From: | Mark Kirkwood <markir(at)paradise(dot)net(dot)nz> |
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To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Greg Stark <gsstark(at)mit(dot)edu>, Luke Lonergan <llonergan(at)greenplum(dot)com>, stange(at)rentec(dot)com, Dave Cramer <pg(at)fastcrypt(dot)com>, Joshua Marsh <icub3d(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org |
Subject: | Re: Hardware/OS recommendations for large databases ( |
Date: | 2005-11-24 22:07:50 |
Message-ID: | 438639B6.5090902@paradise.net.nz |
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Lists: | pgsql-performance |
Tom Lane wrote:
> Greg Stark <gsstark(at)mit(dot)edu> writes:
>
>>Last I heard the reason count(*) was so expensive was because its state
>>variable was a bigint. That means it doesn't fit in a Datum and has to be
>>alloced and stored as a pointer. And because of the Aggregate API that means
>>it has to be allocated and freed for every tuple processed.
>
>
> There's a hack in 8.1 to avoid the palloc overhead (courtesy of Neil
> Conway IIRC).
>
It certainly makes quite a difference as I measure it:
doing select(1) from a 181000 page table (completely uncached) on my PIII:
8.0 : 32 s
8.1 : 25 s
Note that the 'fastcount()' function takes 21 s in both cases - so all
the improvement seems to be from the count overhead reduction.
Cheers
Mark
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