PostgreSQL创建自定义函数
PostgreSQL 支持创建自定义函数，相比于 SQL 对表进行简单增删改查操作，自定义函数能够实现各种复杂的逻辑。PostgreSQL 自定义函数支持的语法主要包括 SQL，plsql，C语言函数等。
1. 创建函数的语法
CREATE [ OR REPLACE ] FUNCTION name ( [ [ argmode ] [ argname ] argtype [ { DEFAULT | = } default_expr ] [, ...] ] ) [ RETURNS rettype | RETURNS TABLE ( column_name column_type [, ...] ) ] { LANGUAGE lang_name | TRANSFORM { FOR TYPE type_name } [, ... ] | WINDOW | { IMMUTABLE | STABLE | VOLATILE } | [ NOT ] LEAKPROOF | { CALLED ON NULL INPUT | RETURNS NULL ON NULL INPUT | STRICT } | { [ EXTERNAL ] SECURITY INVOKER | [ EXTERNAL ] SECURITY DEFINER } | PARALLEL { UNSAFE | RESTRICTED | SAFE } | COST execution_cost | ROWS result_rows | SUPPORT support_function | SET configuration_parameter { TO value | = value | FROM CURRENT } | AS 'definition' | AS 'obj_file', 'link_symbol' } ...
