Horton

Re: WIP: Avoid creation of the free space map for small tables

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Robert Haas <robertmhaas@gmail.com>, jcnaylor@gmail.com, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2018-11-03T04:48:02Z
Lists: pgsql-hackers 
On Fri, Nov 2, 2018 at 7:37 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Robert Haas <robertmhaas@gmail.com> writes:
> > That's not what I'm saying.  If we don't have the FSM, we have to
> > check every page of the table.  If there's a workload where that
> > happens a lot on a table that is just under the size threshold for
> > creating the FSM, then it's likely to be a worst case for this patch.
>
> Hmm, you're assuming something not in evidence: why would that be the
> algorithm?  On a FSM-less table, I'd be inclined to just check the
> last page and then grow the table if the tuple doesn't fit there.
> This would, in many cases, soon result in a FSM being created, but
> I think that's just fine.  The point of the change is to optimize
> for cases where a table *never* gets more than a few inserts.  Not, IMO,
> for cases where a table gets a lot of churn but never has a whole lot of
> live tuples.  In the latter scenario we are far better off having a FSM.
>

In the past, you seem to have suggested an approach to try each block
[1] for small tables which don't have FSM.  I think if we do what you
are suggesting now, then we don't need to worry much about any
regression and code will be somewhat simpler, but OTOH, I don't see
much harm in trying every block if we keep the threshold as no more
than 4.  That would address more cases.

[1] - https://www.postgresql.org/message-id/11360.1345502641%40sss.pgh.pa.us

-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com

Commits

  1. Improve code comments in b0eaa4c51b.

  2. During pg_upgrade, conditionally skip transfer of FSMs.

  3. Add more tests for FSM.

  4. Doc: Update the documentation for FSM behavior for small tables.

  5. Make FSM test portable.

  6. Avoid creation of the free space map for small heap relations, take 2.

  7. Move page initialization from RelationAddExtraBlocks() to use, take 2.

  8. Avoid creation of the free space map for small heap relations.

  9. In bootstrap mode, don't allow the creation of files if they don't already