8.21. 概念型別

PostgreSQL 型別系統包含許多專用項目，這些項目統稱為概念型別(pseudo-type)。概念型別不能用作實際的欄位資料型別，但可以用來宣告函數的參數或結果型別。各個概念型別在於函數的行為不只是對應於簡單地獲取或回傳特定 SQL 資料型別的值的情況下很有用。Table 8.27 列出了現有的概念型別。

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).
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.
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.
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 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 types anyelement, anyarray, anynonarray, anyenum, and anyrange.

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.