Horton

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

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Amit Kapila <amit.kapila16@gmail.com>, John Naylor <jcnaylor@gmail.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2018-11-02T14:38:45Z
Lists: pgsql-hackers 
On Fri, Nov 2, 2018 at 10:07 AM 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?

I think it's in evidence, in the form of several messages mentioning a
flag called try_every_block.

Just checking the last page of the table doesn't sound like a good
idea to me.  I think that will just lead to a lot of stupid bloat.  It
seems likely that checking every page of the table is fine for npages
<= 3, and that would still be win in a very significant number of
cases, since lots of instances have many empty or tiny tables.  I was
merely reacting to the suggestion that the approach should be used for
npages <= 32; that threshold sounds way too high.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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