On Tue, 2008-09-02 at 14:03 +0300, Heikki Linnakangas wrote: > Simon Riggs wrote: > > 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. > > I mean, can you how me an SQL query of what's left after removing the > join? Certainly just removing the join and the WHERE clause doesn't give > the same answer.
Yes, it does select a.col2 from a left outer join b on a.col1 = b.col1 where b.col2 = 1; is logically equivalent to select a.col2 from a; and hence removing the join produces a SeqScan plan, whereas the equivalent join can in some circumstances be faster. I discovered this, I didn't think of it in advance. > Or is it something that can't be expressed with SQL? > What's the filter in the SeqScan? There is no filter in the SeqScan. Try some queries and you'll see what I mean. I've said its a dead end and that I spent hours thinking that, so please think about this... -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support -- Sent via pgsql-patches mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-patches