Message-ID: <4DE5D8B3.1070403@2ndQuadrant.com>
Date: Wed, 01 Jun 2011 02:14:11 -0400
From: Greg Smith <greg@2ndQuadrant.com>
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To: pgsql-performance@postgresql.org
Subject: Re: Delete performance
References: <6940CBA1-BB1C-4030-B425-2DE9135CEAE3@gmail.com>
In-Reply-To: <6940CBA1-BB1C-4030-B425-2DE9135CEAE3@gmail.com>
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On 05/30/2011 08:08 PM, Jarrod Chesney wrote:
> 	My database uses joined table inheritance and my server version is 9.0
> I have about 120,000 records in the table that everything else inherits from, if i truncate-cascaded this table it happens almost instantly. If i run 30,000 prepared "DELETE FROM xxx WHERE "ID" = ?" commands it takes close to 10 minutes.
>
> My foreign keys to the base table are all set with "ON DELETE CASCADE".

You may also want to make them DEFERRABLE and then use "SET CONSTRAINTS 
ALL DEFERRABLE" so that the constraint checking all happens at one 
time.  This will cause more memory to be used, but all the constraint 
related work will happen in a batch.

You mentioned inheritance.  That can cause some unexpected problems 
sometimes.  You might want to do:

EXPLAIN DELETE FROM ...

To see how this is executing.  EXPLAIN works fine on DELETE statements, 
too, and it may highlight something strange about how the deletion is 
happening.  If you can, use EXPLAIN ANALYZE, but note that this will 
actually execute the statement--the deletion will happen, it's not just 
a test.

There may be a problem with the query plan for the deletion that's 
actually causing the issue here, such as missing the right indexes.  If 
you have trouble reading it, http://explain.depesz.com/ is a good web 
resources to help break down where the time is going.

-- 
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support  www.2ndQuadrant.us
"PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books