Horton

Re: BitmapHeapScan streaming read user and prelim refactoring

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: Tomas Vondra <tomas.vondra@enterprisedb.com>, Thomas Munro <thomas.munro@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Andres Freund <andres@anarazel.de>, Pg Hackers <pgsql-hackers@postgresql.org>, Nazir Bilal Yavuz <byavuz81@gmail.com>
Date: 2024-04-06T14:59:07Z
Lists: pgsql-hackers 
Melanie Plageman <melanieplageman@gmail.com> writes:
> On Sat, Apr 06, 2024 at 02:51:45AM +0200, Tomas Vondra wrote:
>> * The one question I'm somewhat unsure about is why Tom chose to use the
>> "wrong" recheck flag in the 2017 commit, when the correct recheck flag
>> is readily available. Surely that had a reason, right? But I can't think
>> of one ...

> See above.

Hi, I hadn't been paying attention to this thread, but Melanie pinged
me off-list about this question.  I think it's just a flat-out
oversight in 7c70996eb.  Looking at the mailing list thread
(particularly [1][2]), it seems that Alexander hadn't really addressed
the question of when to prefetch at all, but just skipped prefetch if
the current page was skippable:

+		/*
+		 * If we did not need to fetch the current page,
+		 * we probably will not need to fetch the next.
+		 */
+		return;

It looks like I noticed that we could check the appropriate VM bits,
but failed to notice that we could easily check the appropriate
recheck flag as well.  Feel free to change it.

			regards, tom lane

[1] https://www.postgresql.org/message-id/a6434d5c-ed8d-b09c-a7c3-b2d1677e35b3%40postgrespro.ru
[2] https://www.postgresql.org/message-id/5974.1509573988%40sss.pgh.pa.us

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.