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 5-21 lists the existing
pseudo-types.
Table 5-21. Pseudo-Types
| Type name | Description |
|---|
| record | Identifies a function returning an unspecified row type |
| any | Indicates that a function accepts any input data type whatever |
| anyarray | Indicates that a function accepts any array data type |
| void | Indicates that a function returns no value |
| trigger | A trigger function is declared to return trigger |
| language_handler | A procedural language call handler is declared to return language_handler |
| 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 |
| opaque | An 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 data 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 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.
