Horton

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

Peter Geoghegan <pg@bowt.ie>

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From: Peter Geoghegan <pg@bowt.ie>
To: Tomas Vondra <tomas@vondra.me>
Cc: Mark Dilger <mark.dilger@enterprisedb.com>, Heikki Linnakangas <hlinnaka@iki.fi>, pgsql-hackers@lists.postgresql.org, Matthias van de Meent <boekewurm+postgres@gmail.com>
Date: 2025-05-09T13:59:09Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. nbtree: Always set skipScan flag on rescan.

  2. meson: Build numeric.c with -ftree-vectorize.

  3. Fix "variable not found in subplan target lists" in semijoin de-duplication.

  4. Revert "nbtree: Remove useless row compare arg."

  5. nbtree: Remove useless row compare arg.

  6. Prevent premature nbtree array advancement.

  7. nbtree: tighten up array recheck rules.

  8. Avoid treating nonrequired nbtree keys as required.

  9. Adjust overstrong nbtree skip array assertion.

  10. Make NULL tuple values always advance skip arrays.

  11. Avoid extra index searches through preprocessing.

  12. Improve nbtree skip scan primitive scan scheduling.

  13. Further optimize nbtree search scan key comparisons.

  14. Add nbtree skip scan optimization.

  15. Improve nbtree array primitive scan scheduling.

  16. nbtree: Make BTMaxItemSize into object-like macro.

  17. Show index search count in EXPLAIN ANALYZE, take 2.

  18. Make parallel nbtree index scans use an LWLock.

  19. Show index search count in EXPLAIN ANALYZE.

  20. Avoid nbtree parallel scan currPos confusion.

  21. nbtree: Remove useless 'strat' local variable.

  22. Normalize nbtree truncated high key array behavior.

  23. Refactor handling of nbtree array redundancies.

  24. Fix nbtree pgstats accounting with parallel scans.

  25. Avoid parallel nbtree index scan hangs with SAOPs.

  26. Show Parallel Bitmap Heap Scan worker stats in EXPLAIN ANALYZE

  27. Enhance nbtree ScalarArrayOp execution.

  28. Skip checking of scan keys required for directional scan in B-tree

  29. Instead of using a numberOfRequiredKeys count to distinguish required

On Fri, May 9, 2025 at 9:42 AM Peter Geoghegan <pg@bowt.ie> wrote:
> I'm rather puzzled as to why this happens, then. I expect that nbtree
> preprocessing will be able to use its usual single index column/index
> key fast path here -- the "We can short-circuit most of the work if
> there's just one key" path in _bt_preprocess_keys (and I expect that
> _bt_num_array_keys() quickly determines that no skip arrays should be
> added, preventing array preprocessing from ever really starting).

You've been testing commit 92fe23d9 ("Add nbtree skip scan
optimization") here, but I think you should test commit 8a510275
("Further optimize nbtree search scan key comparisons") instead. The
former commit's commit message says that there big regressions, that
the latter commit should go on to fix. Note that these two commits
were pushed together, as a unit. All of my performance validation work
was for the patch series as a whole, not for any individual commit.

I don't actually think that this kind of scan would have been affected
by those known regressions -- since they don't use array keys. But it
is definitely true that the queries that you're looking at very much
rely on the optimization from commit 8a510275 (or its predecessor
optimization, the "pstate.prechecked" optimization). As I said, my
performance validation didn't target individual commits.

-- 
Peter Geoghegan