Re: Performance query about large tables, lots of concurrent access

Tom Lane wrote:
> Karl Wright <kwright(at)metacarta(dot)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