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Re: Measuring relation free space

From: Noah Misch <noah(at)leadboat(dot)com>
To: Jaime Casanova <jaime(at)2ndquadrant(dot)com>
Cc: Bernd Helmle <mailings(at)oopsware(dot)de>, Greg Smith <greg(at)2ndquadrant(dot)com>,pgsql-hackers(at)postgresql(dot)org
Subject: Re: Measuring relation free space
Date: 2012-01-21 01:33:30
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Lists: pgsql-hackers
On Fri, Jan 20, 2012 at 07:03:22PM -0500, Jaime Casanova wrote:
> On Wed, Jan 18, 2012 at 7:01 PM, Noah Misch <noah(at)leadboat(dot)com> wrote:
> > On Wed, Jan 18, 2012 at 09:46:20AM -0500, Jaime Casanova wrote:
> >>
> >> ignoring all non-leaf pages still gives a considerable difference
> >> between pgstattuple and relation_free_space()
> >
> > pgstattuple() counts the single B-tree meta page as always-full, while
> > relation_free_space() skips it for all purposes. ?For tiny indexes, that can
> > shift the percentage dramatically.
> >
> ok, i will reformulate the question. why is fine ignoring non-leaf
> pages but is not fine to ignore the meta page?

pgstattuple() figures the free_percent by adding up all space available to
hold tuples and dividing that by the simple size of the relation.  Non-leaf
pages and the meta page get identical treatment: both never hold tuples, so
they do not contribute to the free space.

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