Horton

Re: Unhappy about API changes in the no-fsm-for-small-rels patch

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Robert Haas <robertmhaas@gmail.com>, John Naylor <john.naylor@2ndquadrant.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2019-05-07T13:34:42Z
Lists: pgsql-hackers 
Amit Kapila <amit.kapila16@gmail.com> writes:
> On Mon, May 6, 2019 at 8:57 PM Andres Freund <andres@anarazel.de> wrote:
>> On 2019-05-06 11:10:15 -0400, Robert Haas wrote:
>>> I think it's legitimate to question whether sending additional
>>> invalidation messages as part of the design of this feature is a good
>>> idea.

>> I don't think it's an actual problem. We'd only do so when creating an
>> FSM, or when freeing up additional space that'd otherwise not be visible
>> to other backends.

> The other place we need to consider for this is when one of the
> backends updates its map (due to unavailability of space in the
> existing set of pages).  We can choose not to send invalidation in
> this case, but then different backends need to identify the same thing
> themselves and reconstruct the map again.

I'm inclined to wonder why bother with invals at all.  The odds are
quite good that no other backend will care (which, I imagine, is the
reasoning behind why the original patch was designed like it was).
A table that has a lot of concurrent write activity on it is unlikely
to stay small enough to not have a FSM for long.

The approach I'm imagining here is not too different from Robert's
"just search the table's pages every time" straw-man.  Backends would
cache the results of their own searches, but not communicate about it.

			regards, tom lane

Commits

  1. Revert "Avoid the creation of the free space map for small heap relations".

  2. Improve code comments in b0eaa4c51b.

  3. During pg_upgrade, conditionally skip transfer of FSMs.

  4. Add more tests for FSM.

  5. Fix typos in commit 8586bf7ed8.

  6. tableam: introduce table AM infrastructure.

  7. Clear the local map when not used.

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

  9. Make FSM test portable.

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