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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: Gather.", "/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/2/paragraphs/0": "The source callbacks below can coordinate execution or collect worker instrumentation. 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This is reflected in the plan as a Finalize Aggregate node.", "/versions/10/sections/4/paragraphs/0": "nodeGather.c Support routines for scanning a plan via multiple workers.", "/versions/10/sections/4/paragraphs/1": "A Gather executor launches parallel workers to run multiple copies of a plan. It can also run the plan itself, if the workers are not available or have not started up yet. It then merges all of the results it produces and the results from the workers into a single output stream. Therefore, it will normally be used with a plan where running multiple copies of the same plan does not produce duplicate output, such as parallel-aware SeqScan.", "/versions/10/sections/4/paragraphs/2": "Alternatively, a Gather node can be configured to use just one worker and the single-copy flag can be set. In this case, the Gather node will run the plan in one worker and will not execute the plan itself. In this case, it simply returns whatever tuples were returned by the worker. 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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": "Executor implementation notes", "/versions/11/sections/5/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. 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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: Gather.", "/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: none extracted from this node implementation.", "/versions/12/sections/3/paragraphs/0": "PostgreSQL supports parallel aggregation by aggregating in two stages. First, each process participating in the parallel portion of the query performs an aggregation step, producing a partial result for each group of which that process is aware. This is reflected in the plan as a Partial Aggregate node. Second, the partial results are transferred to the leader via Gather or Gather Merge . Finally, the leader re-aggregates the results across all workers in order to produce the final result. This is reflected in the plan as a Finalize Aggregate node.", "/versions/12/sections/4/paragraphs/0": "nodeGather.c Support routines for scanning a plan via multiple workers.", "/versions/12/sections/4/paragraphs/1": "A Gather executor launches parallel workers to run multiple copies of a plan. It can also run the plan itself, if the workers are not available or have not started up yet. It then merges all of the results it produces and the results from the workers into a single output stream. Therefore, it will normally be used with a plan where running multiple copies of the same plan does not produce duplicate output, such as parallel-aware SeqScan.", "/versions/12/sections/4/paragraphs/2": "Alternatively, a Gather node can be configured to use just one worker and the single-copy flag can be set. In this case, the Gather node will run the plan in one worker and will not execute the plan itself. In this case, it simply returns whatever tuples were returned by the worker. 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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": "Executor implementation notes", "/versions/13/sections/5/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: Gather.", "/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. 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This is reflected in the plan as a Finalize Aggregate node.", "/versions/13/sections/4/paragraphs/0": "nodeGather.c Support routines for scanning a plan via multiple workers.", "/versions/13/sections/4/paragraphs/1": "A Gather executor launches parallel workers to run multiple copies of a plan. It can also run the plan itself, if the workers are not available or have not started up yet. It then merges all of the results it produces and the results from the workers into a single output stream. Therefore, it will normally be used with a plan where running multiple copies of the same plan does not produce duplicate output, such as parallel-aware SeqScan.", "/versions/13/sections/4/paragraphs/2": "Alternatively, a Gather node can be configured to use just one worker and the single-copy flag can be set. In this case, the Gather node will run the plan in one worker and will not execute the plan itself. In this case, it simply returns whatever tuples were returned by the worker. 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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: Gather.", "/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. 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This is reflected in the plan as a Finalize Aggregate node.", "/versions/14/sections/4/paragraphs/0": "nodeGather.c Support routines for scanning a plan via multiple workers.", "/versions/14/sections/4/paragraphs/1": "A Gather executor launches parallel workers to run multiple copies of a plan. It can also run the plan itself, if the workers are not available or have not started up yet. It then merges all of the results it produces and the results from the workers into a single output stream. Therefore, it will normally be used with a plan where running multiple copies of the same plan does not produce duplicate output, such as parallel-aware SeqScan.", "/versions/14/sections/4/paragraphs/2": "Alternatively, a Gather node can be configured to use just one worker and the single-copy flag can be set. In this case, the Gather node will run the plan in one worker and will not execute the plan itself. In this case, it simply returns whatever tuples were returned by the worker. 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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": "Executor implementation notes", "/versions/16/sections/5/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: Gather.", "/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: none extracted from this node implementation.", "/versions/16/sections/3/paragraphs/0": "PostgreSQL supports parallel aggregation by aggregating in two stages. First, each process participating in the parallel portion of the query performs an aggregation step, producing a partial result for each group of which that process is aware. This is reflected in the plan as a Partial Aggregate node. Second, the partial results are transferred to the leader via Gather or Gather Merge . Finally, the leader re-aggregates the results across all workers in order to produce the final result. This is reflected in the plan as a Finalize Aggregate node.", "/versions/16/sections/4/paragraphs/0": "nodeGather.c Support routines for scanning a plan via multiple workers.", "/versions/16/sections/4/paragraphs/1": "A Gather executor launches parallel workers to run multiple copies of a plan. It can also run the plan itself, if the workers are not available or have not started up yet. It then merges all of the results it produces and the results from the workers into a single output stream. Therefore, it will normally be used with a plan where running multiple copies of the same plan does not produce duplicate output, such as parallel-aware SeqScan.", "/versions/16/sections/4/paragraphs/2": "Alternatively, a Gather node can be configured to use just one worker and the single-copy flag can be set. In this case, the Gather node will run the plan in one worker and will not execute the plan itself. In this case, it simply returns whatever tuples were returned by the worker. 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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": "Executor implementation notes", "/versions/17/sections/5/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: Gather.", "/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: none extracted from this node implementation.", "/versions/17/sections/3/paragraphs/0": "PostgreSQL supports parallel aggregation by aggregating in two stages. First, each process participating in the parallel portion of the query performs an aggregation step, producing a partial result for each group of which that process is aware. This is reflected in the plan as a Partial Aggregate node. Second, the partial results are transferred to the leader via Gather or Gather Merge . Finally, the leader re-aggregates the results across all workers in order to produce the final result. This is reflected in the plan as a Finalize Aggregate node.", "/versions/17/sections/4/paragraphs/0": "nodeGather.c Support routines for scanning a plan via multiple workers.", "/versions/17/sections/4/paragraphs/1": "A Gather executor launches parallel workers to run multiple copies of a plan. It can also run the plan itself, if the workers are not available or have not started up yet. It then merges all of the results it produces and the results from the workers into a single output stream. Therefore, it will normally be used with a plan where running multiple copies of the same plan does not produce duplicate output, such as parallel-aware SeqScan.", "/versions/17/sections/4/paragraphs/2": "Alternatively, a Gather node can be configured to use just one worker and the single-copy flag can be set. In this case, the Gather node will run the plan in one worker and will not execute the plan itself. In this case, it simply returns whatever tuples were returned by the worker. 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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": "Executor implementation notes", "/versions/18/sections/5/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: Gather.", "/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. 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This is reflected in the plan as a Finalize Aggregate node.", "/versions/18/sections/4/paragraphs/0": "nodeGather.c Support routines for scanning a plan via multiple workers.", "/versions/18/sections/4/paragraphs/1": "A Gather executor launches parallel workers to run multiple copies of a plan. It can also run the plan itself, if the workers are not available or have not started up yet. It then merges all of the results it produces and the results from the workers into a single output stream. Therefore, it will normally be used with a plan where running multiple copies of the same plan does not produce duplicate output, such as parallel-aware SeqScan.", "/versions/18/sections/4/paragraphs/2": "Alternatively, a Gather node can be configured to use just one worker and the single-copy flag can be set. In this case, the Gather node will run the plan in one worker and will not execute the plan itself. In this case, it simply returns whatever tuples were returned by the worker. 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So we can't trivially rely on the slot type being fixed for expressions evaluated within this node.", "/versions/18/sections/4/paragraphs/4": "Without projections result slot type is not trivially known, see comment above.", "/versions/18/tables/0/columns/0/label": "Text-format label", "/versions/18/tables/0/columns/1/label": "Structured node identity"}, "fallback_fields": [], "source_language": "en", "original_snapshot_sha256": "af9e759baadd6acbac8d5f3225142db82b91afd08b2106deb709b090b359ccf6"}, "evidence_kind": "source and documentation", "explain_names": ["Gather"], "partial_modes": [], "comparison_data": {"node_tag": "T_Gather", "strategies": [], "text_names": ["Gather"], "initializer": "ExecInitGather", "partial_modes": [], "memory_mechanism": "unclassified", "parallel_callbacks": []}, "comparison_hash": "6fbd3ac9f7150ee72d4ae315dded3c145ee461d926c8ce6a04a1b8c6c31deee4", "explain_prefixes": ["Parallel", "Async"], "runtime_verified": false, "source_inventory": {"explain": 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"label": "max_parallel_workers_per_gather"}, {"url": "/wiki/guc/parallel_leader_participation/?v=19", "label": "parallel_leader_participation"}], "release": {"ref": "PostgreSQL 19beta4 source archive", "label": "19beta4", "major": "19", "channel": "preview", "revision": "83157ee9c599d03b2f7a3d73ef3a56ec24e0e79cc2b3501a64d1364f56398c86", "source_url": "https://ftp.postgresql.org/pub/source/v19beta4/postgresql-19beta4.tar.bz2", "source_snapshot_utc": ""}, "sources": [{"url": "https://ftp.postgresql.org/pub/source/v19beta4/postgresql-19beta4.tar.bz2", "line": 1450, "path": "src/backend/commands/explain.c", "label": "src/backend/commands/explain.c:1450", "sha256": "8b115b1c194a4b54ae630209a741e293b1df49a9052f10b2de9ca092a48998e3", "archive_sha256": "83157ee9c599d03b2f7a3d73ef3a56ec24e0e79cc2b3501a64d1364f56398c86"}, {"url": "https://ftp.postgresql.org/pub/source/v19beta4/postgresql-19beta4.tar.bz2", "line": 355, "path": "src/backend/executor/execProcnode.c", "label": "src/backend/executor/execProcnode.c:355", "sha256": "5e39b2037bed672da55104229ecc32da5abde44c26bcad01479edcfa044d09ed", "archive_sha256": "83157ee9c599d03b2f7a3d73ef3a56ec24e0e79cc2b3501a64d1364f56398c86"}, {"url": "https://ftp.postgresql.org/pub/source/v19beta4/postgresql-19beta4.tar.bz2", "path": "src/backend/executor/nodeGather.c", "label": "src/backend/executor/nodeGather.c", "sha256": "fa073da7b81e09398273b25eb0c04bdbe9140a12cad0dbc0d82a612dfb51a489", "archive_sha256": "83157ee9c599d03b2f7a3d73ef3a56ec24e0e79cc2b3501a64d1364f56398c86"}, {"url": "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/parallel-plans.html#PARALLEL-AGGREGATION", "path": "parallel-plans.html", "label": "PostgreSQL 19beta4 \u00b7 parallel-plans", "sha256": "f99fee3456a48a5ce2c399d6b35ab93183c2c9012e8ed60c53c0c253f79c3756", "language": "en", "original_url": "/docs/19/parallel-plans.html#PARALLEL-AGGREGATION"}, {"url": "https://pg.center/docs/19/using-explain.html#USING-EXPLAIN-ANALYZE", "path": "using-explain.html", "label": "PostgreSQL 19beta4 \u00b7 using-explain", "sha256": "52f111fbd213e2200146319d01a9dcc2ea90001617d2a28edc52f7954c2dd70b", "language": "en", "original_url": "/docs/19/using-explain.html#USING-EXPLAIN-ANALYZE"}], "node_tag": "T_Gather", "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\uff1aGather.", "\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\uff1anone extracted from this node implementation."]}, {"title": "\u540c\u7248\u672c\u624b\u518c\u8bf4\u660e", "paragraphs": ["PostgreSQL \u901a\u8fc7\u5206\u4e24\u4e2a\u9636\u6bb5\u8fdb\u884c\u805a\u5408\u6765\u652f\u6301\u5e76\u884c\u805a\u5408\u3002\u9996\u5148\uff0c\u6bcf\u4e2a\u53c2\u4e0e\u67e5\u8be2\u5e76\u884c\u90e8\u5206\u7684\u8fdb\u7a0b\u6267\u884c\u4e00\u4e2a\u805a\u5408\u6b65\u9aa4\uff0c\u4e3a\u8be5\u8fdb\u7a0b\u6240\u89c1\u5230\u7684\u6bcf\u4e2a\u5206\u7ec4\u4ea7\u751f\u4e00\u4e2a\u90e8\u5206\u7ed3\u679c\u3002\u8fd9\u5728\u8ba1\u5212\u4e2d\u4f53\u73b0\u4e3a\u4e00\u4e2a Partial Aggregate \u8282\u70b9\u3002\u7136\u540e\uff0c\u90e8\u5206\u7ed3\u679c\u901a\u8fc7 Gather \u6216 Gather Merge \u4f20\u9001\u7ed9\u9886\u5bfc\u8005\u3002\u6700\u540e\uff0c\u9886\u5bfc\u8005\u4f1a\u628a\u6765\u81ea\u6240\u6709\u5de5\u4f5c\u8fdb\u7a0b\u7684\u7ed3\u679c\u518d\u6b21\u805a\u5408\uff0c\u4ee5\u4ea7\u751f\u6700\u7ec8\u7ed3\u679c\u3002\u8fd9\u5728\u8ba1\u5212\u4e2d\u4f53\u73b0\u4e3a\u4e00\u4e2a Finalize Aggregate \u8282\u70b9\u3002"]}, {"title": "\u672c\u7248\u624b\u518c\u4e2d\u7684\u793a\u4f8b", "blocks": [{"code": "SET min_parallel_table_scan_size = 0;\nSET parallel_tuple_cost = 0;\nSET parallel_setup_cost = 0;\n\nEXPLAIN ANALYZE SELECT *\nFROM tenk1 t1, tenk2 t2\nWHERE t1.unique1 < 10 AND t1.unique2 = t2.unique2;\n                                                                QUERY PLAN\n------------------------------------------------------------------------------------------------------------------------------------------\n Gather  (cost=4.65..70.96 rows=10 width=488) (actual time=1.161..11.655 rows=10.00 loops=1)\n   Workers Planned: 2\n   Workers Launched: 2\n   Buffers: shared hit=78 read=6\n   ->  Nested Loop  (cost=4.65..70.96 rows=4 width=488) (actual time=0.247..0.317 rows=3.33 loops=3)\n         Buffers: shared hit=78 read=6\n         ->  Parallel Bitmap Heap Scan on tenk1 t1  (cost=4.36..39.31 rows=4 width=244) (actual time=0.228..0.249 rows=3.33 loops=3)\n               Recheck Cond: (unique1 < 10)\n               Heap Blocks: exact=10\n               Buffers: shared hit=54\n               ->  Bitmap Index Scan on tenk1_unique1  (cost=0.00..4.36 rows=10 width=0) (actual time=0.438..0.439 rows=10.00 loops=1)\n                     Index Cond: (unique1 < 10)\n                     Index Searches: 1\n                     Buffers: shared hit=2\n         ->  Index Scan using tenk2_unique2 on tenk2 t2  (cost=0.29..7.90 rows=1 width=244) (actual time=0.016..0.017 rows=1.00 loops=10)\n               Index Cond: (unique2 = t1.unique2)\n               Index Searches: 10\n               Buffers: shared hit=24 read=6\n Planning:\n   Buffers: shared hit=327 read=3\n Planning Time: 4.781 ms\n Execution Time: 11.858 ms\n(22 rows)", "source": {"url": "/docs/19/using-explain.html#USING-EXPLAIN-ANALYZE", "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", "\u5e76\u884c\u6267\u884c\u4e5f\u4f1a\u5bfc\u81f4\u8282\u70b9\u88ab\u6267\u884c\u591a\u6b21\u3002\u8fd9\u540c\u6837\u4f1a\u7531 loops \u503c\u4f53\u73b0\u3002\u6211\u4eec\u53ef\u4ee5\u8c03\u6574\u4e00\u4e9b\u89c4\u5212\u5668\u8bbe\u7f6e\uff0c\u8ba9\u89c4\u5212\u5668\u4e3a\u4e0a\u9762\u7684\u67e5\u8be2\u9009\u62e9\u5e76\u884c\u8ba1\u5212\uff1a"]}]}, {"title": "\u6267\u884c\u5668\u5b9e\u73b0\u8bf4\u660e", "paragraphs": ["nodeGather.c\uff1a\u901a\u8fc7\u591a\u4e2a\u5de5\u4f5c\u8fdb\u7a0b\u626b\u63cf\u8ba1\u5212\u7684\u652f\u6301\u4f8b\u7a0b\u3002", "Gather \u6267\u884c\u5668\u542f\u52a8\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\uff0c\u8fd0\u884c\u4e00\u4e2a\u8ba1\u5212\u7684\u591a\u4e2a\u526f\u672c\u3002\u5982\u679c\u5de5\u4f5c\u8fdb\u7a0b\u4e0d\u53ef\u7528\u6216\u5c1a\u672a\u542f\u52a8\uff0c\u5b83\u4e5f\u53ef\u4ee5\u81ea\u884c\u8fd0\u884c\u6b64\u8ba1\u5212\u3002\u7136\u540e\uff0c\u5b83\u5c06\u81ea\u8eab\u548c\u5de5\u4f5c\u8fdb\u7a0b\u4ea7\u751f\u7684\u5168\u90e8\u7ed3\u679c\u5408\u5e76\u4e3a\u5355\u4e2a\u8f93\u51fa\u6d41\u3002\u56e0\u6b64\uff0cGather \u901a\u5e38\u7528\u4e8e\u540c\u65f6\u8fd0\u884c\u591a\u4e2a\u526f\u672c\u4e5f\u4e0d\u4f1a\u4ea7\u751f\u91cd\u590d\u8f93\u51fa\u7684\u8ba1\u5212\uff0c\u4f8b\u5982\u53ef\u611f\u77e5\u5e76\u884c\u7684 SeqScan\u3002", "\u53e6\u4e00\u79cd\u914d\u7f6e\u65b9\u5f0f\u662f\u8ba9 Gather \u8282\u70b9\u53ea\u4f7f\u7528\u4e00\u4e2a\u5de5\u4f5c\u8fdb\u7a0b\uff0c\u5e76\u8bbe\u7f6e single-copy \u6807\u5fd7\u3002\u6b64\u65f6\uff0cGather \u8ba9\u4e00\u4e2a\u5de5\u4f5c\u8fdb\u7a0b\u8fd0\u884c\u8ba1\u5212\uff0c\u81ea\u8eab\u4e0d\u6267\u884c\u8ba1\u5212\uff0c\u800c\u662f\u76f4\u63a5\u8fd4\u56de\u5de5\u4f5c\u8fdb\u7a0b\u4ea7\u751f\u7684\u5143\u7ec4\u3002\u5982\u679c\u65e0\u6cd5\u83b7\u5f97\u5de5\u4f5c\u8fdb\u7a0b\uff0cGather \u4f1a\u81ea\u884c\u8fd0\u884c\u8ba1\u5212\u5e76\u8fd4\u56de\u7ed3\u679c\u3002\u56e0\u6b64\uff0c\u5355\u526f\u672c Gather \u8282\u70b9\u4f7f\u7528\u7684\u8ba1\u5212\u4e0d\u5fc5\u5177\u5907\u5e76\u884c\u611f\u77e5\u80fd\u529b\u3002", "\u9886\u5bfc\u8fdb\u7a0b\u53ef\u80fd\u76f4\u63a5\u8bbf\u95ee ExecProcNode \u7684\u7ed3\u679c\uff08need_to_scan_locally \u4e3a\u771f\u65f6\uff09\uff0c\u4e5f\u53ef\u80fd\u901a\u8fc7\u5143\u7ec4\u961f\u5217\u63a5\u6536\u5de5\u4f5c\u8fdb\u7a0b\u7684\u7ed3\u679c\u3002\u56e0\u6b64\uff0c\u4e0d\u80fd\u7b80\u5355\u5047\u5b9a\u5728\u6b64\u8282\u70b9\u5185\u8ba1\u7b97\u8868\u8fbe\u5f0f\u65f6\uff0c\u5143\u7ec4\u69fd\u7c7b\u578b\u59cb\u7ec8\u56fa\u5b9a\u3002", "\u6ca1\u6709\u6295\u5f71\u65f6\uff0c\u7ed3\u679c\u69fd\u4f4d\u7c7b\u578b\u5e76\u975e\u663e\u800c\u6613\u89c1\uff0c\u53c2\u89c1\u4e0a\u65b9\u6ce8\u91ca\u3002"]}, {"code": "case T_Gather:\n\t\t\tpname = sname = \"Gather\";\n\t\t\tbreak;", "title": "\u6838\u5fc3\u6e90\u7801\u4e2d\u7684 EXPLAIN \u6807\u8bc6"}], "strategies": [], "description": ["\u6c47\u96c6\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u53ca\u53c2\u4e0e\u6267\u884c\u7684\u4e3b\u8fdb\u7a0b\u6240\u4ea7\u751f\u7684\u5143\u7ec4\u3002"], "localization": {"status": "complete", "sources": [{"url": "/docs/19/parallel-plans.html", "method": "same-major semantic node", "sha256": "c162417a8d7d4bf4b4b79ba445207cd01020c24ca11178e13a9fa3d7d8ca8e2e", "language": "zh", "matched_nodes": ["#PARALLEL-PLANS/div[5]/p[1]"]}, {"url": "/docs/19/using-explain.html", "method": "same-major semantic node", "sha256": "7cc78f59bf29afb1811ece53e9d5c572a3f634d66d7422fb8761f19bc6811492", "language": "zh", "matched_nodes": ["#USING-EXPLAIN/div[8]/p[5]"]}], "language": "zh", "original_text": {"/versions/19/description/0": "Combines tuples from parallel workers and any participating leader execution.", "/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": "One parallel child plan", "/versions/19/facts/3/label": "Output", "/versions/19/facts/3/value": "Combined tuples without a merge-order guarantee", "/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: Gather.", "/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: none extracted from this node implementation.", "/versions/19/sections/3/paragraphs/0": "PostgreSQL supports parallel aggregation by aggregating in two stages. First, each process participating in the parallel portion of the query performs an aggregation step, producing a partial result for each group of which that process is aware. This is reflected in the plan as a Partial Aggregate node. Second, the partial results are transferred to the leader via Gather or Gather Merge . Finally, the leader re-aggregates the results across all workers in order to produce the final result. This is reflected in the plan as a Finalize Aggregate node.", "/versions/19/sections/5/paragraphs/0": "nodeGather.c Support routines for scanning a plan via multiple workers.", "/versions/19/sections/5/paragraphs/1": "A Gather executor launches parallel workers to run multiple copies of a plan. It can also run the plan itself, if the workers are not available or have not started up yet. It then merges all of the results it produces and the results from the workers into a single output stream. Therefore, it will normally be used with a plan where running multiple copies of the same plan does not produce duplicate output, such as parallel-aware SeqScan.", "/versions/19/sections/5/paragraphs/2": "Alternatively, a Gather node can be configured to use just one worker and the single-copy flag can be set. In this case, the Gather node will run the plan in one worker and will not execute the plan itself. In this case, it simply returns whatever tuples were returned by the worker. If a worker cannot be obtained, then it will run the plan itself and return the results. Therefore, a plan used with a single-copy Gather node need not be parallel-aware.", "/versions/19/sections/5/paragraphs/3": "Leader may access ExecProcNode result directly (if need_to_scan_locally), or from workers via tuple queue. So we can't trivially rely on the slot type being fixed for expressions evaluated within this node.", "/versions/19/sections/5/paragraphs/4": "Without projections result slot type is not trivially known, see comment above.", "/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": "Parallel execution will also cause nodes to be executed more than once. This is also reported with the loops value. We can change some planner settings to make the planner pick a parallel plan for the above query:"}, "fallback_fields": [], "source_language": "en", "original_snapshot_sha256": "3030497833e4f371699f2efc02efc8a2a37c22e02fc468fc126cbdfdd8ee0ec7"}, "evidence_kind": "source and documentation", "explain_names": ["Gather"], "partial_modes": [], "comparison_data": {"node_tag": "T_Gather", "strategies": [], "text_names": ["Gather"], "initializer": "ExecInitGather", "partial_modes": [], "memory_mechanism": "unclassified", "parallel_callbacks": []}, "comparison_hash": "6fbd3ac9f7150ee72d4ae315dded3c145ee461d926c8ce6a04a1b8c6c31deee4", "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/nodeGather.c"}, "parallel_callbacks": []}, "20": {"facts": [{"label": "\u6838\u5fc3\u8282\u70b9\u6807\u7b7e", "value": "T_Gather"}, 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time=1.161..11.655 rows=10.00 loops=1)\n   Workers Planned: 2\n   Workers Launched: 2\n   Buffers: shared hit=78 read=6\n   ->  Nested Loop  (cost=4.65..70.96 rows=4 width=488) (actual time=0.247..0.317 rows=3.33 loops=3)\n         Buffers: shared hit=78 read=6\n         ->  Parallel Bitmap Heap Scan on tenk1 t1  (cost=4.36..39.31 rows=4 width=244) (actual time=0.228..0.249 rows=3.33 loops=3)\n               Recheck Cond: (unique1 < 10)\n               Heap Blocks: exact=10\n               Buffers: shared hit=54\n               ->  Bitmap Index Scan on tenk1_unique1  (cost=0.00..4.36 rows=10 width=0) (actual time=0.438..0.439 rows=10.00 loops=1)\n                     Index Cond: (unique1 < 10)\n                     Index Searches: 1\n                     Buffers: shared hit=2\n         ->  Index Scan using tenk2_unique2 on tenk2 t2  (cost=0.29..7.90 rows=1 width=244) (actual time=0.016..0.017 rows=1.00 loops=10)\n               Index Cond: (unique2 = t1.unique2)\n               Index Searches: 10\n               Buffers: shared hit=24 read=6\n Planning:\n   Buffers: shared hit=327 read=3\n Planning Time: 4.781 ms\n Execution Time: 11.858 ms\n(22 rows)", "source": {"url": "/docs/devel/using-explain.html#USING-EXPLAIN-ANALYZE", "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", "\u5e76\u884c\u6267\u884c\u4e5f\u4f1a\u5bfc\u81f4\u8282\u70b9\u88ab\u6267\u884c\u591a\u6b21\u3002\u8fd9\u540c\u6837\u4f1a\u7531 loops \u503c\u4f53\u73b0\u3002\u6211\u4eec\u53ef\u4ee5\u8c03\u6574\u4e00\u4e9b\u89c4\u5212\u5668\u8bbe\u7f6e\uff0c\u8ba9\u89c4\u5212\u5668\u4e3a\u4e0a\u9762\u7684\u67e5\u8be2\u9009\u62e9\u5e76\u884c\u8ba1\u5212\uff1a"]}]}, {"title": "\u6267\u884c\u5668\u5b9e\u73b0\u8bf4\u660e", "paragraphs": 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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: Gather.", "/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: none extracted from this node implementation.", "/versions/20/sections/3/paragraphs/0": "PostgreSQL supports parallel aggregation by aggregating in two stages. First, each process participating in the parallel portion of the query performs an aggregation step, producing a partial result for each group of which that process is aware. This is reflected in the plan as a Partial Aggregate node. Second, the partial results are transferred to the leader via Gather or Gather Merge . Finally, the leader re-aggregates the results across all workers in order to produce the final result. This is reflected in the plan as a Finalize Aggregate node.", "/versions/20/sections/5/paragraphs/0": "nodeGather.c Support routines for scanning a plan via multiple workers.", "/versions/20/sections/5/paragraphs/1": "A Gather executor launches parallel workers to run multiple copies of a plan. It can also run the plan itself, if the workers are not available or have not started up yet. It then merges all of the results it produces and the results from the workers into a single output stream. Therefore, it will normally be used with a plan where running multiple copies of the same plan does not produce duplicate output, such as parallel-aware SeqScan.", "/versions/20/sections/5/paragraphs/2": "Alternatively, a Gather node can be configured to use just one worker and the single-copy flag can be set. In this case, the Gather node will run the plan in one worker and will not execute the plan itself. In this case, it simply returns whatever tuples were returned by the worker. If a worker cannot be obtained, then it will run the plan itself and return the results. Therefore, a plan used with a single-copy Gather node need not be parallel-aware.", "/versions/20/sections/5/paragraphs/3": "Leader may access ExecProcNode result directly (if need_to_scan_locally), or from workers via tuple queue. So we can't trivially rely on the slot type being fixed for expressions evaluated within this node.", "/versions/20/sections/5/paragraphs/4": "Without projections result slot type is not trivially known, see comment above.", "/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": "Parallel execution will also cause nodes to be executed more than once. This is also reported with the loops value. 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\u8282\u70b9\u53ea\u4f7f\u7528\u4e00\u4e2a\u5de5\u4f5c\u8fdb\u7a0b\uff0c\u5e76\u8bbe\u7f6e single-copy \u6807\u5fd7\u3002\u6b64\u65f6\uff0cGather \u8ba9\u4e00\u4e2a\u5de5\u4f5c\u8fdb\u7a0b\u8fd0\u884c\u8ba1\u5212\uff0c\u81ea\u8eab\u4e0d\u6267\u884c\u8ba1\u5212\uff0c\u800c\u662f\u76f4\u63a5\u8fd4\u56de\u5de5\u4f5c\u8fdb\u7a0b\u4ea7\u751f\u7684\u5143\u7ec4\u3002\u5982\u679c\u65e0\u6cd5\u83b7\u5f97\u5de5\u4f5c\u8fdb\u7a0b\uff0cGather \u4f1a\u81ea\u884c\u8fd0\u884c\u8ba1\u5212\u5e76\u8fd4\u56de\u7ed3\u679c\u3002\u56e0\u6b64\uff0c\u5355\u526f\u672c Gather \u8282\u70b9\u4f7f\u7528\u7684\u8ba1\u5212\u4e0d\u5fc5\u5177\u5907\u5e76\u884c\u611f\u77e5\u80fd\u529b\u3002", "\u9886\u5bfc\u8fdb\u7a0b\u53ef\u80fd\u76f4\u63a5\u8bbf\u95ee ExecProcNode \u7684\u7ed3\u679c\uff08need_to_scan_locally \u4e3a\u771f\u65f6\uff09\uff0c\u4e5f\u53ef\u80fd\u901a\u8fc7\u5143\u7ec4\u961f\u5217\u63a5\u6536\u5de5\u4f5c\u8fdb\u7a0b\u7684\u7ed3\u679c\u3002\u56e0\u6b64\uff0c\u4e0d\u80fd\u7b80\u5355\u5047\u5b9a\u5728\u6b64\u8282\u70b9\u5185\u8ba1\u7b97\u8868\u8fbe\u5f0f\u65f6\uff0c\u5143\u7ec4\u69fd\u7c7b\u578b\u59cb\u7ec8\u56fa\u5b9a\u3002", "\u6ca1\u6709\u6295\u5f71\u65f6\uff0c\u7ed3\u679c\u69fd\u4f4d\u7c7b\u578b\u5e76\u975e\u663e\u800c\u6613\u89c1\uff0c\u53c2\u89c1\u4e0a\u65b9\u6ce8\u91ca\u3002"]}, {"code": "case T_Gather:\n\t\t\tpname = sname = \"Gather\";\n\t\t\tbreak;", "title": "\u6838\u5fc3\u6e90\u7801\u4e2d\u7684 EXPLAIN \u6807\u8bc6"}], "strategies": [], "description": ["\u6c47\u96c6\u5e76\u884c\u5de5\u4f5c\u8fdb\u7a0b\u53ca\u53c2\u4e0e\u6267\u884c\u7684\u4e3b\u8fdb\u7a0b\u6240\u4ea7\u751f\u7684\u5143\u7ec4\u3002"], "localization": {"status": "complete", "sources": [{"url": "/docs/18/parallel-plans.html", "method": "same-major semantic node", "sha256": "7fa104623c0b4cd22acf0929e40518fbed8fe368af3e3b124cf693bd7ab69f0c", "language": "zh", "matched_nodes": ["#PARALLEL-PLANS/div[5]/p[1]"]}], "language": "zh", "original_text": {"/versions/18/description/0": "Combines tuples from parallel workers and any participating leader execution.", "/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": "One parallel child plan", "/versions/18/facts/3/label": "Output", "/versions/18/facts/3/value": "Combined tuples without a merge-order guarantee", "/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": "Executor implementation notes", "/versions/18/sections/5/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: Gather.", "/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: none extracted from this node implementation.", "/versions/18/sections/3/paragraphs/0": "PostgreSQL supports parallel aggregation by aggregating in two stages. First, each process participating in the parallel portion of the query performs an aggregation step, producing a partial result for each group of which that process is aware. This is reflected in the plan as a Partial Aggregate node. Second, the partial results are transferred to the leader via Gather or Gather Merge . Finally, the leader re-aggregates the results across all workers in order to produce the final result. This is reflected in the plan as a Finalize Aggregate node.", "/versions/18/sections/4/paragraphs/0": "nodeGather.c Support routines for scanning a plan via multiple workers.", "/versions/18/sections/4/paragraphs/1": "A Gather executor launches parallel workers to run multiple copies of a plan. It can also run the plan itself, if the workers are not available or have not started up yet. It then merges all of the results it produces and the results from the workers into a single output stream. Therefore, it will normally be used with a plan where running multiple copies of the same plan does not produce duplicate output, such as parallel-aware SeqScan.", "/versions/18/sections/4/paragraphs/2": "Alternatively, a Gather node can be configured to use just one worker and the single-copy flag can be set. In this case, the Gather node will run the plan in one worker and will not execute the plan itself. In this case, it simply returns whatever tuples were returned by the worker. If a worker cannot be obtained, then it will run the plan itself and return the results. Therefore, a plan used with a single-copy Gather node need not be parallel-aware.", "/versions/18/sections/4/paragraphs/3": "Leader may access ExecProcNode result directly (if need_to_scan_locally), or from workers via tuple queue. So we can't trivially rely on the slot type being fixed for expressions evaluated within this node.", "/versions/18/sections/4/paragraphs/4": "Without projections result slot type is not trivially known, see comment above.", "/versions/18/tables/0/columns/0/label": "Text-format label", "/versions/18/tables/0/columns/1/label": "Structured node identity"}, "fallback_fields": [], "source_language": "en", "original_snapshot_sha256": "af9e759baadd6acbac8d5f3225142db82b91afd08b2106deb709b090b359ccf6"}, "evidence_kind": "source and documentation", "explain_names": ["Gather"], "partial_modes": [], "comparison_data": {"node_tag": "T_Gather", "strategies": [], "text_names": ["Gather"], "initializer": "ExecInitGather", "partial_modes": [], "memory_mechanism": "unclassified", "parallel_callbacks": []}, "comparison_hash": "6fbd3ac9f7150ee72d4ae315dded3c145ee461d926c8ce6a04a1b8c6c31deee4", "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/nodeGather.c"}, "parallel_callbacks": []}, "comparison": {"left": "17", "right": "18", "status": "unchanged", "diff": ""}}