From: Itai Zukerman <zukerman@math-hat.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-sql@postgresql.org
Subject: Re: count(*), EXISTS, indexes
Date: Fri, 11 Apr 2003 17:22:33 -0400
Message-ID: <87vfxkrap2.fsf@matt.w80.math-hat.com> (raw)
In-Reply-To: <13918.1050095452@sss.pgh.pa.us>
References: <874r54sr8q.fsf@matt.w80.math-hat.com>
<13918.1050095452@sss.pgh.pa.us>
>> 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);
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.
> 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.
I don't think indexing B will help speed-wise if it still does the seq
scan through A.
>> 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?
Oh, I just meant, "the DISTINCT A.x that are returned by:
SELECT DISTINCT A.x FROM A,B WHERE A.x=B.x
". Sorry for my lack of clarity...
--
Itai Zukerman <http://www.math-hat.com/~zukerman/;
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-sql@postgresql.org
Cc: zukerman@math-hat.com, tgl@sss.pgh.pa.us
Subject: Re: count(*), EXISTS, indexes
In-Reply-To: <87vfxkrap2.fsf@matt.w80.math-hat.com>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox