On Tue, 2008-09-02 at 13:41 +0300, Heikki Linnakangas wrote: > Simon Riggs wrote: > > On Tue, 2008-09-02 at 13:20 +0300, Heikki Linnakangas wrote: > >> Simon Riggs wrote: > >>> It turns out that a join like this > >>> > >>> select a.col2 > >>> from a left outer join b on a.col1 = b.col1 > >>> where b.col2 = 1; > >>> > >>> can be cheaper if we don't remove the join, when there is an index on > >>> a.col1 and b.col2, because the presence of b allows the values returned > >>> from b to be used for an index scan on a. > >> Umm, you *can't* remove that join. > > > > Yes, you can. The presence or absence of rows in b is not important to > > the result of the query because of the "left outer join". > > > > I spent nearly a whole day going down that deadend also. > > Oh. How does the query look like after removing the join, then?
Same answer, just slower. Removing the join makes the access to a into a SeqScan, whereas it was a two-table index plan when both tables present. The two table plan is added by the immediately preceding call add_... - i.e. that plan is only added during join time not during planning of base relations. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support -- Sent via pgsql-patches mailing list (pgsql-patches@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-patches
