Re: generic plans and "initial" pruning

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. 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.

Thanks for the patch! I think the new coding is indeed a bit easier to
follow.

--
Álvaro Herrera Breisgau, Deutschland — https://www.EnterpriseDB.com/
<inflex> really, I see PHP as like a strange amalgamation of C, Perl, Shell
<crab> inflex: you know that "amalgam" means "mixture with mercury",
more or less, right?
<crab> i.e., "deadly poison"