Horton

Re: BitmapHeapScan streaming read user and prelim refactoring

Melanie Plageman <melanieplageman@gmail.com>

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From: Melanie Plageman <melanieplageman@gmail.com>
To: Thomas Munro <thomas.munro@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: 2025-02-19T21:14:30Z
Lists: pgsql-hackers

Attachments

On Thu, Feb 13, 2025 at 5:51 PM Thomas Munro <thomas.munro@gmail.com> wrote:
>
> On Fri, Feb 14, 2025 at 5:52 AM Melanie Plageman
> <melanieplageman@gmail.com> wrote:
> > On Thu, Feb 13, 2025 at 11:28 AM Tomas Vondra <tomas@vondra.me> wrote:
> > > On 2/13/25 17:01, Melanie Plageman wrote:
> > > I know it's not changing how much memory we allocate (compared to
> > > master). I haven't thought about the GinScanEntry - yes, flexible array
> > > member would make this a bit more complex.
> >
> > Oh, I see. I didn't understand Thomas' proposal. I don't know how hard
> > it would be to make tidbitmap allocate the offsets on-demand. I'd need
> > to investigate more. But probably not worth it for this patch.
>
> No, what I meant was:  It would be nice to try to hold only one
> uncompressed result set in memory at a time, like we achieved in the
> vacuum patches.  The consumer expands them from a tiny object when the
> associated buffer pops out the other end.  That should be possible
> here too, right, because the bitmap is immutable and long lived, so
> you should be able to stream (essentially) pointers into its internal
> guts.  The current patch streams the uncompressed data itself, and
> thus has to reserve space for the maximum possible amount of it, and
> also forces you to think about fixed sizes.

Attached v30 makes the tuple offset extraction happen later as you
suggested. It turns out that none of the users need to worry much
about allocating and freeing -- I was able to have all users make an
offsets array on the stack. Personally I don't think we should worry
about making a smaller array when fewer offsets are needed.

Even without the read stream API, in bitmap heap scan, delaying
extraction of the offsets lets us skip doing so for the prefetch
iterator -- so that's kind of cool. Anyway, we are going to get rid of
the prefetch iterator, so I won't belabor the point.

I was worried the patch would be too big of a change to the tidbitmap
API for this late in the cycle, but it turned out to be a small patch.
I'd be interested in what folks think.

- Melanie

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.