From: Josh Berkus <josh@agliodbs.com>
To: pgsql-performance@postgresql.org
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Craig James <craig_james@emolecules.com>
Subject: Re: count * performance issue
Date: Fri, 7 Mar 2008 17:27:35 -0800
Message-ID: <200803071727.36146.josh@agliodbs.com> (raw)
In-Reply-To: <18451.1204824562@sss.pgh.pa.us>
References: <f966c2ee0803052142l6a105678jfe2da92a0d9371@mail.gmail.com>
<47D00DB2.7040007@emolecules.com>
<18451.1204824562@sss.pgh.pa.us>
Tom,
> > Count() on Oracle and MySQL is almost instantaneous, even for very
> > large tables. So why can't Postgres do what they do?
>
> AFAIK the above claim is false for Oracle. They have the same
> transactional issues we do.
Nope. Oracle's MVCC is implemented through rollback segments, rather than
non-overwriting the way ours is. So Oracle can just do a count(*) on the
index, then check the rollback segment for any concurrent
update/delete/insert activity and adjust the count. This sucks if there's
a *lot* of concurrent activity, but in the usual case it's pretty fast.
I've been thinking that when we apply the Dead Space Map we might be able
to get a similar effect in PostgreSQL. That is, just do a count over the
index, and visit only the heap pages flagged in the DSM. Again, for a
heavily updated table this wouldn't have any benefit, but for most cases
it would be much faster.
--
--Josh
Josh Berkus
PostgreSQL @ Sun
San Francisco
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To: pgsql-performance@postgresql.org
Cc: josh@agliodbs.com, tgl@sss.pgh.pa.us, craig_james@emolecules.com
Subject: Re: count * performance issue
In-Reply-To: <200803071727.36146.josh@agliodbs.com>
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