From: Mark Mielke <mark@mark.mielke.cc>
To: josh@agliodbs.com
Cc: pgsql-performance@postgresql.org, Tom Lane <tgl@sss.pgh.pa.us>
Cc: Craig James <craig_james@emolecules.com>
Subject: Re: count * performance issue
Date: Sat, 08 Mar 2008 00:31:42 -0500
Message-ID: <47D224BE.1060001@mark.mielke.cc> (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 wrote:
>>> 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 read the "almost instantaneous" against "the above claim is false" and
"Nope.", and I am not sure from the above whether you are saying that
Oracle keeps an up-to-date count for the index (which might make it
instantaneous?), or whether you are saying it still has to scan the
index - which can take time if the index is large (therefore not
instantaneous).
Cheers,
mark
--
Mark Mielke <mark@mielke.cc>
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To: pgsql-performance@postgresql.org
Cc: mark@mark.mielke.cc, josh@agliodbs.com, tgl@sss.pgh.pa.us, craig_james@emolecules.com
Subject: Re: count * performance issue
In-Reply-To: <47D224BE.1060001@mark.mielke.cc>
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