Horton

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

Tomas Vondra <tomas@vondra.me>

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From: Tomas Vondra <tomas@vondra.me>
To: Peter Geoghegan <pg@bowt.ie>
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: 2025-05-09T17:19:31Z
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 5/9/25 18:36, Peter Geoghegan wrote:
> On Fri, May 9, 2025 at 12:28 PM Tomas Vondra <tomas@vondra.me> wrote:
>> Not sure if it matters, but this uses index-only scans, and the pages
>> are all-visible, so maybe it's not much more expensive.
> 
> You're still going to have to scan 85 full index pages on your
> pgbench_accounts.bid index -- that's pretty expensive, even with an
> index-only scan.
> 

Not sure I understand. Why would it need to scan 85 index pages? There's
only 100 matching tuples total, spread over the 100 partitions. We'll
need to scan maybe 1 page per partition.

> Even if there was some kind of really obvious regression in
> preprocessing (which seems almost impossible), I'd still expect it to
> be drowned out for queries such as these.
> 
>> Not sure, I did multiple tests with different queries, and it'd be a bit
>> strange if this was the only one affected. Not impossible.
> 
> The only thing that substantially differs between this
> pgbench_accounts.bid query and traditional pgbench SELECT queries (on
> pgbench_accounts.aid) is 1.) this query is quite a bit more expensive
> at execution time, and 2.) that it involves the use of partitioning.
> 
> I made sure to test pgbench SELECT quite thoroughly -- that certainly
> wasn't regressed. So it seems very likely to have something to do with
> partitioning.
> 

Yeah. This type of query amplifies any overhead in the index scan,
because it does one for every partition. I haven't seen the regression
without the partitioning.


regards

-- 
Tomas Vondra