Horton

Re: BitmapHeapScan streaming read user and prelim refactoring

Thomas Munro <thomas.munro@gmail.com>

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From: Thomas Munro <thomas.munro@gmail.com>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: Tomas Vondra <tomas@vondra.me>, Nazir Bilal Yavuz <byavuz81@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Andres Freund <andres@anarazel.de>, Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2025-02-21T00:32:20Z
Lists: pgsql-hackers 
On Fri, Feb 21, 2025 at 11:17 AM Melanie Plageman
<melanieplageman@gmail.com> wrote:
> [v31-0001-Delay-extraction-of-TIDBitmap-per-page-offsets.patch]

Nice patch, seems like a straightforward win with the benefits you
explained: less work done under a lock, less work done in the second
iterator if the rest of this stuff doesn't make it in yet, and less
space used assuming it does.  My only comment is that I'm not sure
about this type:

+/*
+ * The tuple OffsetNumbers extracted from a single page in the bitmap.
+ */
+typedef OffsetNumber TBMOffsets[TBM_MAX_TUPLES_PER_PAGE];

It's used for a stack object, but it's also used as a degraded pointer here:

+extern void tbm_extract_page_tuple(TBMIterateResult *iteritem,
+
TBMOffsets offsets);

Maybe a personal style question but degraded pointers aren't my
favourite C feature.  A couple of alternatives, I am not sure how good
they are:  Would this output variable be better wrapped up in a new
struct for better API clarity?  If so, shouldn't such a struct also
hold ntuples itself, since that really goes with the offsets it's
holding?  If so, could iteritem->ntuples then be replaced with a
boolean iteritem->lossy, since all you really need to be able to check
is whether it has offsets to give you, instead of using -1 and -2 for
that?  Or as a variation without a new struct, would it be better to
take that output array as a plain pointer to OffsetNumber and
max_offsets, and return ntuples (ie how many were filled in, cf commit
f6bef362), for the caller to use in a loop or stash somewhere, with an
additional comment that TBM_MAX_TUPLES_PER_PAGE is guaranteed to be
enough, for convenience eg for arrays on the stack?

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.