Re: generic plans and "initial" pruning

On Fri, Dec 9, 2022 at 8:37 PM Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:
> On 2022-Dec-09, Amit Langote wrote:
>
> > Pruning will be done afresh on every fetch of a given cached plan when
> > CheckCachedPlan() is called on it, so the part_prune_results_list part
> > will be discarded and rebuilt as many times as the plan is executed.
> > You'll find a description around CachedPlanSavePartitionPruneResults()
> > that's in v12.
>
> I see.
>
> In that case, a separate container struct seems warranted.

I thought about this today and played around with some container struct ideas.

Though, I started feeling like putting all the new logic being added
by this patch into plancache.c at the heart of GetCachedPlan() and
tweaking its API in kind of unintuitive ways may not have been such a
good idea to begin with. So I started thinking again about your
GetRunnablePlan() wrapper idea and thought maybe we could do something
with it. Let's say we name it GetCachedPlanLockPartitions() and put
the logic that does initial pruning with the new
ExecutorDoInitialPruning() in it, instead of in the normal
GetCachedPlan() path. Any callers that call GetCachedPlan() instead
call GetCachedPlanLockPartitions() with either the List ** parameter
as now or some container struct if that seems better. Whether
GetCachedPlanLockPartitions() needs to do anything other than return
the CachedPlan returned by GetCachedPlan() can be decided by the
latter setting, say, CachedPlan.has_unlocked_partitions. That will be
done by AcquireExecutorLocks() when it sees containsInitialPrunnig in
any of the PlannedStmts it sees, locking only the
PlannedStmt.minLockRelids set (which is all relations where no pruning
is needed!), leaving the partition locking to
GetCachedPlanLockPartitions(). If the CachedPlan is invalidated
during the partition locking phase, it calls GetCachedPlan() again;
maybe some refactoring is needed to avoid too much useless work in
such cases.

Thoughts?

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