Horton

Re: BitmapHeapScan streaming read user and prelim refactoring

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Tomas Vondra <tomas@vondra.me>
Cc: Melanie Plageman <melanieplageman@gmail.com>, 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: 2025-02-10T21:24:25Z
Lists: pgsql-hackers 
On Mon, Feb 10, 2025 at 1:11 PM Tomas Vondra <tomas@vondra.me> wrote:
> Certainly for the "localized" regressions, and cases when bitmapheapscan
> would not be picked. The eic=1 case makes me a bit more nervous, because
> it's default and affects NVMe storage. Would be good to know why is
> that, or perhaps consider bumping up eic default. Not sure.

I'm relatively upset by the fact that effective_io_concurrency is
measured in units that are, AFAIUI, completely stupid. The original
idea was that you would set it to the number of spindles you have. But
from what I have heard, that didn't actually work: you needed to set
it to a significantly higher number. But in 2025, you probably don't
even have spindles any more, because you're probably on SSD or some
other modern storage medium rather than a rotating hard drive. And if
you do have spindles, do you know how many you have? Is that even a
meaningful concept?

I am not saying that it's this patch's job to replace
effective_io_concurrency with something better. I think adopting the
streaming read interface is pretty important, and if it works out that
we should also change the default effective_io_concurrency from 1 to 2
or 17 or 42715 in the same release, fine. But I think that in the
slightly longer term, it would be a really good idea for someone to
propose a more sensible model. It's hard to think of a clearer case of
a parameter being rendered meaningless by the march of technology. The
closest analogue that comes to mind is our sorting implementation used
to have a hard coded number of tape drives, when the underlying
implementation was a bunch of files all on the same filesystem, but
that wasn't a user-settable parameter.

-- 
Robert Haas
EDB: http://www.enterprisedb.com

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.