8.21. Pseudo-Types

8.21. Pseudo-Types
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The PostgreSQL type system contains a number of special-purpose entries that are collectively called pseudo-types . A pseudo-type cannot be used as a column data type, but it can be used to declare a function's argument or result type. Each of the available pseudo-types is useful in situations where a function's behavior does not correspond to simply taking or returning a value of a specific SQL data type. Table 8.27 lists the existing pseudo-types.

Table 8.27. Pseudo-Types

Name	Description
`any`	Indicates that a function accepts any input data type.
`anyelement`	Indicates that a function accepts any data type (see Section 36.2.5 ).
`anyarray`	Indicates that a function accepts any array data type (see Section 36.2.5 ).
`anynonarray`	Indicates that a function accepts any non-array data type (see Section 36.2.5 ).
`anyenum`	Indicates that a function accepts any enum data type (see Section 36.2.5 and Section 8.7 ).
`anyrange`	Indicates that a function accepts any range data type (see Section 36.2.5 and Section 8.17 ).
`anymultirange`	Indicates that a function accepts any multirange data type (see Section 36.2.5 and Section 8.17 ).
`anycompatible`	Indicates that a function accepts any data type, with automatic promotion of multiple arguments to a common data type (see Section 36.2.5 ).
`anycompatiblearray`	Indicates that a function accepts any array data type, with automatic promotion of multiple arguments to a common data type (see Section 36.2.5 ).
`anycompatiblenonarray`	Indicates that a function accepts any non-array data type, with automatic promotion of multiple arguments to a common data type (see Section 36.2.5 ).
`anycompatiblerange`	Indicates that a function accepts any range data type, with automatic promotion of multiple arguments to a common data type (see Section 36.2.5 and Section 8.17 ).
`anycompatiblemultirange`	Indicates that a function accepts any multirange data type, with automatic promotion of multiple arguments to a common data type (see Section 36.2.5 and Section 8.17 ).
`cstring`	Indicates that a function accepts or returns a null-terminated C string.
`internal`	Indicates that a function accepts or returns a server-internal data type.
`language_handler`	A procedural language call handler is declared to return `language_handler` .
`fdw_handler`	A foreign-data wrapper handler is declared to return `fdw_handler` .
`table_am_handler`	A table access method handler is declared to return `table_am_handler` .
`index_am_handler`	An index access method handler is declared to return `index_am_handler` .
`tsm_handler`	A tablesample method handler is declared to return `tsm_handler` .
`record`	Identifies a function taking or returning an unspecified row type.
`trigger`	A trigger function is declared to return `trigger.`
`event_trigger`	An event trigger function is declared to return `event_trigger.`
`pg_ddl_command`	Identifies a representation of DDL commands that is available to event triggers.
`void`	Indicates that a function returns no value.
`unknown`	Identifies a not-yet-resolved type, e.g., of an undecorated string literal.

Functions coded in C (whether built-in or dynamically loaded) can be declared to accept or return any of these pseudo-types. It is up to the function author to ensure that the function will behave safely when a pseudo-type is used as an argument type.

Functions coded in procedural languages can use pseudo-types only as allowed by their implementation languages. At present most procedural languages forbid use of a pseudo-type as an argument type, and allow only void and record as a result type (plus trigger or event_trigger when the function is used as a trigger or event trigger). Some also support polymorphic functions using the polymorphic pseudo-types, which are shown above and discussed in detail in Section 36.2.5 .

The internal pseudo-type is used to declare functions that are meant only to be called internally by the database system, and not by direct invocation in an SQL query. If a function has at least one internal -type argument then it cannot be called from SQL . To preserve the type safety of this restriction it is important to follow this coding rule: do not create any function that is declared to return internal unless it has at least one internal argument.

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