8.21. Pseudo-Types
The Postgres Pro 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 40.2.5). |
anyarray | Indicates that a function accepts any array data type (see Section 40.2.5). |
anynonarray | Indicates that a function accepts any non-array data type (see Section 40.2.5). |
anyenum | Indicates that a function accepts any enum data type (see Section 40.2.5 and Section 8.7). |
anyrange | Indicates that a function accepts any range data type (see Section 40.2.5 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 40.2.5). |
anycompatiblearray | Indicates that a function accepts any array data type, with automatic promotion of multiple arguments to a common data type (see Section 40.2.5). |
anycompatiblenonarray | Indicates that a function accepts any non-array data type, with automatic promotion of multiple arguments to a common data type (see Section 40.2.5). |
anycompatiblerange | Indicates that a function accepts any range data type, with automatic promotion of multiple arguments to a common data type (see Section 40.2.5 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 40.2.5.
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.