Re: generic plans and "initial" pruning

On Tue, Apr 5, 2022 at 7:00 PM Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:
> On 2022-Apr-05, Amit Langote wrote:
> > While at it, maybe it's better to rename ExecInitPruningContext() to
> > InitPartitionPruneContext(), which I've done in the attached updated
> > patch.
>
> Good call. I had changed that name too, but yours seems a better
> choice.
>
> I made a few other cosmetic changes and pushed.

Thanks!

> I'm afraid this will
> cause a few conflicts with your 0004 -- hopefully these should mostly be
> minor.
>
> One change that's not completely cosmetic is a change in the test on
> whether to call PartitionPruneFixSubPlanMap or not. Originally it was:
>
> if (partprune->do_exec_prune &&
> bms_num_members( ... ))
> do_stuff();
>
> which meant that bms_num_members() is only evaluated if do_exec_prune.
> However, the do_exec_prune bit is an optimization (we can skip doing
> that stuff if it's not going to be used), but the other test is more
> strict: the stuff is completely irrelevant if no plans have been
> removed, since the data structure does not need fixing. So I changed it
> to be like this
>
> if (bms_num_members( .. ))
> {
> /* can skip if it's pointless */
> if (do_exec_prune)
> do_stuff();
> }
>
> I think that it is clearer to the human reader this way; and I think a
> smart compiler may realize that the test can be reversed and avoid
> counting bits when it's pointless.
>
> So your 0004 patch should add the new condition to the outer if(), since
> it's a critical consideration rather than an optimization:
> if (partprune && bms_num_members())
> {
> /* can skip if pointless */
> if (do_exec_prune)
> do_stuff()
> }
>
> Now, if we disagree and think that counting bits in the BMS when it's
> going to be discarded by do_exec_prune being false, then we can flip
> that back as originally and a more explicit comment. With no evidence,
> I doubt it matters.

I agree that counting bits in the outer condition makes this easier to
read, so see no problem with keeping it that way.

Will post the rebased main patch soon, whose rewrite I'm close to
being done with.

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