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Re: gprof SELECT COUNT(*) results

From: Simon Riggs <simon(at)2ndquadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Christopher Kings-Lynne <chriskl(at)familyhealth(dot)com(dot)au>,Qingqing Zhou <zhouqq(at)cs(dot)toronto(dot)edu>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: gprof SELECT COUNT(*) results
Date: 2005-11-26 17:57:01
Message-ID: 1133027821.2906.156.camel@localhost.localdomain (view raw or flat)
Thread:
Lists: pgsql-hackers
On Sat, 2005-11-26 at 11:53 -0500, Tom Lane wrote:
> Christopher Kings-Lynne <chriskl(at)familyhealth(dot)com(dot)au> writes:
> >> ...and for emphasis: this optimization of SeqScans is not possible with
> >> any other database system, so its a big win for PostgreSQL.
> 
> > With any other db system?  That's a big call.  Why?
> 
> One could equally well spin it negatively, as "this optimization is not
> needed with any other database" ...

Why is it spin to call it a big win? You amaze me. After all the time
we've spent investigating tuning the buffer manager, to reduce the
locking required by a SeqScan to about 1% of what it was before is just
way cool. When did you last spend a few hours tuning an internals
feature (not the optimizer) and have something go 50% faster? Probably
at least a month :-)

Thinking more about other systems, ISTM that Oracle can do this, as can
any MVCC based system. OTOH DB2 and SQLServer take block level read
locks, so they can do this too, but at major loss of concurrency and
threat of deadlock. Having said that, *any* system that chose not to do
this would be severely sub-optimal with buffer manager contention. So
AFAICS they all need this optimization.

So, I think I'll take back the claim of uniqueness, but I'm happy with
the accuracy of the statement that this is a big win for PostgreSQL. And
a bigger win than most on the poor benighted Xeon, where reducing memory
contention/traffic seems to be crucial.

Best Regards, Simon Riggs


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