Horton

Re: BitmapHeapScan streaming read user and prelim refactoring

Mark Dilger <mark.dilger@enterprisedb.com>

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From: Mark Dilger <mark.dilger@enterprisedb.com>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: Pg Hackers <pgsql-hackers@postgresql.org>, Andres Freund <andres@anarazel.de>, Thomas Munro <thomas.munro@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Nazir Bilal Yavuz <byavuz81@gmail.com>
Date: 2024-02-14T04:34:20Z
Lists: pgsql-hackers 

> On Feb 13, 2024, at 3:11 PM, Melanie Plageman <melanieplageman@gmail.com> wrote:

Thanks for the patch...

> Attached is a patch set which refactors BitmapHeapScan such that it
> can use the streaming read API [1]. It also resolves the long-standing
> FIXME in the BitmapHeapScan code suggesting that the skip fetch
> optimization should be pushed into the table AMs. Additionally, it
> moves table scan initialization to after the index scan and bitmap
> initialization.
> 
> patches 0001-0002 are assorted cleanup needed later in the set.
> patches 0003 moves the table scan initialization to after bitmap creation
> patch 0004 is, I think, a bug fix. see [2].
> patches 0005-0006 push the skip fetch optimization into the table AMs
> patches 0007-0009 change the control flow of BitmapHeapNext() to match
> that required by the streaming read API
> patch 0010 is the streaming read code not yet in master
> patch 0011 is the actual bitmapheapscan streaming read user.
> 
> patches 0001-0009 apply on top of master but 0010 and 0011 must be
> applied on top of a commit before a 21d9c3ee4ef74e2 (until a rebased
> version of the streaming read API is on the mailing list).

I followed your lead and applied them to 6a8ffe812d194ba6f4f26791b6388a4837d17d6c.  `make check` worked fine, though I expect you know that already.

> The caveat is that these patches introduce breaking changes to two
> table AM functions for bitmapheapscan: table_scan_bitmap_next_block()
> and table_scan_bitmap_next_tuple().

You might want an independent perspective on how much of a hassle those breaking changes are, so I took a stab at that.  Having written a custom proprietary TAM for postgresql 15 here at EDB, and having ported it and released it for postgresql 16, I thought I'd try porting it to the the above commit with your patches.  Even without your patches, I already see breaking changes coming from commit f691f5b80a85c66d715b4340ffabb503eb19393e, which creates a similar amount of breakage for me as does your patches.  Dealing with the combined breakage might amount to a day of work, including testing, half of which I think I've already finished.  In other words, it doesn't seem like a big deal.

Were postgresql 17 shaping up to be compatible with TAMs written for 16, your patch would change that qualitatively, but since things are already incompatible, I think you're in the clear.

> A TBMIterateResult used to be threaded through both of these functions
> and used in BitmapHeapNext(). This patch set removes all references to
> TBMIterateResults from BitmapHeapNext. Because the streaming read API
> requires the callback to specify the next block, BitmapHeapNext() can
> no longer pass a TBMIterateResult to table_scan_bitmap_next_block().
> 
> More subtly, table_scan_bitmap_next_block() used to return false if
> there were no more visible tuples on the page or if the block that was
> requested was not valid. With these changes,
> table_scan_bitmap_next_block() will only return false when the bitmap
> has been exhausted and the scan can end. In order to use the streaming
> read API, the user must be able to request the blocks it needs without
> requiring synchronous feedback per block. Thus, this table AM function
> must change its meaning.
> 
> I think the way the patches are split up could be improved. I will
> think more about this. There are also probably a few mistakes with
> which comments are updated in which patches in the set.

I look forward to the next version of the patch set.  Thanks again for working on this.

—
Mark Dilger
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.