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>, Heikki Linnakangas <hlinnaka@iki.fi>
Cc: Andres Freund <andres@anarazel.de>, Pg Hackers <pgsql-hackers@postgresql.org>, Thomas Munro <thomas.munro@gmail.com>, Nazir Bilal Yavuz <byavuz81@gmail.com>
Date: 2024-03-20T18:13:05Z
Lists: pgsql-hackers 
On 3/18/24 16:19, Melanie Plageman wrote:
> On Mon, Mar 18, 2024 at 02:10:28PM +0200, Heikki Linnakangas wrote:
>> On 14/02/2024 21:42, Andres Freund wrote:
>>> On 2024-02-13 18:11:25 -0500, Melanie Plageman wrote:
>>>> patch 0004 is, I think, a bug fix. see [2].
>>>
>>> I'd not quite call it a bugfix, it's not like it leads to wrong
>>> behaviour. Seems more like an optimization. But whatever :)
>>
>> It sure looks like bug to me, albeit a very minor one. Certainly not an
>> optimization, it doesn't affect performance in any way, only what EXPLAIN
>> reports. So committed and backported that to all supported branches.
> 
> I've attached v7 rebased over this commit.
>

I've started a new set of benchmarks with v7 (on top of f69319f2f1), but
unfortunately that results in about 15% of the queries failing with:

  ERROR:  prefetch and main iterators are out of sync

Reproducing it is pretty simple (at least on my laptop). Simply apply
0001-0011, and then do this:

======================================================================
create table test_table (a bigint, b bigint, c text) with (fillfactor = 25);

insert into test_table select 10000 * random(), i, md5(random()::text)
from generate_series(1, 1000000) s(i);

create index on test_table(a);

vacuum analyze ;
checkpoint;

set work_mem = '128kB';
set effective_io_concurrency = 8;
set random_page_cost = 2;
set max_parallel_workers_per_gather = 0;

explain select * from test_table where a >= 1 and a <= 512;

explain analyze select * from test_table where a >= 1 and a <= 512;

ERROR:  prefetch and main iterators are out of sync
======================================================================

I haven't investigated this, but it seems to get broken by this patch:

  v7-0009-Make-table_scan_bitmap_next_block-async-friendly.patch

I wonder if there are some additional changes aside from the rebase.


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.