Re: Showing primitive index scan count in EXPLAIN ANALYZE (for skip scan and SAOP scans)

On Sun, Nov 10, 2024 at 2:00 PM Alena Rybakina
<a.rybakina@postgrespro.ru> wrote:
> Or maybe I was affected by fatigue, but I don’t understand this point, to be honest. I see from the documentation and your first letter that it specifies how many times in total the tuple search would be performed during the index execution. Is that not quite right?

Well, nodes that appear on the inner side of a nested loop join (and
in a few other contexts) generally have their row counts (and a few
other things) divided by the total number of executions. The idea is
that we're showing the average across all executions of the node -- if
the user wants the true absolute number, they're expected to multiply
nrows by nloops themselves. This is slightly controversial behavior,
but it is long established (weirdly, we never divide by nloops for
"Buffers").

Initial versions of my patch didn't do this. The latest version does
divide like this, though. In general it isn't all that likely that an
inner index scan would have more than a single primitive index scan,
in any case, so which particular behavior I use here (divide vs don't
divide) is not something that I feel strongly about.

>> Why wouldn't they expect to continue to see the same information (or
>> similar information) when the index searches happen to be coordinated
>> by the index scan node/index AM itself?
>
> To be honest, I don't quite understand this. Can you please explain in more detail?

I just meant that your OR transformation patch is another case where
we shouldn't obscure the count of primitive index scans.

It would be inconsistent of us to allow users to see the number of
index scans today (without your patch), while denying users the
ability to see essentially the same information in the future (with
your patch). The fact that an index scan has its own executor node
today and won't have one tomorrow shouldn't in itself affect
instrumentation of the number of (primitive) index scans shown by
EXPLAIN ANALYZE (it certainly won't affect the instrumentation within
the pg_stat_all_indexes view, as things stand, even without my patch).

-- 
Peter Geoghegan