Date: Fri, 28 Feb 2003 12:34:45 -0800 (PST)
From: Stephan Szabo <sszabo@megazone23.bigpanda.com>
To: Dan Langille <dan@langille.org>
Cc: <pgsql-sql@postgresql.org>
Subject: Re: DELETE FROM  t WHERE EXISTS
In-Reply-To: <3E5F530B.1649.424D34C1@localhost>
Message-ID: <20030228123036.L2988-100000@megazone23.bigpanda.com>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII


On Fri, 28 Feb 2003, Dan Langille wrote:

> Hi folks,
>
> I wanted to delete "old" rows from a table.  These are the rows I
> want to keep:
>
> SELECT *
> FROM clp
> ORDER BY commit_date
> LIMIT 100
>
> So I tried this:
>
> DELETE FROM clp
> WHERE NOT EXISTS (
> SELECT *
> FROM clp
> ORDER BY commit_date
> LIMIT 100);
>
> Uhh uhh, nothing deleted.  I don't understand why.

As long as the inner select returns at least 1 result NOT EXISTS is
going to return false (you haven't correlated the two queries at all).

> OK, I can do this instead:
>
> DELETE from clp
> where commit_log_id NOT in (
> SELECT commit_log_id
> FROM clp
> ORDER BY commit_date
> LIMIT 100);
>
> Can you think of a better way?

Possibly something like:
DELETE FROM clp
 WHERE NOT EXISTS (
  select * from (select * from clp order by commit_date limit 100) tmp
  where tmp.commit_log_id = clp.commit_log_id
 );

But I haven't tried it for stupid errors, and am not sure that it'd end up
being any better than NOT IN anyway.