CREATE OR REPLACE FUNCTION message_store.id(

stream_name varchar

) RETURNS varchar AS $$ DECLARE

_id_separator_position integer;

BEGIN

_id_separator_position := STRPOS(id.stream_name, '-');

IF _id_separator_position = 0 THEN
  RETURN NULL;
END IF;

RETURN SUBSTRING(id.stream_name, _id_separator_position + 1);

END; $$ LANGUAGE plpgsql IMMUTABLE;