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help / color / mirror / Atom feedFrom: Alexander Pyhalov <a.pyhalov@postgrespro.ru>
To: Alexander Korotkov <aekorotkov@gmail.com>
Cc: Matheus Alcantara <matheusssilv97@gmail.com>
Cc: Pgsql Hackers <pgsql-hackers@postgresql.org>
Subject: Re: Asynchronous MergeAppend
Date: Mon, 03 Aug 2026 09:41:33 +0300
Message-ID: <e8d502937d94eb6bd18e38ff1f240a05@postgrespro.ru> (raw)
In-Reply-To: <CAPpHfdsAL9oXrSP25c1XYS-ew0doUST=FU3LWj6aEG0iDw2rSA@mail.gmail.com>
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<CAPpHfdsAL9oXrSP25c1XYS-ew0doUST=FU3LWj6aEG0iDw2rSA@mail.gmail.com>
Alexander Korotkov писал(а) 2026-08-01 00:03:
> Hi!
>
> On Mon, Jul 6, 2026 at 4:42 PM Alexander Pyhalov
> <a.pyhalov@postgrespro.ru> wrote:
>> Alexander Korotkov писал(а) 2026-07-04 02:05:
>>
>> > While discovering a correctness of callback_pending flag reset in
>> > async MergeAppend, I found bug in async plain Append [1]. I've
>> > included the fix as 0001 in the current patchset. Other changes to
>> > patchset includes.
>> >
>>
>> I expect that this issue can affect both MergeAppend and Append, but
>> current test cases don't confirm this.
>> If we revert to the old behavior (setting areq->callback_pending to
>> false), the tests results of Merge Append tests
>> are not changed.
>
> Did you try the test case showed by Gleb [1]. Yet I think it's safe
> to follow the fix by Etsuro. In the revised patchset I put this into
> ExecAppendBaseAsyncProcessPending() and use for both async append and
> async merge append.
Yes, he found this case when we tested your original fix for
ExecReScanAppend() behavior.
I'm fine with following Etsuro's fix.
>
>> > 0003 contains some cleanups
>> > * Unify set_append_references and set_mergeappend_references
>> > (setrefs.c)
>> > * Merge duplicate Append/MergeAppend cases in explain.c
>> > * Merge duplicate Append/MergeAppend cases in
>> > planstate_tree_walker_impl (nodeFuncs.c)
>>
>> This looks good.
>>
>> > 0006 includes following optimizations and fixes
>> > * Fix assertion failure on MergeAppend rescan with an in-flight async
>> > request (same as 0001 but for MergeAppend)
>>
>>
>> > * Don't use ExecProcNode() for async subplans. Despite its
>> > effectiveness, it doesn't works correctly. When postgres_fdw subplan
>> > is executed by ExecProcNode() it interprets the end of async batch as
>> > end of the whole data. That effectively leads to skipping the
>> > remaining dataset after first batch (100 rows). The test is added.
>>
>> Ouch. Luckily we've catched this. postgresIterateForeignScan() doesn't
>> fetch tuples for async scan states...
>>
>> > * Make ExecReScanMergeAppend() clear ms_slots. Otherwise subsequent
>> > scans can use leftover tuples. The test is also added.
>>
>> This seems to happen only in updated version of the patch, where
>> ExecMergeAppendAsyncGetNext() relies on the fact that
>> async requests have been already sent (previously this function
>> firstly
>> set slot to NULL prior to sending requests and processing them).
>
> Yes, thank you for clarification. Are you good with this approach?
Yes, I'm fine with this.
>
>> > * Document that the needrequest fast-skip is a no-op for MergeAppend
>> > (postgres_fdw.c)
>> > * Document why create_merge_append_plan discards
>> > mark_async_capable_plan's result (createplan.c)
>> >
>> Looks good.
>>
>> ExecMergeAppendGetNextSlot() - I'd sligtly prefer to check if mplan is
>> member of as_asyncplans and assert that it's a member of
>> node->as.valid_asyncplans
>> in this case, but I think it doesn't matter much.
>
> OK, I changed to this way.
It seems you've missed the attachment.
--
Best regards,
Alexander Pyhalov,
Postgres Professional
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To: pgsql-hackers@postgresql.org
Cc: a.pyhalov@postgrespro.ru, aekorotkov@gmail.com, matheusssilv97@gmail.com
Subject: Re: Asynchronous MergeAppend
In-Reply-To: <e8d502937d94eb6bd18e38ff1f240a05@postgrespro.ru>
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