-Percent and underscore are only special in contexts where they can be wild
-cards, and this isn't usually the case for data inserted from messages, since
-that isn't likely to be treated as a pattern of any kind. However, pgsql seems
-to allow escaping "on spec". If you use something like "where id="ab\%cd" it
-does treat the string as "ab%cd". So we can safely quote percent and
-underscore. [This is different to MySQL, where you can't do this.]