Horton

Re: BitmapHeapScan streaming read user and prelim refactoring

James Hunter <james.hunter.pg@gmail.com>

View thread View on postgr.es
From: James Hunter <james.hunter.pg@gmail.com>
To: Thomas Munro <thomas.munro@gmail.com>
Cc: Melanie Plageman <melanieplageman@gmail.com>, Tomas Vondra <tomas@vondra.me>, Nazir Bilal Yavuz <byavuz81@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Andres Freund <andres@anarazel.de>, Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2025-04-09T01:46:31Z
Lists: pgsql-hackers 
On Mon, Apr 7, 2025 at 7:34 PM Thomas Munro <thomas.munro@gmail.com> wrote:
>
> On Thu, Feb 13, 2025 at 1:40 PM Melanie Plageman
> <melanieplageman@gmail.com> wrote:
> > Thomas mentioned this to me off-list, and I think he's right. We
> > likely need to rethink the way parallel bitmap heap scan workers get
> > block assignments for reading and prefetching to make it more similar
> > to parallel sequential scan. The workers should probably get
> > assignments of a range of blocks. On master, each worker does end up
> > issuing reads/fadvises for a bunch of blocks in a row -- even though
> > that isn't the intent of the parallel bitmap table scan
> > implementation. We are losing some of that with the patch -- but only
> > because it is behaving as you would expect given the implementation
> > and design. I don't consider that a blocker, though.
>
> I had a crack at this one a few weeks back, and wanted to share some
> thoughts.

Fwiw, I've had reasonably good results, in a similar situation, by
just batching + double-buffering.

Since you're using AIO (as Melanie points out) there should be no real
penalty to having a single worker request the next batch of blocks,
when the current block is used up.

Then you can separate "reading" from "prefetching": whoever reads the
last block in the current batch, prefetches the next batch.

This way, you could preserve your existing "reading" logic, and you
wouldn't need to create several new, related queues.

James

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.