Re: Conflict detection for update_deleted in logical replication

Hello Hayato,

> WaitForOlderSnapshots() waits other transactions which can access older
tuples
> than the specified (=current) transaction, right? I think it does not
solve our issue.

Oh, I actually described the idea a bit incorrectly. The goal isn’t simply
to call WaitForOlderSnapshots(slot.xmin);
rather, it’s to ensure that we wait for slot.xmin in the same way we wait
for regular snapshots (xmin).
The reason WaitForOlderSnapshots is used in ReindexConcurrently and
DefineIndex is to guarantee that any transaction
needing to view rows not included in the index has completed before the
index is marked as valid.
The same logic should apply here — we need to wait for the xmin of slot
used in conflict detection as well.

> From another point of view... this approach must fix REINDEX code, but we
should
> not modify other component of codes as much as possible. This feature is
related
> with the replication so that changes should be closed within the
replication subdir.

One possible solution here would be to register a snapshot with slot.xmin
for the worker backend.
This way, WaitForOlderSnapshots will account for it.

By the way, WaitForOlderSnapshots is also used in partitioning and other
areas for similar reasons,
so these might be good places to check for any related issues.

Best regards,
Mikhail,