From: <terry@ashtonwoodshomes.com>
To: <josh@agliodbs.com>, "'Scott Cain'" <cain@cshl.org>,
	<pgsql-sql@postgresql.org>
Subject: Re: OR vs UNION
Date: Thu, 17 Jul 2003 15:58:35 -0400
Message-ID: <004f01c34c9d$ce9edcc0$2766f30a@development.greatgulfhomes.com>
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In-Reply-To: <200307171200.18626.josh@agliodbs.com>

Actually, I have used a UNION to replace OR's, the case (simpliefied to)
something like this:

Sample 1:
WHERE (f1 = 'v1' OR f1 = '')
  AND (f2 = 'v2' OR f2 = '')

Changed to Sample 2:
WHERE (f1 = 'v1')
  AND (f2 = 'v2')
UNION
WHERE (f1 = 'v1')
  AND (f2 = '')
UNION
WHERE (f1 = '')
  AND (f2 = '')


Note that Sample 1 is actually a simplified version, the queries are not
exactly equivalent.

The point is that sample 2 ran MUCH faster because:
a)  The table was *very* large
b)  The OR clauses of sample 1 prevented the use of an INDEX,

Reason:  It is faster to scan an index 3 times then scan this very large
table once.

I do not know if there is a proof to say that one can *always* replace OR's
with a union, but sometimes certainly, and in this case it made things much
better...

Terry Fielder
Manager Software Development and Deployment
Great Gulf Homes / Ashton Woods Homes
terry@greatgulfhomes.com
Fax: (416) 441-9085


> -----Original Message-----
> From: pgsql-sql-owner@postgresql.org
> [mailto:pgsql-sql-owner@postgresql.org]On Behalf Of Josh Berkus
> Sent: Thursday, July 17, 2003 3:00 PM
> To: Scott Cain; pgsql-sql@postgresql.org
> Subject: Re: [SQL] OR vs UNION
>
>
> Scott,
>
> > I have a query that uses a series of ORs and I have heard
> that sometimes
> > this type of query can be rewritten to use UNION instead and be more
> > efficient.
>
> I'd be interested to know where you heard that; as far as I
> know, it could
> only apply to conditional left outer joins.
>
> >      select distinct
> f.name,fl.fmin,fl.fmax,fl.strand,f.type_id,f.feature_id
> >      from feature f, featureloc fl
> >      where
> >        (f.type_id = 219 OR
> >         f.type_id = 368 OR
> >         f.type_id = 514 OR
> >         f.type_id = 475 OR
> >         f.type_id = 426 OR
> >         f.type_id = 456 OR
> >         f.type_id = 461 OR
> >         f.type_id = 553 OR
> >         f.type_id = 89) and
> >       fl.srcfeature_id = 1 and
> >       f.feature_id  = fl.feature_id and
> >       fl.fmin <= 2491413 and fl.fmax >= 2485521
>
> Certainly a query of the above form would not benefit from
> being a union.
>
> For readability, you could use an IN() statement rather than
> a bunch of ORs
> ... this would not help performance, but would make your
> query easier to
> type/read.
>
> --
> -Josh Berkus
>  Aglio Database Solutions
>  San Francisco
>
>
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