8.20. 概念型別

ThePostgreSQLtype system contains a number of special-purpose entries that are collectively calledpseudo-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 specificSQLdata type.Table 8.25lists the existing pseudo-types.

Table 8.25. Pseudo-Types

Name	Description
any	Indicates that a function accepts any input data type.
anyelement	Indicates that a function accepts any data type (seeSection 37.2.5).
anyarray	Indicates that a function accepts any array data type (seeSection 37.2.5).
anynonarray	Indicates that a function accepts any non-array data type (seeSection 37.2.5).
anyenum	Indicates that a function accepts any enum data type (seeSection 37.2.5andSection 8.7).
anyrange	Indicates that a function accepts any range data type (seeSection 37.2.5andSection 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 returnlanguage_handler.
fdw_handler	A foreign-data wrapper handler is declared to returnfdw_handler.
index_am_handler	An index access method handler is declared to returnindex_am_handler.
tsm_handler	A tablesample method handler is declared to returntsm_handler.
record	Identifies a function taking or returning an unspecified row type.
trigger	A trigger function is declared to returntrigger.
event_trigger	An event trigger function is declared to returnevent_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.
opaque	An obsolete type name that formerly served many of the above purposes.

Functions coded in C (whether built-in or dynamically loaded) can 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 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 onlyvoidandrecordas a result type (plustriggerorevent_triggerwhen the function is used as a trigger or event trigger). Some also support polymorphic functions using the typesanyelement,anyarray,anynonarray,anyenum, andanyrange.

Theinternalpseudo-type is used to declare functions that are meant only to be called internally by the database system, and not by direct invocation in anSQLquery. If a function has at least oneinternal-type argument then it cannot be called fromSQL. 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 returninternalunless it has at least oneinternalargument.