Horton

Re: BitmapHeapScan streaming read user and prelim refactoring

Tomas Vondra <tomas.vondra@enterprisedb.com>

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From: Tomas Vondra <tomas.vondra@enterprisedb.com>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>, Dilip Kumar <dilipbalaut@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Nazir Bilal Yavuz <byavuz81@gmail.com>, Andres Freund <andres@anarazel.de>, Pg Hackers <pgsql-hackers@postgresql.org>, Thomas Munro <thomas.munro@gmail.com>
Date: 2024-03-17T19:21:12Z
Lists: pgsql-hackers

Attachments

On 3/14/24 22:39, Melanie Plageman wrote:
> On Thu, Mar 14, 2024 at 5:26 PM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
>>
>> On 3/14/24 19:16, Melanie Plageman wrote:
>>> On Thu, Mar 14, 2024 at 03:32:04PM +0200, Heikki Linnakangas wrote:
>>>> ...
>>>>
>>>> Ok, committed that for now. Thanks for looking!
>>>
>>> Attached v6 is rebased over your new commit. It also has the "fix" in
>>> 0010 which moves BitmapAdjustPrefetchIterator() back above
>>> table_scan_bitmap_next_block(). I've also updated the Streaming Read API
>>> commit (0013) to Thomas' v7 version from [1]. This has the update that
>>> we theorize should address some of the regressions in the bitmapheapscan
>>> streaming read user in 0014.
>>>
>>
>> Should I rerun the benchmarks with these new patches, to see if it
>> really helps with the regressions?
> 
> That would be awesome!
> 

OK, here's a couple charts comparing the effect of v6 patches to master.
These are from 1M and 10M data sets, same as the runs presented earlier
in this thread (the 10M is still running, but should be good enough for
this kind of visual comparison).

I have results for individual patches, but 0001-0013 behave virtually
the same, so the charts show only 0012 and 0014 (vs master).

Instead of a table with color scale (used before), I used simple scatter
plots as a more compact / concise visualization. It's impossible to
identify patterns (e.g. serial vs. parallel runs), but for the purpose
of this comparison that does not matter.

And then I'll use a chart plotting "relative" time compared to master (I
find it easier to judge the relative difference than with scatter plot).

1) absolute-all - all runs (scatter plot)

2) absolute-optimal - runs where the planner would pick bitmapscan

3) relative-all - all runs (duration relative to master)

4) relative-optimal - relative, runs where bitmapscan would be picked


The 0012 results are a pretty clear sign the "refactoring patches"
behave exactly the same as master. There are a couple outliers (in
either direction), but I'd attribute those to random noise and too few
runs to smooth it out for a particular combination (especially for 10M).

What is even more obvious is that 0014 behaves *VERY* differently. I'm
not sure if this is a good thing or a problem is debatable/unclear. I'm
sure we don't want to cause regressions, but perhaps those are due to
the prefetch issue discussed elsewhere in this thread (identified by
Andres and Melanie). There are also many cases that got much faster, but
the question is whether this is due to better efficiency or maybe the
new code being more aggressive in some way (not sure).

It's however interesting the differences are way more significant (both
in terms of frequency and scale) on the older machine with SATA SSDs.


regards

-- 
Tomas Vondra
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.