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Re: [PATCHES] pg_freespacemap question

From: Mark Kirkwood <markir(at)paradise(dot)net(dot)nz>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Tatsuo Ishii <ishii(at)sraoss(dot)co(dot)jp>, alvherre(at)commandprompt(dot)com, 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 01:51:24
Message-ID: 4414D01C.6050205@paradise.net.nz (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
Tom Lane wrote:
>>Tatsuo Ishii wrote:
>>
>>>BTW, I noticed difference of outputs from pg_freespacemap and
>>>pgstattuple.
>>>
>>>I ran pgbench and inspected "accounts" table by using these tools.
>>>
>>>pg_freespacemap:
>>>sum of bytes: 250712
>>>
>>>pgstattuple:
>>>free_space: 354880
>>>
>>>Shouldn't they be identical?
> 
> 
> vacuum/fsm disregard pages with "uselessly small" amounts of
> free space (less than the average tuple size, IIRC).

Ah - that what I was seeing! Thanks.
> 
> I do notice a rather serious shortcoming of pg_freespacemap in its
> current incarnation, which is that it *only* shows you the per-page free
> space data, and not any of the information that would let you determine
> what the FSM is doing to filter the raw data.  The per-relation
> avgRequest and lastPageCount fields would be interesting for instance.
> Perhaps there should be a second view with one row per relation to
> carry the appropriate data.
> 

Ok - I did wonder about 2 views, but was unsure if the per-relation 
stuff was interesting. Given that it looks like it is interesting, I'll 
see about getting a second view going.

Cheers

Mark

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