From: Heikki Linnakangas <heikki@enterprisedb.com>
To: Aidan Van Dyk <aidan@highrise.ca>
Cc: pgsql-performance@postgresql.org
Subject: Re: PostgreSQL 8.2.3 VACUUM Timings/Performance
Date: Mon, 05 Mar 2007 16:25:27 +0000
Message-ID: <45EC4477.4020702@enterprisedb.com> (raw)
In-Reply-To: <20070305151820.GC25573@yugib.highrise.ca>
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<45EC1931.3020701@enterprisedb.com>
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<45EC2CCF.8020906@enterprisedb.com>
<20070305151820.GC25573@yugib.highrise.ca>
Aidan Van Dyk wrote:
> * Heikki Linnakangas <heikki@enterprisedb.com> [070305 09:46]:
>> In fact, getting rid of vacuum full, or changing it to work like
>> cluster, has been proposed in the past. The use case really is pretty
>> narrow; cluster is a lot faster if there's a lot of unused space in the
>> table, and if there's not, vacuum full isn't going to do much so there's
>> not much point running it in the first place. The reason it exists is
>> largely historical, there hasn't been a pressing reason to remove it either.
>
> I've never used CLUSTER, because I've always heard murmerings of it not
> being completely MVCC safe. From the TODO:
> * CLUSTER
> o Make CLUSTER preserve recently-dead tuples per MVCC
> requirements
Good point, I didn't remember that. Using cluster in an environment like
the OP has, cluster might actually break the consistency of concurrent
transactions.
> But the documents don't mention anything about cluster being unsafe.
Really? <checks docs>. Looks like you're right. Should definitely be
mentioned in the docs.
> AFAIK, Vacuum full doesn't suffer the same MVCC issues that cluster
> does. Is this correct?
That's right. Vacuum full goes to great lengths to be MVCC-safe.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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To: pgsql-performance@postgresql.org
Cc: heikki@enterprisedb.com, aidan@highrise.ca
Subject: Re: PostgreSQL 8.2.3 VACUUM Timings/Performance
In-Reply-To: <45EC4477.4020702@enterprisedb.com>
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