On Sat, Mar 3, 2012 at 10:03 PM, Tom Lane <tgl@xxxxxxxxxxxxx> wrote: > Claus Stadler <cstadler@xxxxxxxxxxxxxxxxxxxxxxxxx> writes: >> Query optimizer glitch: "...WHERE TRUE" condition in union results in >> bad query plan ... > > Yeah, this is because a nonempty WHERE clause defeats simplifying the > UNION ALL into a simple "append relation" (cf is_safe_append_member()). > The planner will eventually figure out that WHERE TRUE is a no-op, > but that doesn't happen till later (and there are good reasons to do > things in that order). > > Sooner or later I'd like to relax the restriction that appendrel members > can't have extra WHERE clauses, but don't hold your breath waiting... Does this comment need updating? * Note: the data structure assumes that append-rel members are single * baserels. This is OK for inheritance, but it prevents us from pulling * up a UNION ALL member subquery if it contains a join. While that could * be fixed with a more complex data structure, at present there's not much * point because no improvement in the plan could result. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
