Message-ID: <4677B831.8060704@metacarta.com>
Date: Tue, 19 Jun 2007 07:04:17 -0400
From: Karl Wright <kwright@metacarta.com>
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To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-performance@postgresql.org
Subject: Re: Performance query about large tables, lots of concurrent
 access
References: <4676DD57.7070304@metacarta.com> <14291.1182221811@sss.pgh.pa.us>
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Tom Lane wrote:
> Karl Wright <kwright@metacarta.com> writes:
>> - At any given time, there are up to 100 of these operations going on at 
>> once against the same database.
> 
> It sounds like your hardware is far past "maxed out".  Which is odd
> since tables with a million or so rows are pretty small for modern
> hardware.  What's the CPU and disk hardware here, exactly?  What do you
> see when watching vmstat or iostat (as appropriate for OS, which you
> didn't mention either)?
> 
> 			regards, tom lane
> 

Yes, I was surprised as well, which is why I decided to post.

The hardware is a Dell 2950, two processor, dual-core each processor, 16 
GB memory, with a RAID disk controller.  The operating system is Debian 
Linux (sarge plus mods, currently using the Postgresql 8.1 backport).

Also, as I said before, I have done extensive query analysis and found 
that the plans for the queries that are taking a long time are in fact 
very reasonable.  Here's an example from the application log of a query 
that took way more time than its plan would seem to indicate it should:

 >>>>>>
[2007-06-18 09:39:49,783]ERROR Found a query that took more than a 
minute: [UPDATE intrinsiclink SET isnew=? WHERE ((jobid=? AND 
childidhash=? AND childid=?)) AND (isnew=? OR isnew=?)]
[2007-06-18 09:39:49,783]ERROR   Parameter 0: 'B'
[2007-06-18 09:39:49,783]ERROR   Parameter 1: '1181766706097'
[2007-06-18 09:39:49,783]ERROR   Parameter 2: 
'7E130F3B688687757187F1638D8776ECEF3009E0'
[2007-06-18 09:39:49,783]ERROR   Parameter 3: 
'http://norwich.openguides.org/?action=index;index_type=category;index_value=Cafe;format=atom'
[2007-06-18 09:39:49,783]ERROR   Parameter 4: 'E'
[2007-06-18 09:39:49,783]ERROR   Parameter 5: 'N'
[2007-06-18 09:39:49,797]ERROR  Plan: Index Scan using i1181764142395 on 
intrinsiclink  (cost=0.00..14177.29 rows=5 width=253)
[2007-06-18 09:39:49,797]ERROR  Plan:   Index Cond: ((jobid = $2) AND 
((childidhash)::text = ($3)::text))
[2007-06-18 09:39:49,797]ERROR  Plan:   Filter: ((childid = ($4)::text) 
AND ((isnew = ($5)::bpchar) OR (isnew = ($6)::bpchar)))
[2007-06-18 09:39:49,797]ERROR
<<<<<<
(The intrinsiclink table above is the "child table" I was referring to 
earlier, with 13,000,000 rows at the moment.)

Overnight I shut things down and ran a VACUUM operation to see if that 
might help.  I'll post again when I find out if indeed that changed any 
performance numbers.  If not, I'll be able to post vmstat output at that 
time.

Karl