Re: logical decoding and replication of sequences, take 2

On 3/18/23 06:35, Amit Kapila wrote:
> On Sat, Mar 18, 2023 at 3:13 AM Tomas Vondra
> <tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
>>
>> ...
>>
>> Clearly, for sequences we can't quite rely on snapshots/slots, we need
>> to get the LSN to decide what changes to apply/skip from somewhere else.
>> I wonder if we can just ignore the queued changes in tablesync, but I
>> guess not - there can be queued increments after reading the sequence
>> state, and we need to apply those. But maybe we could use the page LSN
>> from the relfilenode - that should be the LSN of the last WAL record.
>>
>> Or maybe we could simply add pg_current_wal_insert_lsn() into the SQL we
>> use to read the sequence state ...
>>
>
> What if some Alter Sequence is performed before the copy starts and
> after the copy is finished, the containing transaction rolled back?
> Won't it copy something which shouldn't have been copied?
>

That shouldn't be possible - the alter creates a new relfilenode and
it's invisible until commit. So either it gets committed (and then
replicated), or it remains invisible to the SELECT during sync.

regards

--
Tomas Vondra
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