8.21. 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. 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.