Re: generic plans and "initial" pruning

On Fri, Dec 9, 2022 at 6:52 PM Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:
> On 2022-Dec-09, Amit Langote wrote:
> > On Wed, Dec 7, 2022 at 4:00 AM Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:
> > > I find the API of GetCachedPlans a little weird after this patch.
>
> > David, in his Apr 7 reply on this thread, also sounded to suggest
> > something similar.
> >
> > Hmm, I was / am not so sure if GetCachedPlan() should return something
> > that is not CachedPlan. An idea I had today was to replace the
> > part_prune_results_list output List parameter with, say,
> > QueryInitPruningResult, or something like that and put the current
> > list into that struct. Was looking at QueryEnvironment to come up
> > with *that* name. Any thoughts?
>
> Remind me again why is part_prune_results_list not part of struct
> CachedPlan then? I tried to understand that based on comments upthread,
> but I was unable to find anything.

It used to be part of CachedPlan for a brief period of time (in patch
v12 I posted in [1]), but David, in his reply to [1], said he wasn't
so sure that it belonged there.

> (My first reaction to your above comment was "well, rename GetCachedPlan
> then, maybe to GetRunnablePlan", but then I'm wondering if CachedPlan is
> in any way a structure that must be "immutable" in the way parser output
> is. Looking at the comment at the top of plancache.c it appears to me
> that it isn't, but maybe I'm missing something.)

CachedPlan *is* supposed to be read-only per the comment above
CachedPlanSource definition:

* ...If we are using a generic
* cached plan then it is meant to be re-used across multiple executions, so
* callers must always treat CachedPlans as read-only.

FYI, there was even an idea of putting a PartitionPruneResults for a
given PlannedStmt into the PlannedStmt itself [2], but PlannedStmt is
supposed to be read-only too [3].

Maybe we need some new overarching context when invoking plancache, if
Portal can't already be it, whose struct can be passed to
GetCachedPlan() to put the pruning results in? Perhaps,
GetRunnablePlan() that you floated could be a wrapper for
GetCachedPlan(), owning that new context.

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

[1] https://www.postgresql.org/message-id/CA%2BHiwqH4qQ_YVROr7TY0jSCuGn0oHhH79_DswOdXWN5UnMCBtQ%40mail.gmail.com
[2] https://www.postgresql.org/message-id/CAApHDvp_DjVVkgSV24%2BUF7p_yKWeepgoo%2BW2SWLLhNmjwHTVYQ%40mail.gmail.com
[3] https://www.postgresql.org/message-id/922566.1648784745%40sss.pgh.pa.us