Table functions are functions that produce a set of rows, made up of either base (scalar) data types, or composite (multi-column) data types. They are used like a table, view, or subselect in the FROM clause of a query. Columns returned by table functions may be included in SELECT, JOIN, or WHERE clauses in the same manner as a table, view, or subselect column.
If a table function returns a base data type, the single result column is named for the function. If the function returns a composite type, the result columns get the same names as the individual attributes of the type.
A table function may be aliased in the FROM clause, but it also may be left unaliased. If a function is used in the FROM clause with no alias, the function name is used as the relation name.
Table functions work wherever tables do in SELECT statements. For example
CREATE TABLE foo (fooid int, foosubid int, fooname text); CREATE FUNCTION getfoo(int) RETURNS setof foo AS ' SELECT * FROM foo WHERE fooid = $1; ' LANGUAGE SQL; SELECT * FROM getfoo(1) AS t1; SELECT * FROM foo WHERE foosubid in (select foosubid from getfoo(foo.fooid) z where z.fooid = foo.fooid); CREATE VIEW vw_getfoo AS SELECT * FROM getfoo(1); SELECT * FROM vw_getfoo;
are all valid statements.
In some cases it is useful to define table functions that can return different column sets depending on how they are invoked. To support this, the table function can be declared as returning the pseudo-type record. When such a function is used in a query, the expected row structure must be specified in the query itself, so that the system can know how to parse and plan the query. Consider this example:
SELECT * FROM dblink('dbname=template1', 'select proname, prosrc from pg_proc') AS t1(proname name, prosrc text) WHERE proname LIKE 'bytea%';
The dblink function executes a remote query (see contrib/dblink). It is declared to return record since it might be used for any kind of query. The actual column set must be specified in the calling query so that the parser knows, for example, what * should expand to.