On Mon, Feb 28, 2011 at 2:58 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Craig James <craig_james@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
avoidingthat 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