Re: logical decoding and replication of sequences

On 12/15/21 14:20, Amit Kapila wrote:
> On Tue, Dec 14, 2021 at 7:02 AM Tomas Vondra
> <tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
>>
>> Hi,
>>
>> here's an updated version of the patches, dealing with almost all of the
>> issues (at least in the 0001 and 0002 parts). The main changes:
>>
>> 1) I've removed the 'created' flag from fill_seq_with_data, as
>> discussed. I don't think it's needed by any of the parts (not even 0003,
>> AFAICS). We still need it in xl_seq_rec, though.
>>
>> 2) GetCurrentTransactionId() added to sequence.c are called only with
>> wal_level=logical, to minimize the overhead.
>>
>>
>> There's still one remaining problem, that I already explained in [1].
>> The problem is that with this:
>>
>> BEGIN;
>> SELECT nextval('s') FROM generate_series(1,100);
>> ROLLBACK;
>>
>>
>> The root cause is that pg_current_wal_lsn() uses the LogwrtResult.Write,
>> which is updated by XLogFlush() - but only in RecordTransactionCommit.
>> Which makes sense, because only the committed stuff is "visible".
>>
>> But the non-transactional behavior of sequence decoding disagrees with
>> this, because now some of the changes from aborted transactions may be
>> replicated. Which means the wait_for_catchup() ends up not waiting for
>> the sequence change to be replicated. This is an issue for tests in
>> patch 0003, at least.
>>
>> My concern is this actually affects other places waiting for things
>> getting replicated :-/
>>
>
> By any chance, will this impact synchronous replication as well which
> waits for commits to be replicated?
>

Physical or logical replication? Physical is certainly not replicated.

For logical replication, it's more complicated.

> How is this patch dealing with prepared transaction case where at
> prepare we will send transactional changes and then later if rollback
> prepared happens then the publisher will rollback changes related to
> new relfilenode but subscriber would have already replayed the updated
> seqval change which won't be rolled back?
>

I'm not sure what exact scenario you are describing, but in general we
don't rollback sequence changes anyway, so this should not cause any
divergence between the publisher and subscriber.

Or are you talking about something like ALTER SEQUENCE? I think that
should trigger the transactional behavior for the new relfilenode, so
the subscriber won't see that changes.

regards

--
Tomas Vondra
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