On Mon, Feb 28, 2011 at 2:58 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Craig James <craig_ja...@emolecules.com> writes: >> Then I thought maybe putting a foreign-key constraint on table "my_version" >> would solve the problem: > >> alter table my_version add constraint fk_my_view foreign key(version_id) >> references registry.version(version_id) on delete cascade; > >> That way, the planner would know that every key in table "my_version" has to >> also be in table "version", thus avoiding that part about "forcing the other >> join to be done in toto". But the foreign-key constraint makes no >> difference, it still does the full join and takes 65 seconds. > > That's just wishful thinking I'm afraid. The planner doesn't currently > make any deductions whatsoever from the presence of a foreign key > constraint; and even if it did, I'm not sure that this would help it > decide that a join order constraint could safely be dropped.
I've previously mused on -hackers about teaching the planner the concept of an inner-or-left-join; that is, a join that's guaranteed to return the same results whichever way we choose to implement it. Proving that an inner join is actually inner-or-left would allow the join removal logic to consider removing it altogether, and would allow reordering in cases that aren't otherwise known to be safe. Proving that a left join is actually inner-or-left doesn't help with join removal, but it might allow the join to be reordered. Maybe "non-row-reducing-join" is better terminology than "inner-or-left-join", but in any case I have a suspicion that inner join removal will end up being implemented as a special case of noticing that an inner join falls into this class. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
