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: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: Melanie Plageman <melanieplageman@gmail.com>, Andres Freund <andres@anarazel.de>, Pg Hackers <pgsql-hackers@postgresql.org>, Heikki Linnakangas <hlinnaka@iki.fi>, Nazir Bilal Yavuz <byavuz81@gmail.com>
Date: 2024-03-14T20:58:09Z
Lists: pgsql-hackers 
On Fri, Mar 15, 2024 at 3:18 AM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
> So, IIUC this means (1) the patched code is more aggressive wrt
> prefetching (because we prefetch more data overall, because master would
> prefetch N pages and patched prefetches N ranges, each of which may be
> multiple pages. And (2) it's not easy to quantify how much more
> aggressive it is, because it depends on how we happen to coalesce the
> pages into ranges.
>
> Do I understand this correctly?

Yes.

Parallelism must prevent coalescing here though.  Any parallel aware
executor node that allocates block numbers to workers without trying
to preserve ranges will.  That not only hides the opportunity to
coalesce reads, it also makes (globally) sequential scans look random
(ie locally they are more random), so that our logic to avoid issuing
advice for sequential scan won't work, and we'll inject extra useless
or harmful (?) fadvise calls.  I don't know what to do about that yet,
but it seems like a subject for future research.  Should we recognise
sequential scans with a window (like Linux does), instead of strictly
next-block detection (like some other OSes do)?  Maybe a shared
streaming read that all workers pull blocks from, so it can see what's
going on?  I think the latter would be strictly more like what the ad
hoc BHS prefetching code in master is doing, but I don't know if it'd
be over-engineering, or hard to do for some reason.

Another aspect of per-backend streaming reads in one parallel query
that don't know about each other is that they will all have their own
effective_io_concurrency limit.  That is a version of a problem that
comes up again and again in parallel query, to be solved by the grand
unified resource control system of the future.

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.