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Date: Fri, 12 Oct 2007 14:03:50 -0500
From: "Kevin Grittner" <Kevin.Grittner@wicourts.gov>
To: <theo@flame.co.za>,<pgsql-performance@postgresql.org>
Subject: Re: Performance problems with prepared statements
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>>> On Fri, Oct 12, 2007 at  9:57 AM, in message
<1192201021.6170.47.camel@localhost.localdomain>, Theo Kramer
<theo@flame.co.za> wrote:=20
>=20
> select * from foo where
>   (a =3D a1 and b =3D b1 and c >=3D c1) or
>   (a =3D a1 and b < b1) or
>   (a > a1)
> order by a, b desc, c;
>=20
> I have, however, found that transforming the above into a union based
> query performs substantially better.
=20
Another approach which often performs better is to rearrange the logic
so that the high-order predicate is AND instead of OR:
=20
select * from foo where
      ( a >=3D a1
  and ( a >  a1
   or ( b <=3D b1
  and ( b <  b1
   or ( c >=3D c1 )))))
  order by a, b desc, c;
=20
With the right index and a limit on rows, this can do particularly well.
=20
-Kevin
=20