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: 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: 2024-04-07T00:27:43Z
Lists: pgsql-hackers 
On 4/6/24 23:34, Melanie Plageman wrote:
> ...
>>
>> I realized it makes more sense to add a FIXME (I used XXX. I'm not when
>> to use what) with a link to the message where Andres describes why he
>> thinks it is a bug. If we plan on fixing it, it is good to have a record
>> of that. And it makes it easier to put a clear and accurate comment.
>> Done in 0009.
>>
>>> OK, thanks. If think 0001-0008 are ready to go, with some minor tweaks
>>> per above (tuple vs. tuples etc.), and the question about the recheck
>>> flag. If you can do these tweaks, I'll get that committed today and we
>>> can try to get a couple more patches in tomorrow.
> 
> Attached v19 rebases the rest of the commits from v17 over the first
> nine patches from v18. All patches 0001-0009 are unchanged from v18. I
> have made updates and done cleanup on 0010-0021.
> 

I've pushed 0001-0005, I'll get back to this tomorrow and see how much
more we can get in for v17.

What bothers me on 0006-0008 is that the justification in the commit
messages is "future commit will do something". I think it's fine to have
a separate "prepareatory" patches (I really like how you structured the
patches this way), but it'd be good to have them right before that
"future" commit - I'd like not to have one in v17 and then the "future
commit" in v18, because that's annoying complication for backpatching,
(and probably also when implementing the AM?) etc.

AFAICS for v19, the "future commit" for all three patches (0006-0008) is
0012, which introduces the unified iterator. Is that correct?

Also, for 0008 I'm not sure we could even split it between v17 and v18,
because even if heapam did not use the iterator, what if some other AM
uses it? Without 0012 it'd be a problem for the AM, no?

Would it make sense to move 0009 before these three patches? That seems
like a meaningful change on it's own, right?

FWIW I don't think it's very likely I'll commit the UnifiedTBMIterator
stuff. I do agree with the idea in general, but I think I'd need more
time to think about the details. Sorry about that ...


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.