From: Tom Lane <tgl@sss.pgh.pa.us>
To: josh@agliodbs.com
Cc: pgsql-performance@postgresql.org, Craig James <craig_james@emolecules.com>
Subject: Re: count * performance issue
Date: Fri, 07 Mar 2008 20:38:15 -0500
Message-ID: <13345.1204940295@sss.pgh.pa.us> (raw)
In-Reply-To: <200803071727.36146.josh@agliodbs.com>
References: <f966c2ee0803052142l6a105678jfe2da92a0d9371@mail.gmail.com>
<47D00DB2.7040007@emolecules.com>
<18451.1204824562@sss.pgh.pa.us>
<200803071727.36146.josh@agliodbs.com>
Josh Berkus <josh@agliodbs.com> writes:
> 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.
Well, scanning an index to get a count might be significantly faster
than scanning the main table, but it's hardly "instantaneous". It's
still going to take time proportional to the table size.
Unless they keep a central counter of the number of index entries;
which would have all the same serialization penalties we've talked
about before...
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, josh@agliodbs.com, craig_james@emolecules.com
Subject: Re: count * performance issue
In-Reply-To: <13345.1204940295@sss.pgh.pa.us>
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