Re: Revert: Remove useless self-joins *and* -DREALLOCATE_BITMAPSETS make server crash, regress test fail.

On Thu, May 9, 2024 at 6:40 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> David Rowley <dgrowleyml(at)gmail(dot)com> writes:
> > I'm fine with this one as it's the same as what I already mentioned
> > earlier. I had imagined doing bms_del_member(bms_copy ... but maybe
> > the compiler is able to optimise away the additional store. Likely, it
> > does not matter much as pallocing memory likely adds far more overhead
> > anyway.
>
> I actually wrote it that way to start with, but undid it after
> noticing that the existing code in remove_rel_from_restrictinfo
> does it in separate steps, and thinking that that was good for
> both separation of concerns and a cleaner git history. I too
> can't believe that an extra fetch will be noticeable compared
> to the cost of the adjacent bms_xxx operations.

I also think it seems better to do bms_copy in separate steps, not only
because this keeps consistent with the existing code in
remove_rel_from_restrictinfo, but also because we need to do
bms_del_member twice for each lefthand/righthand relid set.

Speaking of consistency, do you think it would improve the code's
readability if we rearrange the code in remove_rel_from_query so that
the modifications of the same relid set are grouped together, just like
what we do in remove_rel_from_restrictinfo? I mean something like:

sjinf->min_lefthand = bms_copy(sjinf->min_lefthand);
sjinf->min_lefthand = bms_del_member(sjinf->min_lefthand, relid);
sjinf->min_lefthand = bms_del_member(sjinf->min_lefthand, ojrelid);

...

Thanks
Richard