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Re: extending relations more efficiently

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Jeroen Vermeulen <jtv(at)xs4all(dot)nl>, Andres Freund <andres(at)anarazel(dot)de>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: extending relations more efficiently
Date: 2012-05-02 16:26:28
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Lists: pgsql-hackers
Excerpts from Robert Haas's message of mié may 02 08:14:36 -0400 2012:
> On Wed, May 2, 2012 at 7:16 AM, Jeroen Vermeulen <jtv(at)xs4all(dot)nl> wrote:
> > On 2012-05-01 22:06, Robert Haas wrote:
> >> It might also be interesting to provide a mechanism to pre-extend a
> >> relation to a certain number of blocks, though if we did that we'd
> >> have to make sure that autovac got the memo not to truncate those
> >> pages away again.
> >
> > Good point.  And just to check before skipping over it, do we know that
> > autovacuum not leaving enough slack space is not a significant cause of the
> > bottlenecks in the first place?
> I'm not sure exactly what you mean by this: autovacuum doesn't need
> any slack space.  Regular DML operations can certainly benefit from
> slack space, both within each page and overall within the relation.
> But there's no evidence that vacuum is doing too good a job cleaning
> up the mess, forcing the relation to be re-extended.  Rather, the
> usual (and frequent) complaint is that vacuum is leaving way too much
> slack space - i.e. bloat.

Hm.  I see those two things as different -- to me, bloat is unremoved
dead tuples, whereas slack space would be free space that can be reused
by new tuples.  Slack space is useful as it avoids relation extension;
bloat is not.

I wonder, though, if we should set a less-than-100 fillfactor for heap
relations.  Just like default_statistic_target, it seems that the
default value should be a conservative tradeoff between two extremes.
This doesn't help extension for bulk insertions a lot, of course, but
it'd be useful for tables where HOT updates happen with some regularity.

Álvaro Herrera <alvherre(at)commandprompt(dot)com>
The PostgreSQL Company - Command Prompt, Inc.
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

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