5.11. 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 datatype, 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 datatype.

Table 5-22. Pseudo-Types

Type nameDescription
recordIdentifies a function returning an unspecified tuple type
anyIndicates that a function accepts any input datatype whatever
anyarrayIndicates that a function accepts any array datatype
voidIndicates that a function returns no value
triggerA trigger function is declared to return trigger
language_handlerA procedural language call handler is declared to return language_handler
cstringIndicates that a function accepts or returns a null-terminated C string
internalIndicates that a function accepts or returns a server-internal data type
opaqueAn obsolete type name that formerly served all the above purposes

Functions coded in C (whether built-in or dynamically loaded) may be declared to accept or return any of these pseudo datatypes. 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 may use pseudo-types only as allowed by their implementation languages. At present the procedural languages all forbid use of a pseudo-type as argument type, and allow only void as a result type (plus trigger when the function is used as a trigger).

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 a 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.