Re: logical decoding and replication of sequences

On 12/15/21 14:51, Tomas Vondra wrote:
> On 12/15/21 14:20, Amit Kapila wrote:
>> On Tue, Dec 14, 2021 at 7:02 AM Tomas Vondra
>> <tomas.vondra@enterprisedb.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.
> 

Apologies, sent too early ... I think it's more complicated for logical
sync replication, because of a scenario like this:

  BEGIN;
  SELECT nextval('s') FROM generate_series(1,100);  <-- writes WAL
  ROLLBACK;

  SELECT nextval('s');

The first transaction advances the sequence enough to generate a WAL,
which we do every 32 values. But it's rolled back, so it does not update
LogwrtResult.Write, because that happens only at commit.

And then the nextval() generates a value from the sequence without
generating WAL, so it doesn't update the LSN either (IIRC). That'd mean
a sync replication may not wait for this change to reach the subscriber.


regards

-- 
Tomas Vondra
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