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Re: work_mem and shared_buffers

From: "Campbell, Lance" <lance(at)uiuc(dot)edu>
To: "Heikki Linnakangas" <heikki(at)enterprisedb(dot)com>
Cc: <pgsql-performance(at)postgresql(dot)org>
Subject: Re: work_mem and shared_buffers
Date: 2007-11-09 18:08:57
Message-ID: B10E6810AC2A2F4EA7550D072CDE8760197DE5@SAB-FENWICK.sab.uiuc.edu (view raw or flat)
Thread:
Lists: pgsql-performance
How do you know when you should up the value of work_mem?  Just play
with the number.  Is there a query I could do that would tell me if
PostgreSql is performing SQL that could use more memory for sorting?

Thanks,

Lance Campbell
Project Manager/Software Architect
Web Services at Public Affairs
University of Illinois
217.333.0382
http://webservices.uiuc.edu
 

-----Original Message-----
From: Heikki Linnakangas [mailto:hlinnaka(at)gmail(dot)com] On Behalf Of Heikki
Linnakangas
Sent: Friday, November 09, 2007 11:57 AM
To: Campbell, Lance
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: [PERFORM] work_mem and shared_buffers

Campbell, Lance wrote:
> Does the amount of memory allocate to work_mem get subtracted from
> shared_buffers?
> 
> Example:
> 
> If work_mem is 1M and there are 10 connections and shared_buffers is
> 100M then would the total be 90 M left for shared_buffers?
> 
> Or does the amount of memory allocated for work_mem have nothing to do
> with shared_buffers?

No, they're completely separate.

Note that a connection can use more than work_mem of memory. For 
example, if you run a query with multiple Sort or hash-nodes, each such 
node allocates up to work_mem of memory.

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com

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