Re: [HACKERS] pg_freespacemap question

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