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8.21. 伪类型

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PostgreSQL 的类型系统中包含一些特殊用途的条目，统称为伪类型。伪类型不能作为列数据类型使用，但可以用于声明函数的参数类型或结果类型。每一种伪类型都适用于这样的场景：函数的行为并不对应于简单地接受或返回某个特定 SQL 数据类型的值。表 8.27列出了现有的伪类型。

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表 8.27. 伪类型

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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\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
名字说明
any表示一个函数可以接受任意输入数据类型。
anyelement表示一个函数可以接受任意数据类型（参见第 36.2.5 节）。
anyarray表示一个函数可以接受任意数组数据类型（参见第 36.2.5 节）。
anynonarray表示一个函数可以接受任意非数组数据类型（参见第 36.2.5 节）。
anyenum表示一个函数可以接受任意枚举数据类型（参见第 36.2.5 节和第 8.7 节）。
anyrange表示一个函数可以接受任意范围数据类型（参见第 36.2.5 节和第 8.17 节）。
anymultirange表示一个函数可以接受任意多范围数据类型（参见第 36.2.5 节和第 8.17 节）。
anycompatible表示一个函数可以接受任意数据类型，并自动将多个参数提升为公共数据类型（参见第 36.2.5 节）。
anycompatiblearray表示一个函数可以接受任意数组数据类型，并自动将多个参数提升为公共数据类型（参见第 36.2.5 节）。
anycompatiblenonarray表示一个函数可以接受任意非数组数据类型，并自动将多个参数提升为公共数据类型（参见第 36.2.5 节）。
anycompatiblerange表示一个函数可以接受任意范围数据类型，并自动将多个参数提升为公共数据类型（参见第 36.2.5 节和第 8.17 节）。
anycompatiblemultirange表示一个函数可以接受任意多范围数据类型，并自动将多个参数提升为公共数据类型（参见第 36.2.5 节和第 8.17 节）。
cstring表示一个函数接受或返回一个以空字符结尾的 C 字符串。
内部表示一个函数接受或返回一个服务器内部数据类型。
language_handler表示过程语言调用处理器被声明为返回 language_handler。
fdw_handler表示外部数据包装器处理器被声明为返回 fdw_handler。
table_am_handler表示表访问方法处理器被声明为返回 table_am_handler。
index_am_handler表示索引访问方法处理器被声明为返回 index_am_handler。
tsm_handler表示表采样方法处理器被声明为返回 tsm_handler。
record标识一个接受或返回未指定行类型的函数。
trigger触发器函数声明返回 trigger 类型。
event_trigger事件触发器函数声明返回 event_trigger 类型。
pg_ddl_command标识一种可供事件触发器使用的 DDL 命令表示形式。
void表示一个函数不返回值。
unknown（未知）标识一种尚未解析出的类型，例如未修饰的字符串字面值的类型。
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用 C 编写的函数（无论是内置的还是动态装载的）都可以声明为接受或返回这些伪类型中的任意一种。函数作者必须自行确保，当伪类型被用作参数类型时，该函数的行为仍然安全。

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用过程语言编写的函数，只有在其实现语言允许的情况下才能使用伪类型。目前，大多数过程语言都禁止把伪类型用作参数类型，而只允许把 void 和 record 用作结果类型（如果函数被用作触发器或事件触发器，则也允许 trigger 或 event_trigger 作为结果类型）。有些语言还支持借助上面列出的多态伪类型来实现多态函数；详见第 36.2.5 节。

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伪类型 internal 用于声明仅供数据库系统内部调用、而不能在 SQL 查询中直接调用的函数。只要函数至少有一个 internal 类型参数，就不能从 SQL 调用。为保持此限制的类型安全性，必须遵循以下编码规则：除非函数至少有一个 internal 参数，否则不要创建声明为返回 internal 的函数。

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