Horton

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

Andres Freund <andres@anarazel.de>

View thread View on postgr.es
From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@postgresql.org, John Naylor <john.naylor@2ndquadrant.com>, Amit Kapila <amit.kapila16@gmail.com>
Date: 2019-04-16T19:16:25Z
Lists: pgsql-hackers 
Hi,

On 2019-04-16 14:31:25 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > I'm kinda thinking that this is the wrong architecture.
> 
> The bits of that patch that I've looked at seemed like a mess
> to me too.  AFAICT, it's trying to use a single global "map"
> for all relations (strike 1) without any clear tracking of
> which relation the map currently describes (strike 2).

Well, strike 2 basically is not a problem right now, because the map is
cleared whenever a search for a target buffer succeeded. But that has
pretty obvious efficiency issues...


> This can only work at all if an inaccurate map is very fail-soft,
> which I'm not convinced it is

I think it better needs to be fail-soft independent of this the no-fsm
patch. Because the fsm is not WAL logged etc, it's pretty easy to get a
pretty corrupted version. And we better deal with that.


> and in any case it seems pretty inefficient for workloads that insert
> into multiple tables.

As is, it's inefficient for insertions into a *single* relation. The
RelationGetTargetBlock() makes it not crazily expensive, but it's still
plenty expensive.


> I'd have expected any such map to be per-table and be stored in
> the relcache.

Same.

Greetings,

Andres Freund

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.