To: Itai Zukerman <zukerman@math-hat.com>
Cc: pgsql-sql@postgresql.org
Subject: Re: count(*), EXISTS, indexes 
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>
Comments: In-reply-to Itai Zukerman <zukerman@math-hat.com>
	message dated "Fri, 11 Apr 2003 17:22:33 -0400"
Date: Fri, 11 Apr 2003 18:11:58 -0400
Message-ID: <14252.1050099118@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

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