db: fix outer join read lock for PostgreSQL

[mlassnig]

Motivation

PostgreSQL is interpreting the SQL standard correctly, it's actually Oracle and MySQL which are doing non-standard things here. The problem you see is that for an outer join the nullable side cannot be locked for reading, so PostgreSQL stops. This is the last outstanding issue for PostgreSQL compatibility.

Modification

Changing the query for add-rule from "select outer join for update" into something like "select for update union select".

Expected behavior

add-rule will work on PostgreSQL.

[mlassnig]

I suggest raising the priority on this.