When issuing a SELECT DISTINCT ... ORDER BY, PostgreSQL requires that the column being ordered also appears in the SELECT statement. The phpBB Postgres fulltext search backend issues a SELECT DISTINCT when displaying results as topic, but does not add the sorted column to the SELECT, resulting in the error:

ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list LINE 5: ORDER BY t.topic_last_post_time DESC ^ []

To reproduce:

1. Have a forum using PostgreSQL as its database, and the search backend as PostgreSQL Fulltext.
2. Perform any kind of search with 'Display results as:' set to Topics.