8282\u70b9\u6807\u7b7e", "value": "T_SeqScan"}, {"label": "\u7ed3\u6784\u5316 EXPLAIN \u8282\u70b9\u7c7b\u578b", "value": "Seq Scan"}, {"label": "\u8f93\u5165", "value": "\u5173\u7cfb"}, {"label": "\u8f93\u51fa", "value": "\u6ee1\u8db3\u6761\u4ef6\u7684\u5173\u7cfb\u5143\u7ec4"}, {"label": "\u6267\u884c\u5668\u521d\u59cb\u5316\u51fd\u6570", "value": "ExecInitSeqScan"}, {"label": "\u5185\u5b58\u673a\u5236", "value": "unclassified"}], "memory": {"evidence": [{"url": "https://ftp.postgresql.org/pub/source/v16.15/postgresql-16.15.tar.bz2", "path": "src/backend/executor/nodeSeqscan.c", "label": "src/backend/executor/nodeSeqscan.c", "sha256": "a1aca3f4b30d1999d446cc445c687170008683d30c2129ce5ffb2205f3ca88ea", "archive_sha256": "c1575341fa7bd40f5274ea465b34390f4dc64cdd0770af327005caaeb9f6b7ed"}], "mechanism": "unclassified", "description": "\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002", "source_notes": []}, "tables": [{"key": "explain-labels", "rows": [{"label": "Seq Scan", "identity": "Seq Scan"}], "title": "\u672c\u6784\u5efa\u4e2d\u7684 EXPLAIN \u6807\u7b7e", "columns": [{"key": "label", "label": "\u6587\u672c\u683c\u5f0f\u6807\u7b7e"}, {"key": "identity", "label": "\u7ed3\u6784\u5316\u8282\u70b9\u6807\u8bc6"}]}], "related": [{"url": "/wiki/sql/explain/?v=16", "label": "EXPLAIN"}, {"url": "/docs/16/using-explain.html", "label": "\u4f7f\u7528 EXPLAIN"}, {"url": "/docs/16/parallel-plans.html", "label": "\u5e76\u884c\u8ba1\u5212"}, {"url": "/wiki/guc/enable_seqscan/?v=16", "label": "enable_seqscan"}], "release": {"ref": "PostgreSQL 16.15 source archive", "label": "16.15", "major": "16", "channel": "stable", "revision": "c1575341fa7bd40f5274ea465b34390f4dc64cdd0770af327005caaeb9f6b7ed", "source_url": "https://ftp.postgresql.org/pub/source/v16.15/postgresql-16.15.tar.bz2", "source_snapshot_utc": ""}, "sources": [{"url": "https://ftp.postgresql.org/pub/source/v16.15/postgresql-16.15.tar.bz2", "line": 1258, "path": 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"https://ftp.postgresql.org/pub/source/v16.15/postgresql-16.15.tar.bz2", "path": "src/include/nodes/plannodes.h", "label": "src/include/nodes/plannodes.h", "sha256": "97db47353db76326b874589a5ad0a04501cc74cd72e237e7bd956e7472c41f1f", "archive_sha256": "c1575341fa7bd40f5274ea465b34390f4dc64cdd0770af327005caaeb9f6b7ed"}, {"url": "https://pg.center/docs/16/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 16.15 \u00b7 using-explain", "sha256": "bd8b86e5281cf0e52ad6e0e4bb8b6ff6510dac61982b44f0c1dd07d54012db3c", "language": "en", "original_url": "/docs/16/using-explain.html#USING-EXPLAIN-BASICS"}], "node_tag": "T_SeqScan", "sections": [{"title": "EXPLAIN \u540d\u79f0\u4e0e\u5c5e\u6027", "paragraphs": ["\u7ed3\u6784\u5316\u683c\u5f0f\u4f7f\u7528\u4e0a\u8ff0 Node Type\u3002\u6587\u672c\u683c\u5f0f\u540d\u79f0\u8fd8\u53ef\u80fd\u5305\u542b\u64cd\u4f5c\u3001\u7b56\u7565\u3001\u8fde\u63a5\u7c7b\u578b\u3001\u626b\u63cf\u65b9\u5411\u6216\u805a\u5408\u9636\u6bb5\u5c5e\u6027\u3002", "\u6b64\u6e90\u7801\u8bb0\u5f55\u7684\u6587\u672c\u540d\u79f0\uff1aSeq Scan.", "\u5e76\u884c\u611f\u77e5\u4e0e\u5e76\u884c\u5b89\u5168\u662f\u4e0d\u540c\u7684\u8ba1\u5212\u5c5e\u6027\u3002\u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u5185\u8fd0\u884c\u7684\u8282\u70b9\u4e0d\u4e00\u5b9a\u662f\u5e76\u884c\u611f\u77e5\u8282\u70b9\u3002"]}, {"title": "\u5185\u5b58\u4e0e\u4e34\u65f6\u5b58\u50a8", "paragraphs": ["\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002"]}, {"title": "\u5e76\u884c\u6267\u884c\u4e0e\u8fd0\u884c\u4fe1\u606f\u91c7\u96c6", "paragraphs": ["\u4ee5\u4e0b\u6e90\u7801\u56de\u8c03\u53ef\u4ee5\u534f\u8c03\u6267\u884c\u6216\u6536\u96c6\u5de5\u4f5c\u8fdb\u7a0b\u7684\u6d4b\u91cf\u6570\u636e\u3002\u56de\u8c03\u5b58\u5728\u4e0d\u4ee3\u8868\u8be5\u8282\u70b9\u666e\u904d\u652f\u6301\u5171\u4eab\u5e76\u884c\u626b\u63cf\u6216\u5171\u4eab\u72b6\u6001\u3002", "\u6b64\u6784\u5efa\u7684\u56de\u8c03\uff1aExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM."]}, {"title": "\u540c\u7248\u672c\u624b\u518c\u8bf4\u660e", "paragraphs": ["EXPLAIN \u5c06 WHERE \u5b50\u53e5\u663e\u793a\u4e3a Seq Scan \u8282\u70b9\u4e0a\u7684 filter \u6761\u4ef6\u3002\u8fd9\u8868\u793a\u8282\u70b9\u5bf9\u626b\u63cf\u7684\u6bcf\u4e00\u884c\u68c0\u67e5\u6761\u4ef6\uff0c\u4ec5\u8f93\u51fa\u901a\u8fc7\u6761\u4ef6\u7684\u884c\u3002WHERE \u964d\u4f4e\u4e86\u9884\u8ba1\u8f93\u51fa\u884c\u6570\uff0c\u4f46\u626b\u63cf\u4ecd\u9700\u8bbf\u95ee\u5168\u90e8 10000 \u884c\uff0c\u6240\u4ee5\u6210\u672c\u6ca1\u6709\u964d\u4f4e\uff1b\u76f8\u53cd\u8fd8\u7565\u6709\u589e\u52a0\uff08\u51c6\u786e\u8bf4\u589e\u52a0 10000 * cpu_operator_cost\uff09\uff0c\u4ee5\u8ba1\u5165\u68c0\u67e5 WHERE \u6761\u4ef6\u6d88\u8017\u7684\u989d\u5916 CPU \u65f6\u95f4\u3002"]}, {"title": "\u672c\u7248\u624b\u518c\u4e2d\u7684\u793a\u4f8b", "blocks": [{"code": "EXPLAIN SELECT * FROM tenk1;\n\n                         QUERY PLAN\n-------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..458.00 rows=10000 width=244)", "source": {"url": "/docs/16/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 16.15 \u00b7 using-explain", "sha256": "bd8b86e5281cf0e52ad6e0e4bb8b6ff6510dac61982b44f0c1dd07d54012db3c"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 16.15 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u4e0b\u9762\u662f\u4e00\u4e2a\u7b80\u5355\u793a\u4f8b\uff0c\u4ec5\u7528\u6765\u5c55\u793a\u8f93\u51fa\u5f62\u5f0f\uff1a"]}, {"code": "EXPLAIN SELECT * FROM tenk1 WHERE unique1 < 7000;\n\n                         QUERY PLAN\n------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..483.00 rows=7001 width=244)\n   Filter: (unique1 < 7000)", "source": {"url": "/docs/16/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 16.15 \u00b7 using-explain", "sha256": "bd8b86e5281cf0e52ad6e0e4bb8b6ff6510dac61982b44f0c1dd07d54012db3c"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 16.15 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u73b0\u5728\u4fee\u6539\u67e5\u8be2\uff0c\u6dfb\u52a0 WHERE \u6761\u4ef6\uff1a"]}]}, {"title": "\u6267\u884c\u5668\u5b9e\u73b0\u8bf4\u660e", "paragraphs": ["nodeSeqscan.c\uff1a\u652f\u6301\u5173\u7cfb\u987a\u5e8f\u626b\u63cf\u7684\u4f8b\u7a0b\u3002", "ExecSeqScanEstimate \u4f30\u7b97\u5e76\u884c\u626b\u63cf\u6240\u9700\u7684 DSM \u7a7a\u95f4\uff1bExecSeqScanInitializeDSM \u521d\u59cb\u5316\u5e76\u884c\u626b\u63cf\u7684 DSM\uff1bExecSeqScanReInitializeDSM \u4e3a\u65b0\u4e00\u8f6e\u5e76\u884c\u626b\u63cf\u91cd\u65b0\u521d\u59cb\u5316 DSM\uff1bExecSeqScanInitializeWorker \u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u4e2d\u9644\u52a0\u5230 DSM \u4fe1\u606f\u3002", "\u626b\u63cf\u4e0d\u5e76\u884c\u6267\u884c\uff0c\u6216\u539f\u8ba1\u5212\u5e76\u884c\u7684\u626b\u63cf\u4ee5\u4e32\u884c\u65b9\u5f0f\u6267\u884c\u65f6\uff0c\u4f1a\u5230\u8fbe\u6b64\u5904\u3002", "SeqRecheck\uff1a\u5728 EvalPlanQual \u4e2d\u91cd\u65b0\u68c0\u67e5\u5143\u7ec4\u7684\u8bbf\u95ee\u65b9\u6cd5\u4f8b\u7a0b\u3002", "\u4e0e IndexScan \u4e0d\u540c\uff0cSeqScan \u4ece\u4e0d\u5728 heap_beginscan \u4e2d\u4f7f\u7528\u952e\uff08\u8fd9\u5f88\u4e0d\u597d\uff09\uff0c\u56e0\u6b64\u8fd9\u91cc\u4e0d\u68c0\u67e5\u952e\u662f\u5426\u6ee1\u8db3\u6761\u4ef6\u3002"]}, {"code": "case T_SeqScan:\n\t\t\tpname = sname = \"Seq Scan\";\n\t\t\tbreak;", "title": "\u6838\u5fc3\u6e90\u7801\u4e2d\u7684 EXPLAIN \u6807\u8bc6"}], "strategies": [], "description": ["\u901a\u8fc7\u987a\u5e8f\u626b\u63cf\u8bfb\u53d6\u5173\u7cfb\uff0c\u5e76\u5e94\u7528\u626b\u63cf\u6761\u4ef6\u3002"], "localization": {"status": "complete", "sources": [], "language": "zh", "original_text": {"/versions/16/description/0": "Reads a relation through a sequential scan and applies its scan qualifications.", "/versions/16/facts/0/label": "Core node tag", "/versions/16/facts/1/label": "Structured EXPLAIN Node Type", "/versions/16/facts/2/label": "Inputs", "/versions/16/facts/2/value": "A relation", "/versions/16/facts/3/label": "Output", "/versions/16/facts/3/value": "Qualified relation tuples", "/versions/16/facts/4/label": "Executor initializer", "/versions/16/facts/5/label": "Memory mechanism", "/versions/16/tables/0/title": "EXPLAIN labels in this source build", "/versions/16/related/1/label": "Using EXPLAIN", "/versions/16/related/2/label": "Parallel plans", "/versions/16/sections/0/title": "EXPLAIN names and attributes", "/versions/16/sections/1/title": "Memory and temporary storage", "/versions/16/sections/2/title": "Parallel execution and instrumentation", "/versions/16/sections/3/title": "Same-version manual discussion", "/versions/16/sections/4/title": "Examples from this manual build", "/versions/16/sections/5/title": "Executor implementation notes", "/versions/16/sections/6/title": "EXPLAIN identity in core source", "/versions/16/memory/description": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/16/sections/0/paragraphs/0": "Structured formats use the Node Type above. Text-format spellings can also include operation, strategy, join type, scan direction or aggregation-stage attributes.", "/versions/16/sections/0/paragraphs/1": "Text names recorded by this source: Seq Scan.", "/versions/16/sections/0/paragraphs/2": "Parallel-aware and parallel-safe are different plan properties. A node running inside a parallel worker is not necessarily a parallel-aware node.", "/versions/16/sections/1/paragraphs/0": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/16/sections/2/paragraphs/0": "The source callbacks below can coordinate execution or collect worker instrumentation. Their presence is not a blanket claim that this node supports a shared parallel scan or shared state.", "/versions/16/sections/2/paragraphs/1": "Callbacks in this build: ExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM.", "/versions/16/sections/3/paragraphs/0": "Notice that the EXPLAIN output shows the WHERE clause being applied as a \u201c filter \u201d condition attached to the Seq Scan plan node. This means that the plan node checks the condition for each row it scans, and outputs only the ones that pass the condition. The estimate of output rows has been reduced because of the WHERE clause. However, the scan will still have to visit all 10000 rows, so the cost hasn't decreased; in fact it has gone up a bit (by 10000 * cpu_operator_cost , to be exact) to reflect the extra CPU time spent checking the WHERE condition.", "/versions/16/sections/5/paragraphs/0": "nodeSeqscan.c Support routines for sequential scans of relations.", "/versions/16/sections/5/paragraphs/1": "ExecSeqScanEstimate estimates DSM space needed for parallel scan ExecSeqScanInitializeDSM initialize DSM for parallel scan ExecSeqScanReInitializeDSM reinitialize DSM for fresh parallel scan ExecSeqScanInitializeWorker attach to DSM info in parallel worker", "/versions/16/sections/5/paragraphs/2": "We reach here if the scan is not parallel, or if we're serially executing a scan that was planned to be parallel.", "/versions/16/sections/5/paragraphs/3": "SeqRecheck -- access method routine to recheck a tuple in EvalPlanQual", "/versions/16/sections/5/paragraphs/4": "Note that unlike IndexScan, SeqScan never use keys in heap_beginscan (and this is very bad) - so, here we do not check are keys ok or not.", "/versions/16/tables/0/columns/0/label": "Text-format label", "/versions/16/tables/0/columns/1/label": "Structured node identity", "/versions/16/sections/4/blocks/0/paragraphs/0": "Example copied from the PostgreSQL 16.15 manual; it was not executed for this collection.", "/versions/16/sections/4/blocks/0/paragraphs/1": "Here is a trivial example, just to show what the output looks like:", "/versions/16/sections/4/blocks/1/paragraphs/0": "Example copied from the PostgreSQL 16.15 manual; it was not executed for this collection.", "/versions/16/sections/4/blocks/1/paragraphs/1": "Now let's modify the query to add a WHERE condition:"}, "fallback_fields": [], "source_language": "en", "original_snapshot_sha256": "cb1230c1a7caf2aad29e6d67bdc7868b4fc113a09e6cb9b24d4e667bf0b87a7d"}, "evidence_kind": "source and documentation", "explain_names": ["Seq Scan"], "partial_modes": [], "comparison_data": {"node_tag": "T_SeqScan", "strategies": [], "text_names": ["Seq Scan"], "initializer": "ExecInitSeqScan", "partial_modes": [], "memory_mechanism": "unclassified", "parallel_callbacks": ["ExecSeqScanEstimate", "ExecSeqScanInitializeDSM", "ExecSeqScanInitializeWorker", "ExecSeqScanReInitializeDSM"]}, "comparison_hash": "5631c0db91382334dc4ca321ae0335013d10ae1c0c24f3523fcca0184bbbc81b", "explain_prefixes": ["Parallel", "Async"], "runtime_verified": false, "source_inventory": {"explain": "src/backend/commands/explain.c", "executor": "src/backend/executor/execProcnode.c", "implementation": "src/backend/executor/nodeSeqscan.c"}, "parallel_callbacks": ["ExecSeqScanEstimate", "ExecSeqScanInitializeDSM", "ExecSeqScanInitializeWorker", "ExecSeqScanReInitializeDSM"]}, "17": {"facts": [{"label": "\u6838\u5fc3\u8282\u70b9\u6807\u7b7e", "value": "T_SeqScan"}, {"label": "\u7ed3\u6784\u5316 EXPLAIN \u8282\u70b9\u7c7b\u578b", "value": "Seq Scan"}, {"label": "\u8f93\u5165", "value": "\u5173\u7cfb"}, {"label": "\u8f93\u51fa", "value": "\u6ee1\u8db3\u6761\u4ef6\u7684\u5173\u7cfb\u5143\u7ec4"}, {"label": "\u6267\u884c\u5668\u521d\u59cb\u5316\u51fd\u6570", "value": "ExecInitSeqScan"}, {"label": "\u5185\u5b58\u673a\u5236", "value": "unclassified"}], "memory": {"evidence": [{"url": "https://ftp.postgresql.org/pub/source/v17.11/postgresql-17.11.tar.bz2", "path": "src/backend/executor/nodeSeqscan.c", "label": "src/backend/executor/nodeSeqscan.c", "sha256": "6ab025b04660ea6a063dd10834fb7ec2fb091907c6feb694ac4a6cc2281de64c", "archive_sha256": "dd27f2b3c59e73ed14aa3324901242bf69a032a6347805f274e6260322d42979"}], "mechanism": "unclassified", "description": "\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002", "source_notes": []}, "tables": [{"key": "explain-labels", "rows": [{"label": "Seq Scan", "identity": "Seq Scan"}], "title": "\u672c\u6784\u5efa\u4e2d\u7684 EXPLAIN \u6807\u7b7e", "columns": [{"key": "label", "label": "\u6587\u672c\u683c\u5f0f\u6807\u7b7e"}, {"key": "identity", "label": "\u7ed3\u6784\u5316\u8282\u70b9\u6807\u8bc6"}]}], "related": [{"url": "/wiki/sql/explain/?v=17", "label": "EXPLAIN"}, {"url": "/docs/17/using-explain.html", "label": "\u4f7f\u7528 EXPLAIN"}, {"url": "/docs/17/parallel-plans.html", "label": "\u5e76\u884c\u8ba1\u5212"}, {"url": "/wiki/guc/enable_seqscan/?v=17", "label": "enable_seqscan"}], "release": {"ref": "PostgreSQL 17.11 source archive", "label": "17.11", "major": "17", "channel": "stable", "revision": "dd27f2b3c59e73ed14aa3324901242bf69a032a6347805f274e6260322d42979", "source_url": "https://ftp.postgresql.org/pub/source/v17.11/postgresql-17.11.tar.bz2", "source_snapshot_utc": ""}, "sources": [{"url": "https://ftp.postgresql.org/pub/source/v17.11/postgresql-17.11.tar.bz2", "line": 1447, "path": 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"https://ftp.postgresql.org/pub/source/v17.11/postgresql-17.11.tar.bz2", "path": "src/include/nodes/plannodes.h", "label": "src/include/nodes/plannodes.h", "sha256": "d390dd69e2d3f5085beb42b33e46ff0676a2959b916a12b82118a7e545f8e562", "archive_sha256": "dd27f2b3c59e73ed14aa3324901242bf69a032a6347805f274e6260322d42979"}, {"url": "https://pg.center/docs/17/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 17.11 \u00b7 using-explain", "sha256": "8e3422c77496cc53bfc225ccadda3e82eb23c8c362b8eb95e7fb02cdb315ea78", "language": "en", "original_url": "/docs/17/using-explain.html#USING-EXPLAIN-BASICS"}], "node_tag": "T_SeqScan", "sections": [{"title": "EXPLAIN \u540d\u79f0\u4e0e\u5c5e\u6027", "paragraphs": ["\u7ed3\u6784\u5316\u683c\u5f0f\u4f7f\u7528\u4e0a\u8ff0 Node Type\u3002\u6587\u672c\u683c\u5f0f\u540d\u79f0\u8fd8\u53ef\u80fd\u5305\u542b\u64cd\u4f5c\u3001\u7b56\u7565\u3001\u8fde\u63a5\u7c7b\u578b\u3001\u626b\u63cf\u65b9\u5411\u6216\u805a\u5408\u9636\u6bb5\u5c5e\u6027\u3002", "\u6b64\u6e90\u7801\u8bb0\u5f55\u7684\u6587\u672c\u540d\u79f0\uff1aSeq Scan.", "\u5e76\u884c\u611f\u77e5\u4e0e\u5e76\u884c\u5b89\u5168\u662f\u4e0d\u540c\u7684\u8ba1\u5212\u5c5e\u6027\u3002\u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u5185\u8fd0\u884c\u7684\u8282\u70b9\u4e0d\u4e00\u5b9a\u662f\u5e76\u884c\u611f\u77e5\u8282\u70b9\u3002"]}, {"title": "\u5185\u5b58\u4e0e\u4e34\u65f6\u5b58\u50a8", "paragraphs": ["\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002"]}, {"title": "\u5e76\u884c\u6267\u884c\u4e0e\u8fd0\u884c\u4fe1\u606f\u91c7\u96c6", "paragraphs": ["\u4ee5\u4e0b\u6e90\u7801\u56de\u8c03\u53ef\u4ee5\u534f\u8c03\u6267\u884c\u6216\u6536\u96c6\u5de5\u4f5c\u8fdb\u7a0b\u7684\u6d4b\u91cf\u6570\u636e\u3002\u56de\u8c03\u5b58\u5728\u4e0d\u4ee3\u8868\u8be5\u8282\u70b9\u666e\u904d\u652f\u6301\u5171\u4eab\u5e76\u884c\u626b\u63cf\u6216\u5171\u4eab\u72b6\u6001\u3002", "\u6b64\u6784\u5efa\u7684\u56de\u8c03\uff1aExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM."]}, {"title": "\u540c\u7248\u672c\u624b\u518c\u8bf4\u660e", "paragraphs": ["EXPLAIN \u5c06 WHERE \u5b50\u53e5\u663e\u793a\u4e3a Seq Scan \u8282\u70b9\u4e0a\u7684 filter \u6761\u4ef6\u3002\u8fd9\u8868\u793a\u8282\u70b9\u5bf9\u626b\u63cf\u7684\u6bcf\u4e00\u884c\u68c0\u67e5\u6761\u4ef6\uff0c\u4ec5\u8f93\u51fa\u901a\u8fc7\u6761\u4ef6\u7684\u884c\u3002WHERE \u964d\u4f4e\u4e86\u9884\u8ba1\u8f93\u51fa\u884c\u6570\uff0c\u4f46\u626b\u63cf\u4ecd\u9700\u8bbf\u95ee\u5168\u90e8 10000 \u884c\uff0c\u6240\u4ee5\u6210\u672c\u6ca1\u6709\u964d\u4f4e\uff1b\u76f8\u53cd\u8fd8\u7565\u6709\u589e\u52a0\uff08\u51c6\u786e\u8bf4\u589e\u52a0 10000 * cpu_operator_cost\uff09\uff0c\u4ee5\u8ba1\u5165\u68c0\u67e5 WHERE \u6761\u4ef6\u6d88\u8017\u7684\u989d\u5916 CPU \u65f6\u95f4\u3002"]}, {"title": "\u672c\u7248\u624b\u518c\u4e2d\u7684\u793a\u4f8b", "blocks": [{"code": "EXPLAIN SELECT * FROM tenk1;\n\n                         QUERY PLAN\n-------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..445.00 rows=10000 width=244)", "source": {"url": "/docs/17/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 17.11 \u00b7 using-explain", "sha256": "8e3422c77496cc53bfc225ccadda3e82eb23c8c362b8eb95e7fb02cdb315ea78"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 17.11 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u4e0b\u9762\u662f\u4e00\u4e2a\u7b80\u5355\u793a\u4f8b\uff0c\u4ec5\u7528\u6765\u5c55\u793a\u8f93\u51fa\u5f62\u5f0f\uff1a"]}, {"code": "EXPLAIN SELECT * FROM tenk1 WHERE unique1 < 7000;\n\n                         QUERY PLAN\n------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..470.00 rows=7000 width=244)\n   Filter: (unique1 < 7000)", "source": {"url": "/docs/17/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 17.11 \u00b7 using-explain", "sha256": "8e3422c77496cc53bfc225ccadda3e82eb23c8c362b8eb95e7fb02cdb315ea78"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 17.11 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u73b0\u5728\u4fee\u6539\u67e5\u8be2\uff0c\u6dfb\u52a0 WHERE \u6761\u4ef6\uff1a"]}]}, {"title": "\u6267\u884c\u5668\u5b9e\u73b0\u8bf4\u660e", "paragraphs": ["nodeSeqscan.c\uff1a\u652f\u6301\u5173\u7cfb\u987a\u5e8f\u626b\u63cf\u7684\u4f8b\u7a0b\u3002", "ExecSeqScanEstimate \u4f30\u7b97\u5e76\u884c\u626b\u63cf\u6240\u9700\u7684 DSM \u7a7a\u95f4\uff1bExecSeqScanInitializeDSM \u521d\u59cb\u5316\u5e76\u884c\u626b\u63cf\u7684 DSM\uff1bExecSeqScanReInitializeDSM \u4e3a\u65b0\u4e00\u8f6e\u5e76\u884c\u626b\u63cf\u91cd\u65b0\u521d\u59cb\u5316 DSM\uff1bExecSeqScanInitializeWorker \u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u4e2d\u9644\u52a0\u5230 DSM \u4fe1\u606f\u3002", "\u626b\u63cf\u4e0d\u5e76\u884c\u6267\u884c\uff0c\u6216\u539f\u8ba1\u5212\u5e76\u884c\u7684\u626b\u63cf\u4ee5\u4e32\u884c\u65b9\u5f0f\u6267\u884c\u65f6\uff0c\u4f1a\u5230\u8fbe\u6b64\u5904\u3002", "SeqRecheck\uff1a\u5728 EvalPlanQual \u4e2d\u91cd\u65b0\u68c0\u67e5\u5143\u7ec4\u7684\u8bbf\u95ee\u65b9\u6cd5\u4f8b\u7a0b\u3002", "\u4e0e IndexScan \u4e0d\u540c\uff0cSeqScan \u4ece\u4e0d\u5728 heap_beginscan \u4e2d\u4f7f\u7528\u952e\uff08\u8fd9\u5f88\u4e0d\u597d\uff09\uff0c\u56e0\u6b64\u8fd9\u91cc\u4e0d\u68c0\u67e5\u952e\u662f\u5426\u6ee1\u8db3\u6761\u4ef6\u3002"]}, {"code": "case T_SeqScan:\n\t\t\tpname = sname = \"Seq Scan\";\n\t\t\tbreak;", "title": "\u6838\u5fc3\u6e90\u7801\u4e2d\u7684 EXPLAIN \u6807\u8bc6"}], "strategies": [], "description": ["\u901a\u8fc7\u987a\u5e8f\u626b\u63cf\u8bfb\u53d6\u5173\u7cfb\uff0c\u5e76\u5e94\u7528\u626b\u63cf\u6761\u4ef6\u3002"], "localization": {"status": "complete", "sources": [], "language": "zh", "original_text": {"/versions/17/description/0": "Reads a relation through a sequential scan and applies its scan qualifications.", "/versions/17/facts/0/label": "Core node tag", "/versions/17/facts/1/label": "Structured EXPLAIN Node Type", "/versions/17/facts/2/label": "Inputs", "/versions/17/facts/2/value": "A relation", "/versions/17/facts/3/label": "Output", "/versions/17/facts/3/value": "Qualified relation tuples", "/versions/17/facts/4/label": "Executor initializer", "/versions/17/facts/5/label": "Memory mechanism", "/versions/17/tables/0/title": "EXPLAIN labels in this source build", "/versions/17/related/1/label": "Using EXPLAIN", "/versions/17/related/2/label": "Parallel plans", "/versions/17/sections/0/title": "EXPLAIN names and attributes", "/versions/17/sections/1/title": "Memory and temporary storage", "/versions/17/sections/2/title": "Parallel execution and instrumentation", "/versions/17/sections/3/title": "Same-version manual discussion", "/versions/17/sections/4/title": "Examples from this manual build", "/versions/17/sections/5/title": "Executor implementation notes", "/versions/17/sections/6/title": "EXPLAIN identity in core source", "/versions/17/memory/description": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/17/sections/0/paragraphs/0": "Structured formats use the Node Type above. Text-format spellings can also include operation, strategy, join type, scan direction or aggregation-stage attributes.", "/versions/17/sections/0/paragraphs/1": "Text names recorded by this source: Seq Scan.", "/versions/17/sections/0/paragraphs/2": "Parallel-aware and parallel-safe are different plan properties. A node running inside a parallel worker is not necessarily a parallel-aware node.", "/versions/17/sections/1/paragraphs/0": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/17/sections/2/paragraphs/0": "The source callbacks below can coordinate execution or collect worker instrumentation. Their presence is not a blanket claim that this node supports a shared parallel scan or shared state.", "/versions/17/sections/2/paragraphs/1": "Callbacks in this build: ExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM.", "/versions/17/sections/3/paragraphs/0": "Notice that the EXPLAIN output shows the WHERE clause being applied as a \u201c filter \u201d condition attached to the Seq Scan plan node. This means that the plan node checks the condition for each row it scans, and outputs only the ones that pass the condition. The estimate of output rows has been reduced because of the WHERE clause. However, the scan will still have to visit all 10000 rows, so the cost hasn't decreased; in fact it has gone up a bit (by 10000 * cpu_operator_cost , to be exact) to reflect the extra CPU time spent checking the WHERE condition.", "/versions/17/sections/5/paragraphs/0": "nodeSeqscan.c Support routines for sequential scans of relations.", "/versions/17/sections/5/paragraphs/1": "ExecSeqScanEstimate estimates DSM space needed for parallel scan ExecSeqScanInitializeDSM initialize DSM for parallel scan ExecSeqScanReInitializeDSM reinitialize DSM for fresh parallel scan ExecSeqScanInitializeWorker attach to DSM info in parallel worker", "/versions/17/sections/5/paragraphs/2": "We reach here if the scan is not parallel, or if we're serially executing a scan that was planned to be parallel.", "/versions/17/sections/5/paragraphs/3": "SeqRecheck -- access method routine to recheck a tuple in EvalPlanQual", "/versions/17/sections/5/paragraphs/4": "Note that unlike IndexScan, SeqScan never use keys in heap_beginscan (and this is very bad) - so, here we do not check are keys ok or not.", "/versions/17/tables/0/columns/0/label": "Text-format label", "/versions/17/tables/0/columns/1/label": "Structured node identity", "/versions/17/sections/4/blocks/0/paragraphs/0": "Example copied from the PostgreSQL 17.11 manual; it was not executed for this collection.", "/versions/17/sections/4/blocks/0/paragraphs/1": "Here is a trivial example, just to show what the output looks like:", "/versions/17/sections/4/blocks/1/paragraphs/0": "Example copied from the PostgreSQL 17.11 manual; it was not executed for this collection.", "/versions/17/sections/4/blocks/1/paragraphs/1": "Now let's modify the query to add a WHERE condition:"}, "fallback_fields": [], "source_language": "en", "original_snapshot_sha256": "5ad864ee3603e29bd4e8933b1a2df83ff5bb81609e6baf81eccbea34bf46c214"}, "evidence_kind": "source and documentation", "explain_names": ["Seq Scan"], "partial_modes": [], "comparison_data": {"node_tag": "T_SeqScan", "strategies": [], "text_names": ["Seq Scan"], "initializer": "ExecInitSeqScan", "partial_modes": [], "memory_mechanism": "unclassified", "parallel_callbacks": ["ExecSeqScanEstimate", "ExecSeqScanInitializeDSM", "ExecSeqScanInitializeWorker", "ExecSeqScanReInitializeDSM"]}, "comparison_hash": "5631c0db91382334dc4ca321ae0335013d10ae1c0c24f3523fcca0184bbbc81b", "explain_prefixes": ["Parallel", "Async"], "runtime_verified": false, "source_inventory": {"explain": "src/backend/commands/explain.c", "executor": "src/backend/executor/execProcnode.c", "implementation": "src/backend/executor/nodeSeqscan.c"}, "parallel_callbacks": ["ExecSeqScanEstimate", "ExecSeqScanInitializeDSM", "ExecSeqScanInitializeWorker", "ExecSeqScanReInitializeDSM"]}, "18": {"facts": [{"label": "\u6838\u5fc3\u8282\u70b9\u6807\u7b7e", "value": "T_SeqScan"}, {"label": "\u7ed3\u6784\u5316 EXPLAIN \u8282\u70b9\u7c7b\u578b", "value": "Seq Scan"}, {"label": "\u8f93\u5165", "value": "\u5173\u7cfb"}, {"label": "\u8f93\u51fa", "value": "\u6ee1\u8db3\u6761\u4ef6\u7684\u5173\u7cfb\u5143\u7ec4"}, {"label": "\u6267\u884c\u5668\u521d\u59cb\u5316\u51fd\u6570", "value": "ExecInitSeqScan"}, {"label": "\u5185\u5b58\u673a\u5236", "value": "unclassified"}], "memory": {"evidence": [{"url": "https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2", "path": "src/backend/executor/nodeSeqscan.c", "label": "src/backend/executor/nodeSeqscan.c", "sha256": "e6c9800872e17a6cfb6e58613de7e82da082b4d430557a3065a3caa6fe24f34d", "archive_sha256": "555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f"}], "mechanism": "unclassified", "description": "\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002", "source_notes": []}, "tables": [{"key": "explain-labels", "rows": 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"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2", "path": "src/include/nodes/plannodes.h", "label": "src/include/nodes/plannodes.h", "sha256": "52422b327a8049fbbb20d8b96008a0fc0a6fafa60f7eff3c695d5b2e83830120", "archive_sha256": "555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f"}, {"url": "https://pg.center/docs/18/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 18.6 \u00b7 using-explain", "sha256": "60040c30180093418a0affe56dd27dff9df2504b705b38039589e458bf5c31ed", "language": "en", "original_url": "/docs/18/using-explain.html#USING-EXPLAIN-BASICS"}], "node_tag": "T_SeqScan", "sections": [{"title": "EXPLAIN \u540d\u79f0\u4e0e\u5c5e\u6027", "paragraphs": ["\u7ed3\u6784\u5316\u683c\u5f0f\u4f7f\u7528\u4e0a\u8ff0 Node Type\u3002\u6587\u672c\u683c\u5f0f\u540d\u79f0\u8fd8\u53ef\u80fd\u5305\u542b\u64cd\u4f5c\u3001\u7b56\u7565\u3001\u8fde\u63a5\u7c7b\u578b\u3001\u626b\u63cf\u65b9\u5411\u6216\u805a\u5408\u9636\u6bb5\u5c5e\u6027\u3002", "\u6b64\u6e90\u7801\u8bb0\u5f55\u7684\u6587\u672c\u540d\u79f0\uff1aSeq Scan.", "\u5e76\u884c\u611f\u77e5\u4e0e\u5e76\u884c\u5b89\u5168\u662f\u4e0d\u540c\u7684\u8ba1\u5212\u5c5e\u6027\u3002\u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u5185\u8fd0\u884c\u7684\u8282\u70b9\u4e0d\u4e00\u5b9a\u662f\u5e76\u884c\u611f\u77e5\u8282\u70b9\u3002"]}, {"title": "\u5185\u5b58\u4e0e\u4e34\u65f6\u5b58\u50a8", "paragraphs": ["\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002"]}, {"title": "\u5e76\u884c\u6267\u884c\u4e0e\u8fd0\u884c\u4fe1\u606f\u91c7\u96c6", "paragraphs": ["\u4ee5\u4e0b\u6e90\u7801\u56de\u8c03\u53ef\u4ee5\u534f\u8c03\u6267\u884c\u6216\u6536\u96c6\u5de5\u4f5c\u8fdb\u7a0b\u7684\u6d4b\u91cf\u6570\u636e\u3002\u56de\u8c03\u5b58\u5728\u4e0d\u4ee3\u8868\u8be5\u8282\u70b9\u666e\u904d\u652f\u6301\u5171\u4eab\u5e76\u884c\u626b\u63cf\u6216\u5171\u4eab\u72b6\u6001\u3002", "\u6b64\u6784\u5efa\u7684\u56de\u8c03\uff1aExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM."]}, {"title": "\u540c\u7248\u672c\u624b\u518c\u8bf4\u660e", "paragraphs": ["EXPLAIN \u5c06 WHERE \u5b50\u53e5\u663e\u793a\u4e3a Seq Scan \u8282\u70b9\u4e0a\u7684 filter \u6761\u4ef6\u3002\u8fd9\u8868\u793a\u8282\u70b9\u5bf9\u626b\u63cf\u7684\u6bcf\u4e00\u884c\u68c0\u67e5\u6761\u4ef6\uff0c\u4ec5\u8f93\u51fa\u901a\u8fc7\u6761\u4ef6\u7684\u884c\u3002WHERE \u964d\u4f4e\u4e86\u9884\u8ba1\u8f93\u51fa\u884c\u6570\uff0c\u4f46\u626b\u63cf\u4ecd\u9700\u8bbf\u95ee\u5168\u90e8 10000 \u884c\uff0c\u6240\u4ee5\u6210\u672c\u6ca1\u6709\u964d\u4f4e\uff1b\u76f8\u53cd\u8fd8\u7565\u6709\u589e\u52a0\uff08\u51c6\u786e\u8bf4\u589e\u52a0 10000 * cpu_operator_cost\uff09\uff0c\u4ee5\u8ba1\u5165\u68c0\u67e5 WHERE \u6761\u4ef6\u6d88\u8017\u7684\u989d\u5916 CPU \u65f6\u95f4\u3002"]}, {"title": "\u672c\u7248\u624b\u518c\u4e2d\u7684\u793a\u4f8b", "blocks": [{"code": "EXPLAIN SELECT * FROM tenk1;\n\n                         QUERY PLAN\n-------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..445.00 rows=10000 width=244)", "source": {"url": "/docs/18/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 18.6 \u00b7 using-explain", "sha256": "60040c30180093418a0affe56dd27dff9df2504b705b38039589e458bf5c31ed"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 18.6 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u4e0b\u9762\u662f\u4e00\u4e2a\u7b80\u5355\u793a\u4f8b\uff0c\u4ec5\u7528\u6765\u5c55\u793a\u8f93\u51fa\u5f62\u5f0f\uff1a"]}, {"code": "EXPLAIN SELECT * FROM tenk1 WHERE unique1 < 7000;\n\n                         QUERY PLAN\n------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..470.00 rows=7000 width=244)\n   Filter: (unique1 < 7000)", "source": {"url": "/docs/18/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 18.6 \u00b7 using-explain", "sha256": "60040c30180093418a0affe56dd27dff9df2504b705b38039589e458bf5c31ed"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 18.6 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u73b0\u5728\u4fee\u6539\u67e5\u8be2\uff0c\u6dfb\u52a0 WHERE \u6761\u4ef6\uff1a"]}]}, {"title": "\u6267\u884c\u5668\u5b9e\u73b0\u8bf4\u660e", "paragraphs": ["nodeSeqscan.c\uff1a\u652f\u6301\u5173\u7cfb\u987a\u5e8f\u626b\u63cf\u7684\u4f8b\u7a0b\u3002", "ExecSeqScanEstimate \u4f30\u7b97\u5e76\u884c\u626b\u63cf\u6240\u9700\u7684 DSM \u7a7a\u95f4\uff1bExecSeqScanInitializeDSM \u521d\u59cb\u5316\u5e76\u884c\u626b\u63cf\u7684 DSM\uff1bExecSeqScanReInitializeDSM \u4e3a\u65b0\u4e00\u8f6e\u5e76\u884c\u626b\u63cf\u91cd\u65b0\u521d\u59cb\u5316 DSM\uff1bExecSeqScanInitializeWorker \u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u4e2d\u9644\u52a0\u5230 DSM \u4fe1\u606f\u3002", "\u626b\u63cf\u4e0d\u5e76\u884c\u6267\u884c\uff0c\u6216\u539f\u8ba1\u5212\u5e76\u884c\u7684\u626b\u63cf\u4ee5\u4e32\u884c\u65b9\u5f0f\u6267\u884c\u65f6\uff0c\u4f1a\u5230\u8fbe\u6b64\u5904\u3002", "SeqRecheck\uff1a\u5728 EvalPlanQual \u4e2d\u91cd\u65b0\u68c0\u67e5\u5143\u7ec4\u7684\u8bbf\u95ee\u65b9\u6cd5\u4f8b\u7a0b\u3002", "\u4e0e IndexScan \u4e0d\u540c\uff0cSeqScan \u4ece\u4e0d\u5728 heap_beginscan \u4e2d\u4f7f\u7528\u952e\uff08\u8fd9\u5f88\u4e0d\u597d\uff09\uff0c\u56e0\u6b64\u8fd9\u91cc\u4e0d\u68c0\u67e5\u952e\u662f\u5426\u6ee1\u8db3\u6761\u4ef6\u3002"]}, {"code": "case T_SeqScan:\n\t\t\tpname = sname = \"Seq Scan\";\n\t\t\tbreak;", "title": "\u6838\u5fc3\u6e90\u7801\u4e2d\u7684 EXPLAIN \u6807\u8bc6"}], "strategies": [], "description": ["\u901a\u8fc7\u987a\u5e8f\u626b\u63cf\u8bfb\u53d6\u5173\u7cfb\uff0c\u5e76\u5e94\u7528\u626b\u63cf\u6761\u4ef6\u3002"], "localization": {"status": "complete", "sources": [], "language": "zh", "original_text": {"/versions/18/description/0": "Reads a relation through a sequential scan and applies its scan qualifications.", "/versions/18/facts/0/label": "Core node tag", "/versions/18/facts/1/label": "Structured EXPLAIN Node Type", "/versions/18/facts/2/label": "Inputs", "/versions/18/facts/2/value": "A relation", "/versions/18/facts/3/label": "Output", "/versions/18/facts/3/value": "Qualified relation tuples", "/versions/18/facts/4/label": "Executor initializer", "/versions/18/facts/5/label": "Memory mechanism", "/versions/18/tables/0/title": "EXPLAIN labels in this source build", "/versions/18/related/1/label": "Using EXPLAIN", "/versions/18/related/2/label": "Parallel plans", "/versions/18/sections/0/title": "EXPLAIN names and attributes", "/versions/18/sections/1/title": "Memory and temporary storage", "/versions/18/sections/2/title": "Parallel execution and instrumentation", "/versions/18/sections/3/title": "Same-version manual discussion", "/versions/18/sections/4/title": "Examples from this manual build", "/versions/18/sections/5/title": "Executor implementation notes", "/versions/18/sections/6/title": "EXPLAIN identity in core source", "/versions/18/memory/description": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/18/sections/0/paragraphs/0": "Structured formats use the Node Type above. Text-format spellings can also include operation, strategy, join type, scan direction or aggregation-stage attributes.", "/versions/18/sections/0/paragraphs/1": "Text names recorded by this source: Seq Scan.", "/versions/18/sections/0/paragraphs/2": "Parallel-aware and parallel-safe are different plan properties. A node running inside a parallel worker is not necessarily a parallel-aware node.", "/versions/18/sections/1/paragraphs/0": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/18/sections/2/paragraphs/0": "The source callbacks below can coordinate execution or collect worker instrumentation. Their presence is not a blanket claim that this node supports a shared parallel scan or shared state.", "/versions/18/sections/2/paragraphs/1": "Callbacks in this build: ExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM.", "/versions/18/sections/3/paragraphs/0": "Notice that the EXPLAIN output shows the WHERE clause being applied as a \u201c filter \u201d condition attached to the Seq Scan plan node. This means that the plan node checks the condition for each row it scans, and outputs only the ones that pass the condition. The estimate of output rows has been reduced because of the WHERE clause. However, the scan will still have to visit all 10000 rows, so the cost hasn't decreased; in fact it has gone up a bit (by 10000 * cpu_operator_cost , to be exact) to reflect the extra CPU time spent checking the WHERE condition.", "/versions/18/sections/5/paragraphs/0": "nodeSeqscan.c Support routines for sequential scans of relations.", "/versions/18/sections/5/paragraphs/1": "ExecSeqScanEstimate estimates DSM space needed for parallel scan ExecSeqScanInitializeDSM initialize DSM for parallel scan ExecSeqScanReInitializeDSM reinitialize DSM for fresh parallel scan ExecSeqScanInitializeWorker attach to DSM info in parallel worker", "/versions/18/sections/5/paragraphs/2": "We reach here if the scan is not parallel, or if we're serially executing a scan that was planned to be parallel.", "/versions/18/sections/5/paragraphs/3": "SeqRecheck -- access method routine to recheck a tuple in EvalPlanQual", "/versions/18/sections/5/paragraphs/4": "Note that unlike IndexScan, SeqScan never use keys in heap_beginscan (and this is very bad) - so, here we do not check are keys ok or not.", "/versions/18/tables/0/columns/0/label": "Text-format label", "/versions/18/tables/0/columns/1/label": "Structured node identity", "/versions/18/sections/4/blocks/0/paragraphs/0": "Example copied from the PostgreSQL 18.6 manual; it was not executed for this collection.", "/versions/18/sections/4/blocks/0/paragraphs/1": "Here is a trivial example, just to show what the output looks like:", "/versions/18/sections/4/blocks/1/paragraphs/0": "Example copied from the PostgreSQL 18.6 manual; it was not executed for this collection.", "/versions/18/sections/4/blocks/1/paragraphs/1": "Now let's modify the query to add a WHERE condition:"}, "fallback_fields": [], "source_language": "en", "original_snapshot_sha256": "345d63a8fe9cfcb7d8eb953497a0553b8d2171a21b82d194cd9a12154a0a139b"}, "evidence_kind": "source and documentation", "explain_names": ["Seq Scan"], "partial_modes": [], "comparison_data": {"node_tag": "T_SeqScan", "strategies": [], "text_names": ["Seq Scan"], "initializer": "ExecInitSeqScan", "partial_modes": [], "memory_mechanism": "unclassified", "parallel_callbacks": ["ExecSeqScanEstimate", "ExecSeqScanInitializeDSM", "ExecSeqScanInitializeWorker", "ExecSeqScanReInitializeDSM"]}, "comparison_hash": "5631c0db91382334dc4ca321ae0335013d10ae1c0c24f3523fcca0184bbbc81b", "explain_prefixes": ["Parallel", "Async"], "runtime_verified": false, "source_inventory": {"explain": "src/backend/commands/explain.c", "executor": "src/backend/executor/execProcnode.c", "implementation": "src/backend/executor/nodeSeqscan.c"}, "parallel_callbacks": ["ExecSeqScanEstimate", "ExecSeqScanInitializeDSM", "ExecSeqScanInitializeWorker", "ExecSeqScanReInitializeDSM"]}, "19": {"facts": [{"label": "\u6838\u5fc3\u8282\u70b9\u6807\u7b7e", "value": "T_SeqScan"}, {"label": "\u7ed3\u6784\u5316 EXPLAIN \u8282\u70b9\u7c7b\u578b", "value": "Seq Scan"}, {"label": "\u8f93\u5165", "value": "\u5173\u7cfb"}, {"label": "\u8f93\u51fa", "value": "\u6ee1\u8db3\u6761\u4ef6\u7684\u5173\u7cfb\u5143\u7ec4"}, {"label": "\u6267\u884c\u5668\u521d\u59cb\u5316\u51fd\u6570", "value": "ExecInitSeqScan"}, {"label": "\u5185\u5b58\u673a\u5236", "value": "unclassified"}], "memory": {"evidence": [{"url": "https://ftp.postgresql.org/pub/source/v19beta4/postgresql-19beta4.tar.bz2", "path": "src/backend/executor/nodeSeqscan.c", "label": "src/backend/executor/nodeSeqscan.c", "sha256": "fe83e0d0a1cd1b161440b233cf5c85a81b5614ec8b214a7e8069d375a631416d", "archive_sha256": "83157ee9c599d03b2f7a3d73ef3a56ec24e0e79cc2b3501a64d1364f56398c86"}], "mechanism": "unclassified", "description": "\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002", "source_notes": []}, "tables": [{"key": "explain-labels", 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"https://ftp.postgresql.org/pub/source/v19beta4/postgresql-19beta4.tar.bz2", "path": "src/include/nodes/plannodes.h", "label": "src/include/nodes/plannodes.h", "sha256": "1c65d5d6b6c81c71531685843647869bcae630779d815a5036b06e070c6c06c7", "archive_sha256": "83157ee9c599d03b2f7a3d73ef3a56ec24e0e79cc2b3501a64d1364f56398c86"}, {"url": "https://pg.center/docs/19/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 19beta4 \u00b7 using-explain", "sha256": "52f111fbd213e2200146319d01a9dcc2ea90001617d2a28edc52f7954c2dd70b", "language": "en", "original_url": "/docs/19/using-explain.html#USING-EXPLAIN-BASICS"}], "node_tag": "T_SeqScan", "sections": [{"title": "EXPLAIN \u540d\u79f0\u4e0e\u5c5e\u6027", "paragraphs": ["\u7ed3\u6784\u5316\u683c\u5f0f\u4f7f\u7528\u4e0a\u8ff0 Node Type\u3002\u6587\u672c\u683c\u5f0f\u540d\u79f0\u8fd8\u53ef\u80fd\u5305\u542b\u64cd\u4f5c\u3001\u7b56\u7565\u3001\u8fde\u63a5\u7c7b\u578b\u3001\u626b\u63cf\u65b9\u5411\u6216\u805a\u5408\u9636\u6bb5\u5c5e\u6027\u3002", "\u6b64\u6e90\u7801\u8bb0\u5f55\u7684\u6587\u672c\u540d\u79f0\uff1aSeq Scan.", "\u5e76\u884c\u611f\u77e5\u4e0e\u5e76\u884c\u5b89\u5168\u662f\u4e0d\u540c\u7684\u8ba1\u5212\u5c5e\u6027\u3002\u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u5185\u8fd0\u884c\u7684\u8282\u70b9\u4e0d\u4e00\u5b9a\u662f\u5e76\u884c\u611f\u77e5\u8282\u70b9\u3002"]}, {"title": "\u5185\u5b58\u4e0e\u4e34\u65f6\u5b58\u50a8", "paragraphs": ["\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002"]}, {"title": "\u5e76\u884c\u6267\u884c\u4e0e\u8fd0\u884c\u4fe1\u606f\u91c7\u96c6", "paragraphs": ["\u4ee5\u4e0b\u6e90\u7801\u56de\u8c03\u53ef\u4ee5\u534f\u8c03\u6267\u884c\u6216\u6536\u96c6\u5de5\u4f5c\u8fdb\u7a0b\u7684\u6d4b\u91cf\u6570\u636e\u3002\u56de\u8c03\u5b58\u5728\u4e0d\u4ee3\u8868\u8be5\u8282\u70b9\u666e\u904d\u652f\u6301\u5171\u4eab\u5e76\u884c\u626b\u63cf\u6216\u5171\u4eab\u72b6\u6001\u3002", "\u6b64\u6784\u5efa\u7684\u56de\u8c03\uff1aExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanInstrumentEstimate, ExecSeqScanReInitializeDSM, ExecSeqScanRetrieveInstrumentation."]}, {"title": "\u540c\u7248\u672c\u624b\u518c\u8bf4\u660e", "paragraphs": ["EXPLAIN \u5c06 WHERE \u5b50\u53e5\u663e\u793a\u4e3a Seq Scan \u8282\u70b9\u4e0a\u7684 filter \u6761\u4ef6\u3002\u8fd9\u8868\u793a\u8282\u70b9\u5bf9\u626b\u63cf\u7684\u6bcf\u4e00\u884c\u68c0\u67e5\u6761\u4ef6\uff0c\u4ec5\u8f93\u51fa\u901a\u8fc7\u6761\u4ef6\u7684\u884c\u3002WHERE \u964d\u4f4e\u4e86\u9884\u8ba1\u8f93\u51fa\u884c\u6570\uff0c\u4f46\u626b\u63cf\u4ecd\u9700\u8bbf\u95ee\u5168\u90e8 10000 \u884c\uff0c\u6240\u4ee5\u6210\u672c\u6ca1\u6709\u964d\u4f4e\uff1b\u76f8\u53cd\u8fd8\u7565\u6709\u589e\u52a0\uff08\u51c6\u786e\u8bf4\u589e\u52a0 10000 * cpu_operator_cost\uff09\uff0c\u4ee5\u8ba1\u5165\u68c0\u67e5 WHERE \u6761\u4ef6\u6d88\u8017\u7684\u989d\u5916 CPU \u65f6\u95f4\u3002"]}, {"title": "\u672c\u7248\u624b\u518c\u4e2d\u7684\u793a\u4f8b", "blocks": [{"code": "EXPLAIN SELECT * FROM tenk1;\n\n                         QUERY PLAN\n-------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..445.00 rows=10000 width=244)", "source": {"url": "/docs/19/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 19beta4 \u00b7 using-explain", "sha256": "52f111fbd213e2200146319d01a9dcc2ea90001617d2a28edc52f7954c2dd70b"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 19beta4 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u4e0b\u9762\u662f\u4e00\u4e2a\u7b80\u5355\u793a\u4f8b\uff0c\u4ec5\u7528\u6765\u5c55\u793a\u8f93\u51fa\u5f62\u5f0f\uff1a"]}, {"code": "EXPLAIN SELECT * FROM tenk1 WHERE unique1 < 7000;\n\n                         QUERY PLAN\n------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..470.00 rows=7000 width=244)\n   Filter: (unique1 < 7000)", "source": {"url": "/docs/19/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 19beta4 \u00b7 using-explain", "sha256": "52f111fbd213e2200146319d01a9dcc2ea90001617d2a28edc52f7954c2dd70b"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 19beta4 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u73b0\u5728\u4fee\u6539\u67e5\u8be2\uff0c\u6dfb\u52a0 WHERE \u6761\u4ef6\uff1a"]}]}, {"title": "\u6267\u884c\u5668\u5b9e\u73b0\u8bf4\u660e", "paragraphs": ["nodeSeqscan.c\uff1a\u652f\u6301\u5173\u7cfb\u987a\u5e8f\u626b\u63cf\u7684\u4f8b\u7a0b\u3002", "ExecSeqScanEstimate \u4f30\u7b97\u5e76\u884c\u626b\u63cf\u6240\u9700\u7684 DSM \u7a7a\u95f4\uff1bExecSeqScanInitializeDSM \u521d\u59cb\u5316\u5e76\u884c\u626b\u63cf\u7684 DSM\uff1bExecSeqScanReInitializeDSM \u4e3a\u65b0\u4e00\u8f6e\u5e76\u884c\u626b\u63cf\u91cd\u65b0\u521d\u59cb\u5316 DSM\uff1bExecSeqScanInitializeWorker \u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u4e2d\u9644\u52a0\u5230 DSM \u4fe1\u606f\u3002", "\u626b\u63cf\u4e0d\u5e76\u884c\u6267\u884c\uff0c\u6216\u539f\u8ba1\u5212\u5e76\u884c\u7684\u626b\u63cf\u4ee5\u4e32\u884c\u65b9\u5f0f\u6267\u884c\u65f6\uff0c\u4f1a\u5230\u8fbe\u6b64\u5904\u3002", "SeqRecheck\uff1a\u5728 EvalPlanQual \u4e2d\u91cd\u65b0\u68c0\u67e5\u5143\u7ec4\u7684\u8bbf\u95ee\u65b9\u6cd5\u4f8b\u7a0b\u3002", "\u4e0e IndexScan \u4e0d\u540c\uff0cSeqScan \u4ece\u4e0d\u5728 heap_beginscan \u4e2d\u4f7f\u7528\u952e\uff08\u8fd9\u5f88\u4e0d\u597d\uff09\uff0c\u56e0\u6b64\u8fd9\u91cc\u4e0d\u68c0\u67e5\u952e\u662f\u5426\u6ee1\u8db3\u6761\u4ef6\u3002"]}, {"code": "case T_SeqScan:\n\t\t\tpname = sname = \"Seq Scan\";\n\t\t\tbreak;", "title": "\u6838\u5fc3\u6e90\u7801\u4e2d\u7684 EXPLAIN \u6807\u8bc6"}], "strategies": [], "description": ["\u901a\u8fc7\u987a\u5e8f\u626b\u63cf\u8bfb\u53d6\u5173\u7cfb\uff0c\u5e76\u5e94\u7528\u626b\u63cf\u6761\u4ef6\u3002"], "localization": {"status": "complete", "sources": [], "language": "zh", "original_text": {"/versions/19/description/0": "Reads a relation through a sequential scan and applies its scan qualifications.", "/versions/19/facts/0/label": "Core node tag", "/versions/19/facts/1/label": "Structured EXPLAIN Node Type", "/versions/19/facts/2/label": "Inputs", "/versions/19/facts/2/value": "A relation", "/versions/19/facts/3/label": "Output", "/versions/19/facts/3/value": "Qualified relation tuples", "/versions/19/facts/4/label": "Executor initializer", "/versions/19/facts/5/label": "Memory mechanism", "/versions/19/tables/0/title": "EXPLAIN labels in this source build", "/versions/19/related/1/label": "Using EXPLAIN", "/versions/19/related/2/label": "Parallel plans", "/versions/19/sections/0/title": "EXPLAIN names and attributes", "/versions/19/sections/1/title": "Memory and temporary storage", "/versions/19/sections/2/title": "Parallel execution and instrumentation", "/versions/19/sections/3/title": "Same-version manual discussion", "/versions/19/sections/4/title": "Examples from this manual build", "/versions/19/sections/5/title": "Executor implementation notes", "/versions/19/sections/6/title": "EXPLAIN identity in core source", "/versions/19/memory/description": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/19/sections/0/paragraphs/0": "Structured formats use the Node Type above. Text-format spellings can also include operation, strategy, join type, scan direction or aggregation-stage attributes.", "/versions/19/sections/0/paragraphs/1": "Text names recorded by this source: Seq Scan.", "/versions/19/sections/0/paragraphs/2": "Parallel-aware and parallel-safe are different plan properties. A node running inside a parallel worker is not necessarily a parallel-aware node.", "/versions/19/sections/1/paragraphs/0": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/19/sections/2/paragraphs/0": "The source callbacks below can coordinate execution or collect worker instrumentation. Their presence is not a blanket claim that this node supports a shared parallel scan or shared state.", "/versions/19/sections/2/paragraphs/1": "Callbacks in this build: ExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanInstrumentEstimate, ExecSeqScanReInitializeDSM, ExecSeqScanRetrieveInstrumentation.", "/versions/19/sections/3/paragraphs/0": "Notice that the EXPLAIN output shows the WHERE clause being applied as a \u201c filter \u201d condition attached to the Seq Scan plan node. This means that the plan node checks the condition for each row it scans, and outputs only the ones that pass the condition. The estimate of output rows has been reduced because of the WHERE clause. However, the scan will still have to visit all 10000 rows, so the cost hasn't decreased; in fact it has gone up a bit (by 10000 * cpu_operator_cost , to be exact) to reflect the extra CPU time spent checking the WHERE condition.", "/versions/19/sections/5/paragraphs/0": "nodeSeqscan.c Support routines for sequential scans of relations.", "/versions/19/sections/5/paragraphs/1": "ExecSeqScanEstimate estimates DSM space needed for parallel scan ExecSeqScanInitializeDSM initialize DSM for parallel scan ExecSeqScanReInitializeDSM reinitialize DSM for fresh parallel scan ExecSeqScanInitializeWorker attach to DSM info in parallel worker", "/versions/19/sections/5/paragraphs/2": "We reach here if the scan is not parallel, or if we're serially executing a scan that was planned to be parallel.", "/versions/19/sections/5/paragraphs/3": "SeqRecheck -- access method routine to recheck a tuple in EvalPlanQual", "/versions/19/sections/5/paragraphs/4": "Note that unlike IndexScan, SeqScan never use keys in heap_beginscan (and this is very bad) - so, here we do not check are keys ok or not.", "/versions/19/tables/0/columns/0/label": "Text-format label", "/versions/19/tables/0/columns/1/label": "Structured node identity", "/versions/19/sections/4/blocks/0/paragraphs/0": "Example copied from the PostgreSQL 19beta4 manual; it was not executed for this collection.", "/versions/19/sections/4/blocks/0/paragraphs/1": "Here is a trivial example, just to show what the output looks like:", "/versions/19/sections/4/blocks/1/paragraphs/0": "Example copied from the PostgreSQL 19beta4 manual; it was not executed for this collection.", "/versions/19/sections/4/blocks/1/paragraphs/1": "Now let's modify the query to add a WHERE condition:"}, "fallback_fields": [], "source_language": "en", "original_snapshot_sha256": "feeb4c150238344c1f1e0e1e63f15ecf88241894d16289e71eeb8347e24599ac"}, "evidence_kind": "source and documentation", "explain_names": ["Seq Scan"], "partial_modes": [], "comparison_data": {"node_tag": "T_SeqScan", "strategies": [], "text_names": ["Seq Scan"], "initializer": "ExecInitSeqScan", "partial_modes": [], "memory_mechanism": "unclassified", "parallel_callbacks": ["ExecSeqScanEstimate", "ExecSeqScanInitializeDSM", "ExecSeqScanInitializeWorker", "ExecSeqScanInstrumentEstimate", "ExecSeqScanReInitializeDSM", "ExecSeqScanRetrieveInstrumentation"]}, "comparison_hash": "af22b5ba717cf4297e255ac7bbc4ddb497269149637e2e3aaafb72ba70c041d8", "explain_prefixes": ["Parallel", "Async"], "runtime_verified": false, "source_inventory": {"explain": "src/backend/commands/explain.c", "executor": "src/backend/executor/execProcnode.c", "implementation": "src/backend/executor/nodeSeqscan.c"}, "parallel_callbacks": ["ExecSeqScanEstimate", "ExecSeqScanInitializeDSM", "ExecSeqScanInitializeWorker", "ExecSeqScanInstrumentEstimate", "ExecSeqScanReInitializeDSM", "ExecSeqScanRetrieveInstrumentation"]}, "20": {"facts": [{"label": "\u6838\u5fc3\u8282\u70b9\u6807\u7b7e", "value": "T_SeqScan"}, {"label": "\u7ed3\u6784\u5316 EXPLAIN \u8282\u70b9\u7c7b\u578b", "value": "Seq Scan"}, {"label": "\u8f93\u5165", "value": "\u5173\u7cfb"}, {"label": "\u8f93\u51fa", "value": "\u6ee1\u8db3\u6761\u4ef6\u7684\u5173\u7cfb\u5143\u7ec4"}, {"label": "\u6267\u884c\u5668\u521d\u59cb\u5316\u51fd\u6570", "value": "ExecInitSeqScan"}, {"label": "\u5185\u5b58\u673a\u5236", "value": "unclassified"}], "memory": {"evidence": [{"url": "https://ftp.postgresql.org/pub/snapshot/dev/postgresql-snapshot.tar.bz2", "path": "src/backend/executor/nodeSeqscan.c", "label": "src/backend/executor/nodeSeqscan.c", "sha256": "fe83e0d0a1cd1b161440b233cf5c85a81b5614ec8b214a7e8069d375a631416d", "archive_sha256": "4d3346909b201ac1648232cf290462a7070c119326f56196f1f0253ed80fae41"}], "mechanism": "unclassified", "description": 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"channel": "devel", "revision": "4d3346909b201ac1648232cf290462a7070c119326f56196f1f0253ed80fae41", "source_url": "https://ftp.postgresql.org/pub/snapshot/dev/postgresql-snapshot.tar.bz2", "source_snapshot_utc": "26-Sep-2026 20:22"}, "sources": [{"url": "https://ftp.postgresql.org/pub/snapshot/dev/postgresql-snapshot.tar.bz2", "line": 1444, "path": "src/backend/commands/explain.c", "label": "src/backend/commands/explain.c:1444", "sha256": "13402758013520451539427b5993db06d463ca11c4e2d4cc5444e82367688077", "archive_sha256": "4d3346909b201ac1648232cf290462a7070c119326f56196f1f0253ed80fae41"}, {"url": "https://ftp.postgresql.org/pub/snapshot/dev/postgresql-snapshot.tar.bz2", "line": 209, "path": "src/backend/executor/execProcnode.c", "label": "src/backend/executor/execProcnode.c:209", "sha256": "5e39b2037bed672da55104229ecc32da5abde44c26bcad01479edcfa044d09ed", "archive_sha256": "4d3346909b201ac1648232cf290462a7070c119326f56196f1f0253ed80fae41"}, {"url": 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"node_tag": "T_SeqScan", "sections": [{"title": "EXPLAIN \u540d\u79f0\u4e0e\u5c5e\u6027", "paragraphs": ["\u7ed3\u6784\u5316\u683c\u5f0f\u4f7f\u7528\u4e0a\u8ff0 Node Type\u3002\u6587\u672c\u683c\u5f0f\u540d\u79f0\u8fd8\u53ef\u80fd\u5305\u542b\u64cd\u4f5c\u3001\u7b56\u7565\u3001\u8fde\u63a5\u7c7b\u578b\u3001\u626b\u63cf\u65b9\u5411\u6216\u805a\u5408\u9636\u6bb5\u5c5e\u6027\u3002", "\u6b64\u6e90\u7801\u8bb0\u5f55\u7684\u6587\u672c\u540d\u79f0\uff1aSeq Scan.", "\u5e76\u884c\u611f\u77e5\u4e0e\u5e76\u884c\u5b89\u5168\u662f\u4e0d\u540c\u7684\u8ba1\u5212\u5c5e\u6027\u3002\u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u5185\u8fd0\u884c\u7684\u8282\u70b9\u4e0d\u4e00\u5b9a\u662f\u5e76\u884c\u611f\u77e5\u8282\u70b9\u3002"]}, {"title": "\u5185\u5b58\u4e0e\u4e34\u65f6\u5b58\u50a8", "paragraphs": ["\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002"]}, {"title": "\u5e76\u884c\u6267\u884c\u4e0e\u8fd0\u884c\u4fe1\u606f\u91c7\u96c6", "paragraphs": ["\u4ee5\u4e0b\u6e90\u7801\u56de\u8c03\u53ef\u4ee5\u534f\u8c03\u6267\u884c\u6216\u6536\u96c6\u5de5\u4f5c\u8fdb\u7a0b\u7684\u6d4b\u91cf\u6570\u636e\u3002\u56de\u8c03\u5b58\u5728\u4e0d\u4ee3\u8868\u8be5\u8282\u70b9\u666e\u904d\u652f\u6301\u5171\u4eab\u5e76\u884c\u626b\u63cf\u6216\u5171\u4eab\u72b6\u6001\u3002", "\u6b64\u6784\u5efa\u7684\u56de\u8c03\uff1aExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanInstrumentEstimate, ExecSeqScanReInitializeDSM, ExecSeqScanRetrieveInstrumentation."]}, {"title": "\u540c\u7248\u672c\u624b\u518c\u8bf4\u660e", "paragraphs": ["EXPLAIN \u5c06 WHERE \u5b50\u53e5\u663e\u793a\u4e3a Seq Scan \u8282\u70b9\u4e0a\u7684 filter \u6761\u4ef6\u3002\u8fd9\u8868\u793a\u8282\u70b9\u5bf9\u626b\u63cf\u7684\u6bcf\u4e00\u884c\u68c0\u67e5\u6761\u4ef6\uff0c\u4ec5\u8f93\u51fa\u901a\u8fc7\u6761\u4ef6\u7684\u884c\u3002WHERE \u964d\u4f4e\u4e86\u9884\u8ba1\u8f93\u51fa\u884c\u6570\uff0c\u4f46\u626b\u63cf\u4ecd\u9700\u8bbf\u95ee\u5168\u90e8 10000 \u884c\uff0c\u6240\u4ee5\u6210\u672c\u6ca1\u6709\u964d\u4f4e\uff1b\u76f8\u53cd\u8fd8\u7565\u6709\u589e\u52a0\uff08\u51c6\u786e\u8bf4\u589e\u52a0 10000 * cpu_operator_cost\uff09\uff0c\u4ee5\u8ba1\u5165\u68c0\u67e5 WHERE \u6761\u4ef6\u6d88\u8017\u7684\u989d\u5916 CPU \u65f6\u95f4\u3002"]}, {"title": "\u672c\u7248\u624b\u518c\u4e2d\u7684\u793a\u4f8b", "blocks": [{"code": "EXPLAIN SELECT * FROM tenk1;\n\n                         QUERY PLAN\n-------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..445.00 rows=10000 width=244)", "source": {"url": "/docs/devel/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 20devel \u00b7 using-explain", "sha256": "99cfea3035876ea63f88b75ba8c964b51a32a70606544e09f769c0b60a234b31"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 20devel \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u4e0b\u9762\u662f\u4e00\u4e2a\u7b80\u5355\u793a\u4f8b\uff0c\u4ec5\u7528\u6765\u5c55\u793a\u8f93\u51fa\u5f62\u5f0f\uff1a"]}, {"code": "EXPLAIN SELECT * FROM tenk1 WHERE unique1 < 7000;\n\n                         QUERY PLAN\n------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..470.00 rows=7000 width=244)\n   Filter: (unique1 < 7000)", "source": {"url": "/docs/devel/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 20devel \u00b7 using-explain", "sha256": "99cfea3035876ea63f88b75ba8c964b51a32a70606544e09f769c0b60a234b31"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 20devel \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u73b0\u5728\u4fee\u6539\u67e5\u8be2\uff0c\u6dfb\u52a0 WHERE \u6761\u4ef6\uff1a"]}]}, {"title": "\u6267\u884c\u5668\u5b9e\u73b0\u8bf4\u660e", "paragraphs": ["nodeSeqscan.c\uff1a\u652f\u6301\u5173\u7cfb\u987a\u5e8f\u626b\u63cf\u7684\u4f8b\u7a0b\u3002", "ExecSeqScanEstimate \u4f30\u7b97\u5e76\u884c\u626b\u63cf\u6240\u9700\u7684 DSM \u7a7a\u95f4\uff1bExecSeqScanInitializeDSM \u521d\u59cb\u5316\u5e76\u884c\u626b\u63cf\u7684 DSM\uff1bExecSeqScanReInitializeDSM \u4e3a\u65b0\u4e00\u8f6e\u5e76\u884c\u626b\u63cf\u91cd\u65b0\u521d\u59cb\u5316 DSM\uff1bExecSeqScanInitializeWorker \u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u4e2d\u9644\u52a0\u5230 DSM \u4fe1\u606f\u3002", "\u626b\u63cf\u4e0d\u5e76\u884c\u6267\u884c\uff0c\u6216\u539f\u8ba1\u5212\u5e76\u884c\u7684\u626b\u63cf\u4ee5\u4e32\u884c\u65b9\u5f0f\u6267\u884c\u65f6\uff0c\u4f1a\u5230\u8fbe\u6b64\u5904\u3002", "SeqRecheck\uff1a\u5728 EvalPlanQual \u4e2d\u91cd\u65b0\u68c0\u67e5\u5143\u7ec4\u7684\u8bbf\u95ee\u65b9\u6cd5\u4f8b\u7a0b\u3002", "\u4e0e IndexScan \u4e0d\u540c\uff0cSeqScan \u4ece\u4e0d\u5728 heap_beginscan \u4e2d\u4f7f\u7528\u952e\uff08\u8fd9\u5f88\u4e0d\u597d\uff09\uff0c\u56e0\u6b64\u8fd9\u91cc\u4e0d\u68c0\u67e5\u952e\u662f\u5426\u6ee1\u8db3\u6761\u4ef6\u3002"]}, {"code": "case T_SeqScan:\n\t\t\tpname = sname = \"Seq Scan\";\n\t\t\tbreak;", "title": "\u6838\u5fc3\u6e90\u7801\u4e2d\u7684 EXPLAIN \u6807\u8bc6"}], "strategies": [], "description": ["\u901a\u8fc7\u987a\u5e8f\u626b\u63cf\u8bfb\u53d6\u5173\u7cfb\uff0c\u5e76\u5e94\u7528\u626b\u63cf\u6761\u4ef6\u3002"], "localization": {"status": "complete", "sources": [], "language": "zh", "original_text": {"/summary": "Reads a relation through a sequential scan and applies its scan qualifications.", "/category": "Scan", "/versions/20/description/0": "Reads a relation through a sequential scan and applies its scan qualifications.", "/versions/20/facts/0/label": "Core node tag", "/versions/20/facts/1/label": "Structured EXPLAIN Node Type", "/versions/20/facts/2/label": "Inputs", "/versions/20/facts/2/value": "A relation", "/versions/20/facts/3/label": "Output", "/versions/20/facts/3/value": "Qualified relation tuples", "/versions/20/facts/4/label": "Executor initializer", "/versions/20/facts/5/label": "Memory mechanism", "/versions/20/tables/0/title": "EXPLAIN labels in this source build", "/versions/20/related/1/label": "Using EXPLAIN", "/versions/20/related/2/label": "Parallel plans", "/versions/20/sections/0/title": "EXPLAIN names and attributes", "/versions/20/sections/1/title": "Memory and temporary storage", "/versions/20/sections/2/title": "Parallel execution and instrumentation", "/versions/20/sections/3/title": "Same-version manual discussion", "/versions/20/sections/4/title": "Examples from this manual build", "/versions/20/sections/5/title": "Executor implementation notes", "/versions/20/sections/6/title": "EXPLAIN identity in core source", "/versions/20/memory/description": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/20/sections/0/paragraphs/0": "Structured formats use the Node Type above. Text-format spellings can also include operation, strategy, join type, scan direction or aggregation-stage attributes.", "/versions/20/sections/0/paragraphs/1": "Text names recorded by this source: Seq Scan.", "/versions/20/sections/0/paragraphs/2": "Parallel-aware and parallel-safe are different plan properties. A node running inside a parallel worker is not necessarily a parallel-aware node.", "/versions/20/sections/1/paragraphs/0": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/20/sections/2/paragraphs/0": "The source callbacks below can coordinate execution or collect worker instrumentation. Their presence is not a blanket claim that this node supports a shared parallel scan or shared state.", "/versions/20/sections/2/paragraphs/1": "Callbacks in this build: ExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanInstrumentEstimate, ExecSeqScanReInitializeDSM, ExecSeqScanRetrieveInstrumentation.", "/versions/20/sections/3/paragraphs/0": "Notice that the EXPLAIN output shows the WHERE clause being applied as a \u201c filter \u201d condition attached to the Seq Scan plan node. This means that the plan node checks the condition for each row it scans, and outputs only the ones that pass the condition. The estimate of output rows has been reduced because of the WHERE clause. However, the scan will still have to visit all 10000 rows, so the cost hasn't decreased; in fact it has gone up a bit (by 10000 * cpu_operator_cost , to be exact) to reflect the extra CPU time spent checking the WHERE condition.", "/versions/20/sections/5/paragraphs/0": "nodeSeqscan.c Support routines for sequential scans of relations.", "/versions/20/sections/5/paragraphs/1": "ExecSeqScanEstimate estimates DSM space needed for parallel scan ExecSeqScanInitializeDSM initialize DSM for parallel scan ExecSeqScanReInitializeDSM reinitialize DSM for fresh parallel scan ExecSeqScanInitializeWorker attach to DSM info in parallel worker", "/versions/20/sections/5/paragraphs/2": "We reach here if the scan is not parallel, or if we're serially executing a scan that was planned to be parallel.", "/versions/20/sections/5/paragraphs/3": "SeqRecheck -- access method routine to recheck a tuple in EvalPlanQual", "/versions/20/sections/5/paragraphs/4": "Note that unlike IndexScan, SeqScan never use keys in heap_beginscan (and this is very bad) - so, here we do not check are keys ok or not.", "/versions/20/tables/0/columns/0/label": "Text-format label", "/versions/20/tables/0/columns/1/label": "Structured node identity", "/versions/20/sections/4/blocks/0/paragraphs/0": "Example copied from the PostgreSQL 20devel manual; it was not executed for this collection.", "/versions/20/sections/4/blocks/0/paragraphs/1": "Here is a trivial example, just to show what the output looks like:", "/versions/20/sections/4/blocks/1/paragraphs/0": "Example copied from the PostgreSQL 20devel manual; it was not executed for this collection.", "/versions/20/sections/4/blocks/1/paragraphs/1": "Now let's modify the query to add a WHERE condition:"}, "fallback_fields": [], "source_language": "en", "original_snapshot_sha256": "78a2c3b45cba13f29f12b858003ab7dced454b60d2bbc7d555df7013f4d049fc"}, "evidence_kind": "source and documentation", "explain_names": ["Seq Scan"], "partial_modes": [], "comparison_data": 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"path": "src/backend/executor/nodeSeqscan.c", "label": "src/backend/executor/nodeSeqscan.c", "sha256": "e6c9800872e17a6cfb6e58613de7e82da082b4d430557a3065a3caa6fe24f34d", "archive_sha256": "555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f"}, {"url": "https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2", "path": "src/include/nodes/plannodes.h", "label": "src/include/nodes/plannodes.h", "sha256": "52422b327a8049fbbb20d8b96008a0fc0a6fafa60f7eff3c695d5b2e83830120", "archive_sha256": "555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f"}, {"url": "https://pg.center/docs/18/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 18.6 \u00b7 using-explain", "sha256": "60040c30180093418a0affe56dd27dff9df2504b705b38039589e458bf5c31ed", "language": "en", "original_url": "/docs/18/using-explain.html#USING-EXPLAIN-BASICS"}], "node_tag": "T_SeqScan", "sections": [{"title": "EXPLAIN \u540d\u79f0\u4e0e\u5c5e\u6027", "paragraphs": ["\u7ed3\u6784\u5316\u683c\u5f0f\u4f7f\u7528\u4e0a\u8ff0 Node Type\u3002\u6587\u672c\u683c\u5f0f\u540d\u79f0\u8fd8\u53ef\u80fd\u5305\u542b\u64cd\u4f5c\u3001\u7b56\u7565\u3001\u8fde\u63a5\u7c7b\u578b\u3001\u626b\u63cf\u65b9\u5411\u6216\u805a\u5408\u9636\u6bb5\u5c5e\u6027\u3002", "\u6b64\u6e90\u7801\u8bb0\u5f55\u7684\u6587\u672c\u540d\u79f0\uff1aSeq Scan.", "\u5e76\u884c\u611f\u77e5\u4e0e\u5e76\u884c\u5b89\u5168\u662f\u4e0d\u540c\u7684\u8ba1\u5212\u5c5e\u6027\u3002\u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u5185\u8fd0\u884c\u7684\u8282\u70b9\u4e0d\u4e00\u5b9a\u662f\u5e76\u884c\u611f\u77e5\u8282\u70b9\u3002"]}, {"title": "\u5185\u5b58\u4e0e\u4e34\u65f6\u5b58\u50a8", "paragraphs": ["\u672c\u6b21\u62bd\u53d6\u4e0d\u4e3a\u6b64\u8282\u70b9\u8bbe\u5b9a\u7edf\u4e00\u7684\u5185\u5b58\u4e0a\u9650\u6216\u843d\u76d8\u7b56\u7565\u3002\u8bf7\u67e5\u770b\u540c\u4e00\u6784\u5efa\u7684\u5b9e\u73b0\u3001\u76f8\u5173\u8868\u8fbe\u5f0f\u6216\u63d0\u4f9b\u65b9\u3002"]}, {"title": "\u5e76\u884c\u6267\u884c\u4e0e\u8fd0\u884c\u4fe1\u606f\u91c7\u96c6", "paragraphs": ["\u4ee5\u4e0b\u6e90\u7801\u56de\u8c03\u53ef\u4ee5\u534f\u8c03\u6267\u884c\u6216\u6536\u96c6\u5de5\u4f5c\u8fdb\u7a0b\u7684\u6d4b\u91cf\u6570\u636e\u3002\u56de\u8c03\u5b58\u5728\u4e0d\u4ee3\u8868\u8be5\u8282\u70b9\u666e\u904d\u652f\u6301\u5171\u4eab\u5e76\u884c\u626b\u63cf\u6216\u5171\u4eab\u72b6\u6001\u3002", "\u6b64\u6784\u5efa\u7684\u56de\u8c03\uff1aExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM."]}, {"title": "\u540c\u7248\u672c\u624b\u518c\u8bf4\u660e", "paragraphs": ["EXPLAIN \u5c06 WHERE \u5b50\u53e5\u663e\u793a\u4e3a Seq Scan \u8282\u70b9\u4e0a\u7684 filter \u6761\u4ef6\u3002\u8fd9\u8868\u793a\u8282\u70b9\u5bf9\u626b\u63cf\u7684\u6bcf\u4e00\u884c\u68c0\u67e5\u6761\u4ef6\uff0c\u4ec5\u8f93\u51fa\u901a\u8fc7\u6761\u4ef6\u7684\u884c\u3002WHERE \u964d\u4f4e\u4e86\u9884\u8ba1\u8f93\u51fa\u884c\u6570\uff0c\u4f46\u626b\u63cf\u4ecd\u9700\u8bbf\u95ee\u5168\u90e8 10000 \u884c\uff0c\u6240\u4ee5\u6210\u672c\u6ca1\u6709\u964d\u4f4e\uff1b\u76f8\u53cd\u8fd8\u7565\u6709\u589e\u52a0\uff08\u51c6\u786e\u8bf4\u589e\u52a0 10000 * cpu_operator_cost\uff09\uff0c\u4ee5\u8ba1\u5165\u68c0\u67e5 WHERE \u6761\u4ef6\u6d88\u8017\u7684\u989d\u5916 CPU \u65f6\u95f4\u3002"]}, {"title": "\u672c\u7248\u624b\u518c\u4e2d\u7684\u793a\u4f8b", "blocks": [{"code": "EXPLAIN SELECT * FROM tenk1;\n\n                         QUERY PLAN\n-------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..445.00 rows=10000 width=244)", "source": {"url": "/docs/18/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 18.6 \u00b7 using-explain", "sha256": "60040c30180093418a0affe56dd27dff9df2504b705b38039589e458bf5c31ed"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 18.6 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u4e0b\u9762\u662f\u4e00\u4e2a\u7b80\u5355\u793a\u4f8b\uff0c\u4ec5\u7528\u6765\u5c55\u793a\u8f93\u51fa\u5f62\u5f0f\uff1a"]}, {"code": "EXPLAIN SELECT * FROM tenk1 WHERE unique1 < 7000;\n\n                         QUERY PLAN\n------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..470.00 rows=7000 width=244)\n   Filter: (unique1 < 7000)", "source": {"url": "/docs/18/using-explain.html#USING-EXPLAIN-BASICS", "path": "using-explain.html", "label": "PostgreSQL 18.6 \u00b7 using-explain", "sha256": "60040c30180093418a0affe56dd27dff9df2504b705b38039589e458bf5c31ed"}, "paragraphs": ["\u793a\u4f8b\u6458\u81ea PostgreSQL 18.6 \u624b\u518c\uff1b\u672c\u767e\u79d1\u672a\u5b9e\u9645\u6267\u884c\u6b64\u793a\u4f8b\u3002", "\u73b0\u5728\u4fee\u6539\u67e5\u8be2\uff0c\u6dfb\u52a0 WHERE \u6761\u4ef6\uff1a"]}]}, {"title": "\u6267\u884c\u5668\u5b9e\u73b0\u8bf4\u660e", "paragraphs": ["nodeSeqscan.c\uff1a\u652f\u6301\u5173\u7cfb\u987a\u5e8f\u626b\u63cf\u7684\u4f8b\u7a0b\u3002", "ExecSeqScanEstimate \u4f30\u7b97\u5e76\u884c\u626b\u63cf\u6240\u9700\u7684 DSM \u7a7a\u95f4\uff1bExecSeqScanInitializeDSM \u521d\u59cb\u5316\u5e76\u884c\u626b\u63cf\u7684 DSM\uff1bExecSeqScanReInitializeDSM \u4e3a\u65b0\u4e00\u8f6e\u5e76\u884c\u626b\u63cf\u91cd\u65b0\u521d\u59cb\u5316 DSM\uff1bExecSeqScanInitializeWorker \u5728\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u4e2d\u9644\u52a0\u5230 DSM \u4fe1\u606f\u3002", "\u626b\u63cf\u4e0d\u5e76\u884c\u6267\u884c\uff0c\u6216\u539f\u8ba1\u5212\u5e76\u884c\u7684\u626b\u63cf\u4ee5\u4e32\u884c\u65b9\u5f0f\u6267\u884c\u65f6\uff0c\u4f1a\u5230\u8fbe\u6b64\u5904\u3002", "SeqRecheck\uff1a\u5728 EvalPlanQual \u4e2d\u91cd\u65b0\u68c0\u67e5\u5143\u7ec4\u7684\u8bbf\u95ee\u65b9\u6cd5\u4f8b\u7a0b\u3002", "\u4e0e IndexScan \u4e0d\u540c\uff0cSeqScan \u4ece\u4e0d\u5728 heap_beginscan \u4e2d\u4f7f\u7528\u952e\uff08\u8fd9\u5f88\u4e0d\u597d\uff09\uff0c\u56e0\u6b64\u8fd9\u91cc\u4e0d\u68c0\u67e5\u952e\u662f\u5426\u6ee1\u8db3\u6761\u4ef6\u3002"]}, {"code": "case T_SeqScan:\n\t\t\tpname = sname = \"Seq Scan\";\n\t\t\tbreak;", "title": "\u6838\u5fc3\u6e90\u7801\u4e2d\u7684 EXPLAIN \u6807\u8bc6"}], "strategies": [], "description": ["\u901a\u8fc7\u987a\u5e8f\u626b\u63cf\u8bfb\u53d6\u5173\u7cfb\uff0c\u5e76\u5e94\u7528\u626b\u63cf\u6761\u4ef6\u3002"], "localization": {"status": "complete", "sources": [], "language": "zh", "original_text": {"/versions/18/description/0": "Reads a relation through a sequential scan and applies its scan qualifications.", "/versions/18/facts/0/label": "Core node tag", "/versions/18/facts/1/label": "Structured EXPLAIN Node Type", "/versions/18/facts/2/label": "Inputs", "/versions/18/facts/2/value": "A relation", "/versions/18/facts/3/label": "Output", "/versions/18/facts/3/value": "Qualified relation tuples", "/versions/18/facts/4/label": "Executor initializer", "/versions/18/facts/5/label": "Memory mechanism", "/versions/18/tables/0/title": "EXPLAIN labels in this source build", "/versions/18/related/1/label": "Using EXPLAIN", "/versions/18/related/2/label": "Parallel plans", "/versions/18/sections/0/title": "EXPLAIN names and attributes", "/versions/18/sections/1/title": "Memory and temporary storage", "/versions/18/sections/2/title": "Parallel execution and instrumentation", "/versions/18/sections/3/title": "Same-version manual discussion", "/versions/18/sections/4/title": "Examples from this manual build", "/versions/18/sections/5/title": "Executor implementation notes", "/versions/18/sections/6/title": "EXPLAIN identity in core source", "/versions/18/memory/description": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/18/sections/0/paragraphs/0": "Structured formats use the Node Type above. Text-format spellings can also include operation, strategy, join type, scan direction or aggregation-stage attributes.", "/versions/18/sections/0/paragraphs/1": "Text names recorded by this source: Seq Scan.", "/versions/18/sections/0/paragraphs/2": "Parallel-aware and parallel-safe are different plan properties. A node running inside a parallel worker is not necessarily a parallel-aware node.", "/versions/18/sections/1/paragraphs/0": "This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.", "/versions/18/sections/2/paragraphs/0": "The source callbacks below can coordinate execution or collect worker instrumentation. Their presence is not a blanket claim that this node supports a shared parallel scan or shared state.", "/versions/18/sections/2/paragraphs/1": "Callbacks in this build: ExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM.", "/versions/18/sections/3/paragraphs/0": "Notice that the EXPLAIN output shows the WHERE clause being applied as a \u201c filter \u201d condition attached to the Seq Scan plan node. This means that the plan node checks the condition for each row it scans, and outputs only the ones that pass the condition. The estimate of output rows has been reduced because of the WHERE clause. However, the scan will still have to visit all 10000 rows, so the cost hasn't decreased; in fact it has gone up a bit (by 10000 * cpu_operator_cost , to be exact) to reflect the extra CPU time spent checking the WHERE condition.", "/versions/18/sections/5/paragraphs/0": "nodeSeqscan.c Support routines for sequential scans of relations.", "/versions/18/sections/5/paragraphs/1": "ExecSeqScanEstimate estimates DSM space needed for parallel scan ExecSeqScanInitializeDSM initialize DSM for parallel scan ExecSeqScanReInitializeDSM reinitialize DSM for fresh parallel scan ExecSeqScanInitializeWorker attach to DSM info in parallel worker", "/versions/18/sections/5/paragraphs/2": "We reach here if the scan is not parallel, or if we're serially executing a scan that was planned to be parallel.", "/versions/18/sections/5/paragraphs/3": "SeqRecheck -- access method routine to recheck a tuple in EvalPlanQual", "/versions/18/sections/5/paragraphs/4": "Note that unlike IndexScan, SeqScan never use keys in heap_beginscan (and this is very bad) - so, here we do not check are keys ok or not.", "/versions/18/tables/0/columns/0/label": "Text-format label", "/versions/18/tables/0/columns/1/label": "Structured node identity", "/versions/18/sections/4/blocks/0/paragraphs/0": "Example copied from the PostgreSQL 18.6 manual; it was not executed for this collection.", "/versions/18/sections/4/blocks/0/paragraphs/1": "Here is a trivial example, just to show what the output looks like:", "/versions/18/sections/4/blocks/1/paragraphs/0": "Example copied from the PostgreSQL 18.6 manual; it was not executed for this collection.", "/versions/18/sections/4/blocks/1/paragraphs/1": "Now let's modify the query to add a WHERE condition:"}, "fallback_fields": [], "source_language": "en", "original_snapshot_sha256": "345d63a8fe9cfcb7d8eb953497a0553b8d2171a21b82d194cd9a12154a0a139b"}, "evidence_kind": "source and documentation", "explain_names": ["Seq Scan"], "partial_modes": [], "comparison_data": {"node_tag": "T_SeqScan", "strategies": [], "text_names": ["Seq Scan"], "initializer": "ExecInitSeqScan", "partial_modes": [], "memory_mechanism": "unclassified", "parallel_callbacks": ["ExecSeqScanEstimate", "ExecSeqScanInitializeDSM", "ExecSeqScanInitializeWorker", "ExecSeqScanReInitializeDSM"]}, "comparison_hash": "5631c0db91382334dc4ca321ae0335013d10ae1c0c24f3523fcca0184bbbc81b", "explain_prefixes": ["Parallel", "Async"], "runtime_verified": false, "source_inventory": {"explain": "src/backend/commands/explain.c", "executor": "src/backend/executor/execProcnode.c", "implementation": "src/backend/executor/nodeSeqscan.c"}, "parallel_callbacks": ["ExecSeqScanEstimate", "ExecSeqScanInitializeDSM", "ExecSeqScanInitializeWorker", "ExecSeqScanReInitializeDSM"]}, "comparison": {"left": "18", "right": "19", "status": "changed", "diff": "--- PostgreSQL 18\n+++ PostgreSQL 19\n@@ -6,7 +6,9 @@\n     \"ExecSeqScanEstimate\",\n     \"ExecSeqScanInitializeDSM\",\n     \"ExecSeqScanInitializeWorker\",\n-    \"ExecSeqScanReInitializeDSM\"\n+    \"ExecSeqScanInstrumentEstimate\",\n+    \"ExecSeqScanReInitializeDSM\",\n+    \"ExecSeqScanRetrieveInstrumentation\"\n   ],\n   \"partial_modes\": [],\n   \"strategies\": [],"}}