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 37.2.5). | 
| anyarray | Indicates that a function accepts any array data type (see Section 37.2.5). | 
| anynonarray | Indicates that a function accepts any non-array data type (see Section 37.2.5). | 
| anyenum | Indicates that a function accepts any enum data type (see Section 37.2.5 and Section 8.7). | 
| anyrange | Indicates that a function accepts any range data type (see Section 37.2.5 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 37.2.5). | 
| anycompatiblearray | Indicates that a function accepts any array data type, with automatic promotion of multiple arguments to a common data type (see Section 37.2.5). | 
| anycompatiblenonarray | Indicates that a function accepts any non-array data type, with automatic promotion of multiple arguments to a common data type (see Section 37.2.5). | 
| anycompatiblerange | Indicates that a function accepts any range data type, with automatic promotion of multiple arguments to a common data type (see Section 37.2.5 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 37.2.5.
   
    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.