> 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.
Hi, 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?
