Re: Missing update of all_hasnulls in BRIN opclasses

Hi,

It took me a while but I finally got back to reworking this to use the
bt_info bit, as proposed by Alvaro. And it turned out to work great,
because (a) it's a tuple-level flag, i.e. the right place, and (b) it
does not overload existing flags.

This greatly simplified the code in add_values_to_range and (especially)
union_tuples, making it much easier to understand, I think.

One disadvantage is we are unable to see which ranges are empty in
current pageinspect, but 0002 addresses that by adding "empty" column to
the brin_page_items() output. That's a matter for master only, though.
It's a trivial patch and it makes it easier/possible to test this, so we
should consider to squeeze it into PG16.

I did quite a bit of testing - the attached 0003 adds extra tests, but I
don't propose to get this committed as is - it's rather overkill. Maybe
some reduced version of it ...

The hardest thing to test is the union_tuples() part, as it requires
concurrent operations with "correct" timing. Easy to simulate by
breakpoints in GDB, not so much in plain regression/TAP tests.

There's also a stress tests, doing a lot of randomized summarizations,
etc. Without the fix this failed in maybe 30% of runs, now I did ~100
runs without a single failure.

I haven't done any backporting, but I think it should be simpler than
with the earlier approach. I wonder if we need to care about starting to
use the previously unused bit - I don't think so, in the worst case
we'll just ignore it, but maybe I'm missing something (e.g. when using
physical replication).

regards

--
Tomas Vondra
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