http://www.postgresql.org/docs/current/static/extend-type-system.html#EXTEND-TYPES-POLYMORPHIC
先看一个例子：
CREATE OR REPLACE FUNCTION myappend01(anyelement, anyelement) RETURNS anyelement AS $$ SELECT $1 || $2; $$ LANGUAGE SQL;
执行(按 AnynonArray来传递输入参数)：
pgsql=# select myappend01('abc', 'def'); ERROR: could not determine polymorphic type because input has type "unknown" pgsql=#
pgsql=# select myappend01('abc'::text, 'def'::text); myappend01 ------------ abcdef (1 row) pgsql=#
执行(按 AnyArray来传递输入参数)：
pgsql=# select myappend01(ARRAY['mm','nn'], ARRAY['xx','yy']); myappend01 --------------- {mm,nn,xx,yy} (1 row)
可以认为 AnyElement = AnyArray || AnynonArray
再看一个例子：
pgsql=# select myappend02('abc', 'def'); ERROR: could not determine polymorphic type because input has type "unknown" pgsql=# pgsql=# pgsql=# select myappend02('abc'::text, 'def'::text); myappend02 ------------ abcdef (1 row) pgsql=#
pgsql=# select myappend02(ARRAY['aa','bb'], ARRAY['cc','dd']); ERROR: function myappend02(text[], text[]) does not exist LINE 1: select myappend02(ARRAY['aa','bb'], ARRAY['cc','dd']); ^ HINT: No function matches the given name and argument types. You might need to add explicit type casts. pgsql=#
