Horton

Re: generic plans and "initial" pruning

Amit Langote <amitlangote09@gmail.com>

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From: Amit Langote <amitlangote09@gmail.com>
To: Simon Riggs <simon.riggs@enterprisedb.com>
Cc: Robert Haas <robertmhaas@gmail.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2022-01-18T08:10:22Z
Lists: pgsql-hackers 
Hi Simon,

On Tue, Jan 18, 2022 at 4:44 PM Simon Riggs
<simon.riggs@enterprisedb.com> wrote:
> On Tue, 11 Jan 2022 at 16:22, Robert Haas <robertmhaas@gmail.com> wrote:
> > This is just a relatively simple example and I think there are
> > probably a bunch of others. There are a lot of kinds of DDL that could
> > be performed on a partition that gets pruned away: DROP INDEX is just
> > one example.
>
> I haven't followed this in any detail, but this patch and its goal of
> reducing the O(N) drag effect on partition execution time is very
> important. Locking a long list of objects that then get pruned is very
> wasteful, as the results show.
>
> Ideally, we want an O(1) algorithm for single partition access and DDL
> is rare. So perhaps that is the starting point for a safe design -
> invent a single lock or cache that allows us to check if the partition
> hierarchy has changed in any way, and if so, replan, if not, skip
> locks.

Rearchitecting partition locking to be O(1) seems like a project of
non-trivial complexity as Robert mentioned in a related email thread
couple of years ago:

https://www.postgresql.org/message-id/CA%2BTgmoYbtm1uuDne3rRp_uNA2RFiBwXX1ngj3RSLxOfc3oS7cQ%40mail.gmail.com

Pursuing that kind of a project would perhaps have been more
worthwhile if the locking issue had affected more than just this
particular case, that is, the case of running prepared statements over
partitioned tables using generic plans.  Addressing this by
rearchitecting run-time pruning (and plancache to some degree) seemed
like it might lead to this getting fixed in a bounded timeframe.  I
admit that the concerns that Robert has raised about the patch make me
want to reconsider that position, though maybe it's too soon to
conclude.

-- 
Amit Langote
EDB: http://www.enterprisedb.com

Commits

  1. Stamp 19beta1.

  2. Revert "Don't lock partitions pruned by initial pruning"

  3. Ensure first ModifyTable rel initialized if all are pruned

  4. Fix bug in cbc127917 to handle nested Append correctly

  5. Remove unstable test suite added by 525392d57

  6. Don't lock partitions pruned by initial pruning

  7. Fix an oversight in cbc127917 to handle MERGE correctly

  8. Track unpruned relids to avoid processing pruned relations

  9. Perform runtime initial pruning outside ExecInitNode()

  10. Move PartitionPruneInfo out of plan nodes into PlannedStmt

  11. Fix setrefs.c's failure to do expression processing on prune steps.

  12. Remove obsolete executor cleanup code

  13. Revert "Move PartitionPruneInfo out of plan nodes into PlannedStmt"

  14. Move PartitioPruneInfo out of plan nodes into PlannedStmt

  15. Refactor and cleanup runtime partition prune code a little

  16. Remove some unnecessary fields from Plan trees.

  17. Remove more redundant relation locking during executor startup.

  18. Shut down Gather's children before shutting down Gather itself.