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"value": "4 字节"}, {"label": "存储策略", "value": "plain"}, {"label": "输入函数", "value": "regrolein"}, {"label": "输出函数", "value": "regroleout"}, {"label": "手册中的声明", "value": "regrole"}, {"label": "参考资料", "value": "pg_authid"}, {"label": "手册说明", "value": "角色名"}, {"label": "示例值", "value": "smithee"}], "ranges": [], "aliases": [], "catalog": {"oid": "4096", "descr": "registered role", "typacl": "_null_", "typlen": "4", "typelem": "0", "typname": "regrole", "typsend": "regrolesend", "typtype": "b", "typalign": "i", "typarray": "0", "typbyval": "t", "typdelim": "','", "typinput": "regrolein", "typmodin": "-", "typndims": "0", "typowner": "POSTGRES", "typrelid": "0", "typmodout": "-", "typoutput": "regroleout", "typtypmod": "-1", "typanalyze": "-", "typdefault": "_null_", "typnotnull": "f", "typreceive": "regrolerecv", "typstorage": "p", "typbasetype": "0", "typcategory": "N", "typcollation": "0", "typisdefined": "t", "typnamespace": "pg_catalog", "typsubscript": "-", "typdefaultbin": "_null_", 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8.19. 对象标识符类型

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对象标识符（OID）在 PostgreSQL 内部被用作若干系统表的主键。类型 oid 表示一个对象标识符。此外还有若干 oid 的别名类型，其名称均形如 regsomething。表 8.26给出了概要说明。

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oid 目前实现为无符号四字节整数，因此在大型数据库中，甚至在单张大型表中，都不足以保证数据库范围内的唯一性。

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oid 类型本身除比较之外几乎没有其他操作。不过，它可以转换为整数，然后再用标准整数操作符进行处理。（这样做时要注意有符号与无符号可能造成的混淆。）

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除专用输入／输出例程外，OID 别名类型没有自己的运算。这些例程能够接受和显示系统对象的符号名称，而非 oid 类型使用的原始数值。别名类型可简化对象 OID 的查找。例如，要查看表 mytable 对应的 pg_attribute 行，可以写成：

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SELECT * FROM pg_attribute WHERE attrelid = 'mytable'::regclass;\n
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而不是：

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SELECT * FROM pg_attribute\n  WHERE attrelid = (SELECT oid FROM pg_class WHERE relname = 'mytable');\n
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虽然这样看起来也不算太糟，但其实仍然过于简化。如果不同模式中有多个名为 mytable 的表，就必须写一个复杂得多的子查询来选出正确的 OID。regclass 的输入转换器会按照模式路径设置来处理表查找，因此能够自动完成“正确的事情”。同样，把表的 OID 转换成 regclass，也很适合用来以符号形式显示数值 OID。

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表 8.26. 对象标识符类型

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\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
名字参考资料说明示例值
oid任意数字形式的对象标识符564182
regclasspg_class关系名称pg_type
regcollationpg_collation排序规则名称\"POSIX\"
regconfigpg_ts_config文本检索配置english
regdictionarypg_ts_dict文本检索词典简单
regnamespacepg_namespace命名空间名称pg_catalog
regoperpg_operator操作符名称+
regoperatorpg_operator带参数类型的操作符*(integer,​integer) 或 -(NONE,​integer)
regprocpg_proc函数名称sum
regprocedurepg_proc带参数类型的函数sum(int4)
regrolepg_authid角色名smithee
regtypepg_type数据类型名称integer
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所有按命名空间分组的对象对应的 OID 别名类型，都接受模式限定名称；如果某对象在当前搜索路径下未经限定就无法找到，则其输出也会显示为模式限定名称。例如，myschema.mytable 可作为 regclass 的输入（前提是确有此表）。该值输出时可能是 myschema.mytable，也可能只是 mytable，具体取决于当前搜索路径。regproc 和 regoper 只接受唯一（未重载）的输入名称，因此用途有限；多数情况下，regprocedure 或 regoperator 更合适。对于 regoperator，可以通过在未使用的操作数位置写入 NONE 来表示一元操作符。

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这些类型的输入函数允许在各个词元之间插入空白，并且会把大写字母折叠为小写，双引号内除外；这样做是为了让语法规则更接近 SQL 中书写对象名的方式。反过来，如果需要让输出成为有效的 SQL 标识符，输出函数会自动加上双引号。例如，一个名为 Foo（大写 F）且带两个整数参数的函数，其 OID 可以写成 ' \"Foo\" ( int, integer ) '::regprocedure。输出则会是 \"Foo\"(integer,integer)。函数名和参数类型名也都可以带模式限定。

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许多内置的 PostgreSQL 函数接受表的 OID 或其他数据库对象的 OID。为了方便，这些函数会被声明为接受 regclass（或相应的 OID 别名类型）参数。这意味着你不必手工查找对象的 OID，只需把对象名写成字符串字面值即可。例如，nextval(regclass) 函数接受一个序列关系的 OID，因此可以这样调用：

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nextval('foo')              operates on sequence foo\nnextval('FOO')              same as above\nnextval('\"Foo\"')            operates on sequence Foo\nnextval('myschema.foo')     operates on myschema.foo\nnextval('\"myschema\".foo')   same as above\nnextval('foo')              searches search path for foo\n
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注意

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当你把这类函数的参数写成未经修饰的字符串字面值时，它会变成 regclass（或相应类型）的常量。由于这实际上只是一个 OID，所以即便对象后来被重命名、重新分配到其他模式等，它仍会持续指向最初识别到的对象。这种“早绑定”行为，通常很适合列默认值和视图中的对象引用。但有时你可能希望得到“后绑定”行为，也就是在运行时再解析对象引用。要实现后绑定，请强制把该常量存储为 text 常量，而不是 regclass：

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nextval('foo'::text)      foo is looked up at runtime\n
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to_regclass() 及其同类函数也可用于执行运行时查找。参见表 9.76。

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regclass 的另一个实际用途，是查找 information_schema 视图中列出的表的 OID，而这些视图本身并不直接提供此类 OID。例如，你可能想调用 pg_relation_size() 函数，而它需要表的 OID。综合上述规则，正确的写法是：

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SELECT table_schema, table_name,\n       pg_relation_size((quote_ident(table_schema) || '.' ||\n                         quote_ident(table_name))::regclass)\nFROM information_schema.tables\nWHERE ...\n
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quote_ident() 函数会在需要时为标识符添加双引号。看起来似乎更简单的写法

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SELECT pg_relation_size(table_name)\nFROM information_schema.tables\nWHERE ...\n
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并不推荐，因为对于搜索路径之外的表，或者名字需要加引号的表，它都会失败。

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大多数 OID 别名类型还有一个附加特性，就是会创建依赖关系。如果这些类型的常量出现在存储的表达式中（例如列默认表达式或视图），它就会对被引用对象建立依赖。例如，如果某列的默认表达式是 nextval('my_seq'::regclass)，PostgreSQL 就会知道该默认表达式依赖于序列 my_seq，因此系统不允许在尚未移除该默认表达式时删除序列。改用 nextval('my_seq'::text) 则不会建立依赖。（regrole 是这一特性的例外：该类型的常量不允许出现在存储表达式中。）

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系统使用的另一种标识符类型是 xid，即事务（缩写为 xact）标识符。系统列 xmin 和 xmax 使用这种数据类型。事务标识符是 32 位数值。在某些场景中，使用其 64 位变体 xid8。与 xid 值不同，xid8 值严格单调递增，且在数据库集群的整个生命周期内不会被重用。详见第 67.1 节。

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系统使用的第三种标识符类型是 cid，也就是命令标识符。这是系统列 cmin 和 cmax 的数据类型。命令标识符同样是 32 位量。

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系统使用的最后一种标识符类型是 tid，也就是元组标识符（行标识符）。这是系统列 ctid 的数据类型。一个元组 ID 是一对值（块号、块内元组索引），用来标识该行在其所属表中的物理位置。

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（系统列的更多说明见第 5.6 节。）

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