SQL在字符串中查找字串出现第N次的位置

CREATE OR REPLACE FUNCTION find_string_N_time(sourceStr VARCHAR, desStr VARCHAR, appearTime INT)
RETURNS varchar as
$BODY$
DECLARE
v_temp_source_str VARCHAR(1024);
v_position INT := 0;
v_pre_position INT := 0;
BEGIN
v_temp_source_str := sourceStr;
FOR i in 1..appearTime
LOOP
SELECT POSITION(desStr IN v_temp_source_str) INTO v_pre_position;
SELECT SUBSTR(v_temp_source_str, v_position + 1) INTO v_temp_source_str;
v_position := v_position + v_pre_position;

END LOOP;
RETURN v_position;

END;
$BODY$
LANGUAGE plpgsql VOLATILE