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Re: Measuring relation free space

From: Noah Misch <noah(at)leadboat(dot)com>
To: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
Cc: Jaime Casanova <jaime(at)2ndquadrant(dot)com>,Bernd Helmle <mailings(at)oopsware(dot)de>,Greg Smith <greg(at)2ndquadrant(dot)com>,Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Measuring relation free space
Date: 2012-01-24 00:18:59
Message-ID: 20120124001859.GA31986@tornado.leadboat.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Mon, Jan 23, 2012 at 04:56:24PM -0300, Alvaro Herrera wrote:
> Excerpts from Noah Misch's message of vie ene 20 22:33:30 -0300 2012:
> > 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.
> 
> Hm.  Leaf pages hold as much tuples as non-leaf pages, no?  I mean
> for each page element there's a value and a CTID.  In non-leaf those
> CTIDs point to other index pages, one level down the tree; in leaf pages
> they point to the heap.

That distinction seemed important when I sent my last message, but now I agree
that it's largely irrelevant for free space purposes.  If someone feels like
doing it, +1 for making pgstattuple() count non-leaf free space.

Thanks,
nm

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