To: Ken <postgres@kwasnicki.com>
cc: pgsql-sql@postgresql.org
Subject: Re: update query taking 24+ hours 
In-reply-to: <45A9CEA3.3020903@kwasnicki.com> 
References: <45A9CEA3.3020903@kwasnicki.com>
Comments: In-reply-to Ken <postgres@kwasnicki.com>
	message dated "Sat, 13 Jan 2007 22:33:07 -0800"
Date: Sun, 14 Jan 2007 14:30:32 -0500
Message-ID: <12574.1168803032@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Ken <postgres@kwasnicki.com> writes:
> I have postgres 8.1 on a linux box: 2.6Ghz P4, 1.5GB ram, 320GB hard 
> drive.  I'm performing an update between two large tables and so far 
> it's been running for 24+ hours.
> UPDATE Master SET val2=Import.val WHERE Master.x=Import.x AND 
> Master.y=Import.y;

What does EXPLAIN say about that?  (Don't try EXPLAIN ANALYZE,
but a plain EXPLAIN should be quick enough.)

> Both tables have indexes on the x and y columns.  Will that help?

A two-column index would have helped a lot more, probably, although
with so many rows to process I'm not sure that indexes are useful
anyway.  For really bulk updates a hashjoin or sort-and-mergejoin
plan is probably the best bet.

BTW, both of those would require plenty of work_mem to run fast
... what have you got work_mem set to?

And possibly even more to the point, do you have any foreign key
constraints leading into or out of the Master table?

			regards, tom lane