From: Tom Lane <tgl@sss.pgh.pa.us>
To: Itai Zukerman <zukerman@math-hat.com>
Cc: pgsql-sql@postgresql.org
Subject: Re: count(*), EXISTS, indexes
Date: Fri, 11 Apr 2003 17:10:52 -0400
Message-ID: <13918.1050095452@sss.pgh.pa.us> (raw)
In-Reply-To: <874r54sr8q.fsf@matt.w80.math-hat.com>
References: <874r54sr8q.fsf@matt.w80.math-hat.com>
Itai Zukerman <zukerman@math-hat.com> writes:
> Define:
> CREATE TABLE A (x int PRIMARY KEY, real v);
> CREATE TABLE B (x int);
> I'd like to calculate:
> SELECT sum(v) FROM A WHERE EXISTS (SELECT 1 FROM B WHERE A.x=B.x);
> ...but then it won't use the primary key index on A.x.
In CVS tip (7.4-to-be) I think
SELECT sum(v) FROM A WHERE A.x IN (SELECT B.x FROM B);
would probably work well. In current releases I think all you can do is
add an index to B.x and settle for the EXISTS() approach.
> PS. B is relatively small, a few thousand rows, while A has well over
> 500,000 rows. The DISTINCT A.x should be about 10,000-50,000.
BTW, how can a PRIMARY KEY column have fewer DISTINCT values than there
are rows?
regards, tom lane
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To: pgsql-sql@postgresql.org
Cc: tgl@sss.pgh.pa.us, zukerman@math-hat.com
Subject: Re: count(*), EXISTS, indexes
In-Reply-To: <13918.1050095452@sss.pgh.pa.us>
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