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From: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Date: Tue, 26 Aug 2025 11:02:01 +0200
Message-ID: 
 <CADzfLwV+80MfPM=MC5y3nA34djUWuYU6YKcZUO8JjD7_8p7nkg@mail.gmail.com>
Subject: Re: Adding REPACK [concurrently]
To: Antonin Houska <ah@cybertec.at>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>,
 Fujii Masao <masao.fujii@gmail.com>,
	Robert Treat <rob@xzilla.net>,
 Pg Hackers <pgsql-hackers@lists.postgresql.org>
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Antonin Houska <ah@cybertec.at>:

> Although it could work, I think it'd be confusing to consider the transactions
> being replayed as "current" from the point of view of the backend that
> executes REPACK CONCURRENTLY.

Just realized SnapshotDirty is the thing that fits into the role - it
respects not-yet committed transactions, giving enough information to
wait for them.
It is already used in a similar pattern in
check_exclusion_or_unique_constraint and RelationFindReplTupleByIndex.

So, it is easy to detect the case of the race you described previously
and retry + there is no sense to hack around
TransactionIdIsCurrentTransactionId.

BWT, btree + SnapshotDirty has issue [0], but it is a different story
and happens only with concurrent updates which are not present in the
current scope.

[0]: https://www.postgresql.org/message-id/flat/CADzfLwXGhH_qD6RGqPyEeKdmHgr-HpA-tASYdi5onP%2BRyP5TCw%40mail.gmail.com#77f6426ef2d282198f2d930d5334e3fa