Horton

Re: BitmapHeapScan streaming read user and prelim refactoring

Tomas Vondra <tomas@vondra.me>

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From: Tomas Vondra <tomas@vondra.me>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: Nazir Bilal Yavuz <byavuz81@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Thomas Munro <thomas.munro@gmail.com>, Andres Freund <andres@anarazel.de>, Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2025-02-16T12:29:35Z
Lists: pgsql-hackers

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On 2/16/25 02:15, Tomas Vondra wrote:
>
> ...
> 
> OK, I've uploaded the results to the github repository as usual
> 
>   https://github.com/tvondra/bitmapscan-tests/tree/main/20250214-184807
> 
> and I've generated the same PDF reports, with the colored comparison.
> 
> If you compare the pivot tables (I opened the "same" PDF from the two
> runs and flip between them using alt-tab, which makes the interesting
> regions easy to spot), the change is very clear.
> 
> Disabling the sequential detection greatly reduces the scope of
> regressions. That looks pretty great, IMO.
> 
> It also seems to lose some speedups, especially with io_combine_limit=1
> and eic=1. I'm not sure why, if that's expected, etc.
> 
> There still remain areas of regression, but most of them are for cases
> that'd use index scan (tiny fraction of rows scanned), or with
> read-ahead=4096 (and not for the lower settings).
> 
> The read-ahead dependence is actually somewhat interesting, because I
> realized the RAID array has this set to 8192 by default, i.e. even
> higher than 4096 where it regresses. I suppose mdadm does that, or
> something, I don't know how the default is calculated. But I assume it
> depends on the number of devices, so larger arrays might have even
> higher read-ahead values.
> 
> I'm running the benchmarks with ra=8192, shouldn't take more than a
> couple hours.
> 

And done, uploaded to

  https://github.com/tvondra/bitmapscan-tests/tree/main/20250216-014254

The PDFs combine the results with the previous run for other read-ahead
values.

Overall, it seems about the same as for read-ahead 4096. The regressions
are a bit stronger, but nothing surprising.


regards

-- 
Tomas Vondra

Commits

  1. Fix bitmapheapscan incorrect recheck of NULL tuples

  2. Increase default maintenance_io_concurrency to 16

  3. Separate TBM[Shared|Private]Iterator and TBMIterateResult

  4. Improve read_stream.c advice for dense streams.

  5. Increase default effective_io_concurrency to 16

  6. Delay extraction of TIDBitmap per page offsets

  7. Add lossy indicator to TBMIterateResult

  8. Move BitmapTableScan per-scan setup into a helper

  9. Add and use BitmapHeapScanDescData struct

  10. Fix bitmap table scan crash on iterator release

  11. Bitmap Table Scans use unified TBMIterator

  12. Add common interface for TBMIterators

  13. Make table_scan_bitmap_next_block() async-friendly

  14. Move EXPLAIN counter increment to heapam_scan_bitmap_next_block

  15. Refactor tidstore.c iterator buffering.

  16. BitmapHeapScan: Remove incorrect assert and reset field

  17. Change BitmapAdjustPrefetchIterator to accept BlockNumber

  18. BitmapHeapScan: Use correct recheck flag for skip_fetch

  19. BitmapHeapScan: Push skip_fetch optimization into table AM

  20. BitmapHeapScan: postpone setting can_skip_fetch

  21. BitmapHeapScan: begin scan after bitmap creation

  22. Fix EXPLAIN Bitmap heap scan to count pages with no visible tuples

  23. Remove redundant snapshot copying from parallel leader to workers

  24. Remove some obsolete smgrcloseall() calls.

  25. Remove the "snapshot too old" feature.

  26. Compute XID horizon for page level index vacuum on primary.