Horton

Re: BitmapHeapScan streaming read user and prelim refactoring

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Alexander Lakhin <exclusion@gmail.com>
Cc: Melanie Plageman <melanieplageman@gmail.com>, Thomas Munro <thomas.munro@gmail.com>, Tomas Vondra <tomas@vondra.me>, Nazir Bilal Yavuz <byavuz81@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2025-03-22T20:42:42Z
Lists: pgsql-hackers 
Hi,

On 2025-03-22 16:14:11 -0400, Andres Freund wrote:
> On 2025-03-22 22:00:00 +0200, Alexander Lakhin wrote:
> > @@ -24,14 +24,14 @@
> >  SELECT count(*) FROM bmscantest WHERE a = 1 AND b = 1;
> >   count
> >  -------
> > -    23
> > +    18
> >  (1 row)
> 
> Is it possible that this is the bug reported here:
> https://postgr.es/m/873c33c5-ef9e-41f6-80b2-2f5e11869f1c%40garret.ru
> 
> I.e. that the all-visible logic in bitmap heapscans is fundamentally broken?
> 
> I can reproduce different results on a fast machien by just putting a VACUUM
> FREEZE bmscantest after the CREATE INDEXes in bitmapops.sql. After I disable
> the all-visible logic in bitmapheap_stream_read_next(), I can't observe such a
> difference anymore.

Hm, it's clearly related to the all-visible path, but I think the bug is
actually independent of what was reported there. The bug you reported is
perfectly reproducible, without any concurrency, after all.

Here's a smaller reproducer:

CREATE TABLE bmscantest (a int, t text);

INSERT INTO bmscantest
  SELECT (r%53), 'foooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo'
  FROM generate_series(1,70000) r;

CREATE INDEX i_bmtest_a ON bmscantest(a);
set enable_indexscan=false;
set enable_seqscan=false;

-- Lower work_mem to trigger use of lossy bitmaps
set work_mem = 64;

SELECT count(*) FROM bmscantest WHERE a = 1;
vacuum freeze bmscantest;
SELECT count(*) FROM bmscantest WHERE a = 1;

-- clean up
DROP TABLE bmscantest;


The first SELECT reports 1321, the second 572 tuples.

Greetings,

Andres Freund

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.