Re: improve transparency of bitmap-only heap scans

On Sat, Mar 28, 2020 at 8:02 PM James Coleman <jtc331(at)gmail(dot)com> wrote:
>
> On Fri, Mar 27, 2020 at 9:24 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > Yeah, I also see this information could be useful. It seems Tom Lane
> > is not entirely convinced of this. I am not sure if this is the right
> > time to seek more opinions as we are already near the end of CF. So,
> > we should either decide to move this to the next CF if we think of
> > getting the opinion of others or simply reject it and see a better way
> > for EXPLAIN to identify an index-only BMS.
>
> I'm curious if Tom's objection is mostly on the grounds that we should
> be consistent in what's displayed, or that he thinks the information
> is likely to be useless.
>

Yeah, it would be good if he clarifies his position.

> If consistency is the goal you might e.g., do something that just
> changes the node type output, but in favor of changing that, it seems
> to me that showing "how well did the optimization" is actually more
> valuable than "did we do the optimization at all". Additionally I
> think showing it as an optimization of an existing node is actually
> likely less confusing anyway.
>
> One other thing: my understanding is that this actually matches the
> underlying code split too. For the index only scan case, we actually
> have a separate node (it's not just an optimization of the standard
> index scan). There are discussions about whether that's a good thing,
> but it's what we have. In contrast, the bitmap scan actually has it as
> a pure optimization of the existing bitmap heap scan node.
>

I personally see those as valid points. Does anybody else want to
weigh in here, so that we can reach to some conclusion and move ahead
with this CF entry?

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com