Re: lastOverflowedXid does not handle transaction ID wraparound

From: Dmitry Dolgov <9erthalion6(at)gmail(dot)com>
To: Stan Hu <stanhu(at)gmail(dot)com>
Cc: pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: Re: lastOverflowedXid does not handle transaction ID wraparound
Date: 2021-10-17 16:55:21
Message-ID: 20211017165521.74xzmnqisv6rywnc@localhost
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> On Tue, Oct 12, 2021 at 09:53:22PM -0700, Stan Hu wrote:
> I described how PostgreSQL can enter into a suboverflow condition on
> the replica under a number of conditions:
> 1. A long transaction starts.
> 2. A single SAVEPOINT is issued.
> 3. Many rows are updated on the primary, and the same rows are read
> from the replica.
> I noticed that lastOverflowedXid doesn't get cleared even after all
> subtransactions have been completed. On a replica, it only seems to be
> updated via a XLOG_XACT_ASSIGNMENT, but no such message will be sent
> if subtransactions halt. If the XID wraps around again and a long
> transaction starts before lastOverflowedXid, the replica might
> unnecessarily enter in the suboverflow condition again.


that's an interesting finding, thanks for the investigation. I didn't
reproduce it fully (haven't checked the wraparound part), but indeed
lastOverflowedXid is not changing that often, only every
PGPROC_MAX_CACHED_SUBXIDS subtransactions. I wonder what would be side
effects of clearing it when the snapshot is not suboverfloved anymore?

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