Re: BRIN cost estimate

> Not sure what you mean here. I'm not speaking of the brin index am, I
> mean the get_index_stats_hook call which you've added.

I see.  Actually this part was from Alvaro.  I haven't noticed the
get_index_stats_hook call before, but it is still the same coding as
btcostestimate().  btcostestimate() also calls get_index_stats_hook,
and then Asserts nnumbers == 1.

> hmm, before what exactly? before your patch it didn't exist. You
> introduced it into brincostestimate().

I confused by looking at my changes on my repository I made on top of
Alvaro's.  I will rename it on the next version.

> At the very least please write a comment to explain this in the code.
> Right now it looks broken. If I noticed this then one day in the
> future someone else will. If you write a comment then person of the
> future will likely read it, and then not raise any questions about the
> otherwise questionable code.

Will do.

> I do strongly agree that the estimates need improved here. I've
> personally had issues with bad brin estimates before, and I'd like to
> see it improved. I think the patch is not quite complete without it
> also considering stats on expression indexes. If you have time to go
> do that I'd suggest you go ahead with that.

I will look into it this week.