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Date: Wed, 9 Oct 2002 10:26:31 -0700 (PDT)
From: Stephan Szabo <sszabo@megazone23.bigpanda.com>
To: John Pauley <karstdiver@yahoo.com>
Cc: <pgsql-sql@postgresql.org>
Subject: Re: SELECT statement never completes.
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On Wed, 9 Oct 2002, John Pauley wrote:

> pgsql-sql,
>
> We are porting a database from IBM DB2 to PostgreSQL.
> In several related scripts, there is a SELECT
> statement that never completes in Postgres but
> completes in a few seconds using DB2, for example:
>
> Table row count:
> SELECT count(*) FROM tableX;
>  112671
> SELECT count(*) from tableY;
>  314625
>
> This statement does not complete:
> SELECT id FROM tableX WHERE id NOT IN (SELECT id FROM
> tableY);
>
> Any suggestions?

Unfortunately IN <subselect> tends to have poor performance
in postgresql.  Often you can get better performance out
of exists, but not always.
You might want to try:
select id from tableX WHERE NOT EXISTS (select * from
 tableY where tableY.id=tableX.id);
and see if it runs better.