8.21. Pseudo-Types

Pseudo-Types

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. Pseudo-Types lists the existing pseudo-types.

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anyelement Indicates that a function accepts any data type (see Polymorphic Types).

anyarray Indicates that a function accepts any array data type (see Polymorphic Types).

anynonarray Indicates that a function accepts any non-array data type (see Polymorphic Types).

anyenum Indicates that a function accepts any enum data type (see Polymorphic Types and Enumerated Types).

anyrange Indicates that a function accepts any range data type (see Polymorphic Types and Range Types).

anymultirange Indicates that a function accepts any multirange data type (see Polymorphic Types and Range Types).

anycompatible Indicates that a function accepts any data type, with automatic promotion of multiple arguments to a common data type (see Polymorphic Types).

anycompatiblearray Indicates that a function accepts any array data type, with automatic promotion of multiple arguments to a common data type (see Polymorphic Types).

anycompatiblenonarray Indicates that a function accepts any non-array data type, with automatic promotion of multiple arguments to a common data type (see Polymorphic Types).

anycompatiblerange Indicates that a function accepts any range data type, with automatic promotion of multiple arguments to a common data type (see Polymorphic Types and Range Types).

anycompatiblemultirange Indicates that a function accepts any multirange data type, with automatic promotion of multiple arguments to a common data type (see Polymorphic Types and Range Types).

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.

: Pseudo-Types

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 Polymorphic Types.

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.