Re: Adding skip scan (including MDAM style range skip scan) to nbtree

On Fri, Feb 14, 2025 at 6:06 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> Attached is v24, which breaks out these recent changes to primscan
> scheduling into their own commit/patch (namely
> v24-0002-Improve-nbtree-SAOP-primitive-scan-scheduling.patch). The
> primscan scheduling improvement stuff hasn't really changed since
> v23, though (though I did polish it some more). I hope to be able to
> commit this new primscan scheduling patch sooner rather than later
> (though I don't think that it's quite committable yet).

The patch series recently bitrot due to conflicting changes on HEAD,
so I decided to post a new v25 now.

New in v25:

* Fixed a regression with parallel index scans caused by the improved
scheduling logic added by
0002-Improve-nbtree-SAOP-primitive-scan-scheduling.patch.

v24 did not account for how "firstPage=false" calls to _bt_readpage
might originate from _bt_first (not _bt_next) during parallel scans,
even during the first page for the parallel worker's primitive
scan/_bt_first call -- which made the heuristics added to
_bt_advance_array_keys do the wrong thing by not terminating primitive
scan based on faulty information. This was possible via the parallel
scan _bt_readnextpage "seized=true" path, which led to regressions. In
v24 we now pass "firstPage=true" whenever a call to _bt_readpage
happens through _bt_first, no matter the details (for the first
_bt_readpage call's page).

* The additional EXPLAIN ANALYZE logic (that shows "Index Searches:
N") added by 0001-Show-index-search-count-in-EXPLAIN-ANALYZE.patch has
been adjusted, and no longer divides by nloops.

I explain why dividing by nloops has some fairly bad consequences on
the thread I started for the quasi-independent enhancement to EXPLAIN
ANALYZE output:

https://postgr.es/m/CAH2-WzmebSkeKPGw7TEaNw9=Qx-X8fAnFw916Fd2V8VVqYqqaQ@mail.gmail.com

* Polished commit messages.

* Added more test coverage. The LOC covered percentage is now at just
over 90% for nbtutils.c. We now have coverage for almost all of the
new code that advances the scan's skip arrays, including code that
deals with NULL values that is seldom reached.

--
Peter Geoghegan