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From: Hayato Kuroda (Fujitsu) <[email protected]>
To: 'Michail Nikolaev' <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: Amit Kapila <[email protected]>
Subject: RE: [BUG?] check_exclusion_or_unique_constraint false negative
Date: Thu, 1 Aug 2024 05:54:37 +0000
Message-ID: <TYAPR01MB56921C9C3D21B0D62FF76330F5B22@TYAPR01MB5692.jpnprd01.prod.outlook.com> (raw)
In-Reply-To: <CANtu0ohUB9ky45iiMAYN1fGyt82+cg=+UYBom=P7drb+=97G9w@mail.gmail.com>
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Dear Michail,

Thanks for pointing out the issue!

>* RelationFindReplTupleByIndex
>
>Amit, this is why I've included you in this previously solo thread :)
>RelationFindReplTupleByIndex uses DirtySnapshot and may not find some records
>if they are updated by a parallel transaction. This could lead to lost
>deletes/updates, especially in the case of streaming=parallel mode. 
>I'm not familiar with how parallel workers apply transactions, so maybe this
>isn't possible.

IIUC, the issue can happen when two concurrent transactions using DirtySnapshot access
the same tuples, which is not specific to the parallel apply. Consider that two
subscriptions exist and publishers modify the same tuple of the same table.
In this case, two workers access the tuple, so one of the changes may be missed
by the scenario you said. I feel we do not need special treatments for parallel
apply.

Best regards,
Hayato Kuroda
FUJITSU LIMITED

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