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: Tomas Vondra <tomas.vondra@enterprisedb.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: 2024-04-03T22:57:59Z
Lists: pgsql-hackers

Attachments

On Sun, Mar 31, 2024 at 11:45:51AM -0400, Melanie Plageman wrote:
> On Fri, Mar 29, 2024 at 12:05:15PM +0100, Tomas Vondra wrote:
> > 
> > 
> > On 3/29/24 02:12, Thomas Munro wrote:
> > > On Fri, Mar 29, 2024 at 10:43 AM Tomas Vondra
> > > <tomas.vondra@enterprisedb.com> wrote:
> > >> I think there's some sort of bug, triggering this assert in heapam
> > >>
> > >>   Assert(BufferGetBlockNumber(hscan->rs_cbuf) == tbmres->blockno);
> > > 
> > > Thanks for the repro.  I can't seem to reproduce it (still trying) but
> > > I assume this is with Melanie's v11 patch set which had
> > > v11-0016-v10-Read-Stream-API.patch.
> > > 
> > > Would you mind removing that commit and instead applying the v13
> > > stream_read.c patches[1]?  v10 stream_read.c was a little confused
> > > about random I/O combining, which I fixed with a small adjustment to
> > > the conditions for the "if" statement right at the end of
> > > read_stream_look_ahead().  Sorry about that.  The fixed version, with
> > > eic=4, with your test query using WHERE a < a, ends its scan with:
> > > 
> > 
> > I'll give that a try. Unfortunately unfortunately the v11 still has the
> > problem I reported about a week ago:
> > 
> >   ERROR:  prefetch and main iterators are out of sync
> > 
> > So I can't run the full benchmarks :-( but master vs. streaming read API
> > should work, I think.
> 
> Odd, I didn't notice you reporting this ERROR popping up. Now that I
> take a look, v11 (at least, maybe also v10) had this very sill mistake:
> 
>   if (scan->bm_parallel == NULL &&
>       scan->rs_pf_bhs_iterator &&
>       hscan->pfblockno > hscan->rs_base.blockno)
>        elog(ERROR, "prefetch and main iterators are out of sync");
> 
> It errors out if the prefetch block is ahead of the current block --
> which is the opposite of what we want. I've fixed this in attached v12.
> 
> This version also has v13 of the streaming read API. I noticed one
> mistake in my bitmapheap scan streaming read user -- it freed the
> streaming read object at the wrong time. I don't know if this was
> causing any other issues, but it at least is fixed in this version.

Attached v13 is rebased over master (which includes the streaming read
API now). I also reset the streaming read object on rescan instead of
creating a new one each time.

I don't know how much chance any of this has of going in to 17 now, but
I thought I would start looking into the regression repro Tomas provided
in [1].

I'm also not sure if I should try and group the commits into fewer
commits now or wait until I have some idea of whether or not the
approach in 0013 and 0014 is worth pursuing.

- Melanie

[1] https://www.postgresql.org/message-id/5d5954ed-6f43-4f1a-8e19-ece75b2b7362%40enterprisedb.com

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.