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 18:11:58 -0400
Message-ID: <14252.1050099118@sss.pgh.pa.us> (raw)
In-Reply-To: <87vfxkrap2.fsf@matt.w80.math-hat.com>
References: <874r54sr8q.fsf@matt.w80.math-hat.com>
<13918.1050095452@sss.pgh.pa.us>
<87vfxkrap2.fsf@matt.w80.math-hat.com>
Itai Zukerman <zukerman@math-hat.com> writes:
>>> 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);
> Oops. I guess I should've changed the example to "WHERE A.x>=B.x"
> since that's what I'm really doing. Oh, well, my goof.
Anything else you didn't bother to mention? Because in that case I
think your query reduces to
SELECT sum(v) FROM A WHERE A.x >= (SELECT min(x) FROM B);
which ought to run reasonably well (I'm sure it'll still want to
do a seqscan --- but not a nested loop).
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: <14252.1050099118@sss.pgh.pa.us>
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