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Re: extending relations more efficiently

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Alvaro Herrera <alvherre(at)commandprompt(dot)com>, 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 16:45:35
Message-ID: 13539.1335977135@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Wed, May 2, 2012 at 12:26 PM, Alvaro Herrera
> <alvherre(at)commandprompt(dot)com> wrote:
>> 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 guess I think of bloat as including both unremoved dead tuples and
> unwanted internal free space.  If you create a giant table, delete 9
> out of every 10 tuples, and vacuum, the table is still "bloated", IMV.

The difficulty is to tell the difference between useless free space and
useful free space.  If there's a reasonable probability of putting new
data into a given chunk of free space in the near future, it's not
bloat.

			regards, tom lane

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