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Re: [PATCHES] pg_freespacemap question

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Mark Kirkwood <markir(at)paradise(dot)net(dot)nz>
Cc: Christopher Kings-Lynne <chriskl(at)familyhealth(dot)com(dot)au>,Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Tatsuo Ishii <ishii(at)sraoss(dot)co(dot)jp>,peter_e(at)gmx(dot)net, pgsql-hackers(at)postgresql(dot)org,pgsql-patches(at)postgresql(dot)org
Subject: Re: [PATCHES] pg_freespacemap question
Date: 2006-03-13 13:44:12
Message-ID: 20060313134412.GD8274@surnet.cl (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
Mark Kirkwood wrote:
> Christopher Kings-Lynne wrote:
> >>The point here is that if tuples require 50 bytes, and there are 20
> >>bytes free on a page, pgstattuple counts 20 free bytes while FSM
> >>ignores the page.  Recording that space in the FSM will not improve
> >>matters, it'll just risk pushing out FSM records for pages that do
> >>have useful amounts of free space.
> >
> >Maybe an overloaded pgstattuple function that allows you to request FSM 
> >behavior?
> 
> That's a nice idea - could also do equivalently by adding an extra 
> column "usable_free_space" or some such, and calculating this using FSM 
> logic.

The current pgstattuple function scans the whole table, so I don't think
this is a good idea.  Re: the overloaded function, I think the behaviors
are different enough to merit a separate function, with a different
name.

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.

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