Date: Thu, 26 May 2005 16:22:03 +0100
From: Sam Mason <sam@samason.me.uk>
To: pgsql-performance@postgresql.org
Subject: Optimising queries involving unions
Message-ID: <20050526152203.GI15205@colo.samason.me.uk>
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Hi,

I've got a query that I think the query optimiser should be able
to work it's magic on but it doesn't!  I've had a look around and
asked on the IRC channel and found that the current code doesn't
attempt to optimise for what I'm asking it to do at the moment.
Here's a bad example:

  SELECT u.txt
  FROM smalltable t, (
    SELECT id, txt FROM largetable1
    UNION ALL
    SELECT id, txt FROM largetable2) u
  WHERE t.id = u.id
    AND t.foo = 'bar';

I was hoping that "smalltable" would get moved up into the union,
but it doesn't at the moment and the database does a LOT of extra
work.  In this case, I can manually do quite a couple of transforms
to move things around and it does the right thing:

  SELECT txt
  FROM (
    SELECT l.id as lid, r.id as rid, r.foo, l.txt
      FROM largetable1 l, smalltable r
    UNION ALL
    SELECT l.id as lid, r.id as rid, r.foo, l.txt
      FROM largetable1 l, smalltable r)
  WHERE foo = 'bar';
    AND lid = rid

The optimiser is intelligent enough to move the where clauses up
into the union and end end up with a reasonably optimal query.
Unfortunatly, in real life, the query is much larger and reorganising
everything manually isn't really feasible!

Is this a good place to ask about this or is it more in the realm
of the hackers mailing list?

Thanks,
  Sam