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: 2024-05-11T19:18:21Z
Lists: pgsql-hackers 
On 5/10/24 21:48, Melanie Plageman wrote:
> On Thu, May 2, 2024 at 5:37 PM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
>>
>>
>>
>> On 4/24/24 22:46, Melanie Plageman wrote:
>>> On Tue, Apr 23, 2024 at 6:43 PM Tomas Vondra
>>> <tomas.vondra@enterprisedb.com> wrote:
>>>>
>>>> On 4/23/24 18:05, Melanie Plageman wrote:
>>>>> 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).
>>>
>>> I suppose it's true that other tests in this file use joins to test
>>> other code. I guess if we limited join.sql to containing tests of join
>>> implementation, it would be rather small. I just imagined it would be
>>> nice if tests were grouped by what they were testing -- not how they
>>> were testing it.
>>>
>>>> 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.
>>>
>>> You mean I should explain in the test comment why I included the
>>> EXPLAIN plan output? (because it needs to contain a bitmapheapscan to
>>> actually be testing anything)
>>>
>>
>> No, I meant that the comment before the test describes a couple
>> requirements the plan needs to meet (no need to process all inner
>> tuples, bitmapscan eligible for skip_fetch on outer side, ...), but it
>> does not explain why we're testing that plan.
>>
>> I could get to that by doing git-blame to see what commit added this
>> code, and then read the linked discussion. Perhaps that's enough, but
>> maybe the comment could say something like "verify we properly discard
>> tuples on rescans" or something like that?
> 
> Attached is v3. I didn't use your exact language because the test
> wouldn't actually verify that we properly discard the tuples. Whether
> or not the empty tuples are all emitted, it just resets the counter to
> 0. I decided to go with "exercise" instead.
> 

I did go over the v3 patch, did a bunch of tests, and I think it's fine
and ready to go. The one thing that might need some minor tweaks is the
commit message.

1) Isn't the subject "Remove incorrect assert" a bit misleading, as the
patch does not simply remove an assert, but replaces it with a reset of
the field the assert used to check? (The commit message does not mention
this either, at least not explicitly.)

2) The "heap AM-specific bitmap heap scan code" sounds a bit strange to
me, isn't the first "heap" unnecessary?


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.