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.
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