实现代码如下:
CREATE OR REPLACE FUNCTION substring_index(varchar, varchar, integer) RETURNS varchar AS $$ DECLARE tokens varchar[]; length integer ; indexnum integer; BEGIN tokens := pg_catalog.string_to_array($1, $2); length := pg_catalog.array_upper(tokens, 1); indexnum := length - ($3 * -1) + 1; IF $3 >= 0 THEN RETURN pg_catalog.array_to_string(tokens[1:$3], $2); ELSE RETURN pg_catalog.array_to_string(tokens[indexnum:length], $2); END IF; END; $$ IMMUTABLE STRICT LANGUAGE PLPGSQL;