Horton

Re: BitmapHeapScan streaming read user and prelim refactoring

Tomas Vondra <tomas.vondra@enterprisedb.com>

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From: Tomas Vondra <tomas.vondra@enterprisedb.com>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: Michael Paquier <michael@paquier.xyz>, 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: 2024-04-23T22:43:38Z
Lists: pgsql-hackers 

On 4/23/24 18:05, Melanie Plageman wrote:
> On Mon, Apr 22, 2024 at 1:01 PM Melanie Plageman
> <melanieplageman@gmail.com> wrote:
>>
>> On Thu, Apr 18, 2024 at 5:39 AM Tomas Vondra
>> <tomas.vondra@enterprisedb.com> wrote:
>>>
>>> On 4/18/24 09:10, Michael Paquier wrote:
>>>> On Sun, Apr 07, 2024 at 10:54:56AM -0400, Melanie Plageman wrote:
>>>>> I've added an open item [1], because what's one open item when you can
>>>>> have two? (me)
>>>>
>>>> And this is still an open item as of today.  What's the plan to move
>>>> forward here?
>>>
>>> AFAIK the plan is to replace the asserts with actually resetting the
>>> rs_empty_tuples_pending field to 0, as suggested by Melanie a week ago.
>>> I assume she was busy with the post-freeze AM reworks last week, so this
>>> was on a back burner.
>>
>> yep, sorry. Also I took a few days off. I'm just catching up today. I
>> want to pop in one of Richard or Tomas' examples as a test, since it
>> seems like it would add some coverage. I will have a patch soon.
> 
> The patch with a fix is attached. I put the test in
> src/test/regress/sql/join.sql. It isn't the perfect location because
> it is testing something exercisable with a join but not directly
> related to the fact that it is a join. I also considered
> src/test/regress/sql/select.sql, but it also isn't directly related to
> the query being a SELECT query. If there is a better place for a test
> of a bitmap heap scan edge case, let me know.
> 

I don't see a problem with adding this to join.sql - why wouldn't this
count as something related to a join? Sure, it's not like this code
matters only for joins, but if you look at join.sql that applies to a
number of other tests (e.g. there are a couple btree tests).

That being said, it'd be good to explain in the comment why we're
testing this particular plan, not just what the plan looks like.

> One other note: there is some concurrency effect in the parallel
> schedule group containing "join" where you won't trip the assert if
> all the tests in that group in the parallel schedule are run. But, if
> you would like to verify that the test exercises the correct code,
> just reduce the group containing "join".
> 

That is ... interesting. Doesn't that mean that most test runs won't
actually detect the problem? That would make the test a bit useless.


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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.