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: 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: 2024-04-07T14:41:59Z
Lists: pgsql-hackers 

On 4/7/24 16:24, Melanie Plageman wrote:
> On Sun, Apr 7, 2024 at 7:38 AM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
>>
>>
>>
>> On 4/7/24 06:17, Melanie Plageman wrote:
>>> On Sun, Apr 07, 2024 at 02:27:43AM +0200, Tomas Vondra wrote:
>>>> On 4/6/24 23:34, Melanie Plageman wrote:
>>>>> ...
>>>>>>
>>>>>> I realized it makes more sense to add a FIXME (I used XXX. I'm not when
>>>>>> to use what) with a link to the message where Andres describes why he
>>>>>> thinks it is a bug. If we plan on fixing it, it is good to have a record
>>>>>> of that. And it makes it easier to put a clear and accurate comment.
>>>>>> Done in 0009.
>>>>>>
>>>>>>> OK, thanks. If think 0001-0008 are ready to go, with some minor tweaks
>>>>>>> per above (tuple vs. tuples etc.), and the question about the recheck
>>>>>>> flag. If you can do these tweaks, I'll get that committed today and we
>>>>>>> can try to get a couple more patches in tomorrow.
>>>>>
>>>>> Attached v19 rebases the rest of the commits from v17 over the first
>>>>> nine patches from v18. All patches 0001-0009 are unchanged from v18. I
>>>>> have made updates and done cleanup on 0010-0021.
>>>>>
>>>>
>>>> I've pushed 0001-0005, I'll get back to this tomorrow and see how much
>>>> more we can get in for v17.
>>>
>>> Thanks! I thought about it a bit more, and I got worried about the
>>>
>>>       Assert(scan->rs_empty_tuples_pending == 0);
>>>
>>> in heap_rescan() and heap_endscan().
>>>
>>> I was worried if we don't complete the scan it could end up tripping
>>> incorrectly.
>>>
>>> I tried to come up with a query which didn't end up emitting all of the
>>> tuples on the page (using a LIMIT clause), but I struggled to come up
>>> with an example that qualified for the skip fetch optimization and also
>>> returned before completing the scan.
>>>
>>> I could work a bit harder tomorrow to try and come up with something.
>>> However, I think it might be safer to just change these to:
>>>
>>>       scan->rs_empty_tuples_pending = 0
>>>
>>
>> Hmmm, good point. I haven't tried, but wouldn't something like "SELECT 1
>> FROM t WHERE column = X LIMIT 1" do the trick? Probably in a join, as a
>> correlated subquery?
> 
> Unfortunately (or fortunately, I guess) that exact thing won't work
> because even constant values in the target list disqualify it for the
> skip fetch optimization.
> 
> Being a bit too lazy to look at planner code this morning, I removed
> the target list requirement like this:
> 
> -           need_tuples = (node->ss.ps.plan->qual != NIL ||
> -                          node->ss.ps.plan->targetlist != NIL);
> +           need_tuples = (node->ss.ps.plan->qual != NIL);
> 
> And can easily trip the assert with this:
> 
> create table foo (a int);
> insert into foo select i from generate_series(1,10)i;
> create index on foo(a);
> vacuum foo;
> select 1 from (select 2 from foo limit 3);
> 
> Anyway, I don't know if we could find a query that does actually hit
> this. The only bitmap heap scan queries in the regress suite that meet
> the
>    BitmapHeapScanState->ss.ps.plan->targetlist == NIL
> condition are aggregates (all are count(*)).
> 
> I'll dig a bit more later, but do you think this is worth adding an
> open item for? Even though I don't have a repro yet?
> 

Try this:

create table t (a int, b int) with (fillfactor=10);
insert into t select mod((i/22),2), (i/22) from generate_series(0,1000)
S(i);
create index on t(a);
vacuum analyze t;

set enable_indexonlyscan = off;
set enable_seqscan = off;
explain (analyze, verbose) select 1 from (values (1)) s(x) where exists
(select * from t where a = x);

KABOOM!

#2  0x000078a16ac5fafe in __GI_raise (sig=sig@entry=6) at
../sysdeps/posix/raise.c:26
#3  0x000078a16ac4887f in __GI_abort () at abort.c:79
#4  0x0000000000bb2c5a in ExceptionalCondition (conditionName=0xc42ba8
"scan->rs_empty_tuples_pending == 0", fileName=0xc429c8 "heapam.c",
lineNumber=1090) at assert.c:66
#5  0x00000000004f68bb in heap_endscan (sscan=0x19af3a0) at heapam.c:1090
#6  0x000000000077a94c in table_endscan (scan=0x19af3a0) at
../../../src/include/access/tableam.h:1001

So yeah, this assert is not quite correct. It's not breaking anything at
the moment, so we can fix it now or add it as an open item.


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.