问GreenPlum -- greenplum中的“concat”函数

EN

select concat('a', 'b');
ERROR:  function concat(unknown, unknown) does not exist at character 8
HINT:  No function matches the given name and argument types. You may need to add explicit type casts.
LINE 1: select concat('a', 'b');
               ^

CREATE OR REPLACE FUNCTION my_concat(VARIADIC arr VARCHAR[] ) RETURNS VARCHAR AS  $$  SELECT array_to_string(arr, '');  $$  LANGUAGE SQL;
ERROR:  syntax error at or near "VARCHAR" at character 51
LINE 1: CREATE OR REPLACE FUNCTION my_concat(VARIADIC arr VARCHAR[] ...
                                                          ^