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From: Mark Lewis <mark.lewis@mir3.com>
To: Craig James <craig_james@emolecules.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: count * performance issue
Date: Thu, 6 Mar 2008 08:16:31 -0800
Message-ID: <1204820047.9048.902.camel@archimedes> (raw)
In-Reply-To: <47D00DB2.7040007@emolecules.com>
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	<47D00DB2.7040007@emolecules.com>

On Thu, 2008-03-06 at 07:28 -0800, Craig James wrote:
...
> My question is: What do the other databases do that Postgres can't do, and why not?
> 
> Count() on Oracle and MySQL is almost instantaneous, even for very large tables. So why can't Postgres do what they do?
...

I can vouch that Oracle can still take linear time to perform a
count(*), at least in some cases.

I have also seen count(*) fast in some cases too... my understanding is
that they maintain a list of "interested transactions" on a per-relation
basis.  Perhaps they do an optimization based on the index size if there
are no pending DML transactions?

-- Mark





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