Re: logical decoding and replication of sequences, take 2

On 11/27/23 13:08, Hayato Kuroda (Fujitsu) wrote:
> Dear Amit, Tomas,
>
>>>>
>>>> I am wondering that instead of building the infrastructure to know
>>>> whether a particular change is transactional on the decoding side,
>>>> can't we have some flag in the WAL record to note whether the change
>>>> is transactional or not? I have discussed this point with my colleague
>>>> Kuroda-San and we thought that it may be worth exploring whether we
>>>> can use rd_createSubid/rd_newRelfilelocatorSubid in RelationData to
>>>> determine if the sequence is created/changed in the current
>>>> subtransaction and then record that in WAL record. By this, we need to
>>>> have additional information in the WAL record like XLOG_SEQ_LOG but we
>>>> can probably do it only with wal_level as logical.
>>>>
>>>
>>> I may not understand the proposal exactly, but it's not enough to know
>>> if it was created in the same subxact. It might have been created in
>>> some earlier subxact in the same top-level xact.
>>>
>>
>> We should be able to detect even some earlier subxact or top-level
>> xact based on rd_createSubid/rd_newRelfilelocatorSubid.
>
> Here is a small PoC patchset to help your understanding. Please see attached
> files.
>
> 0001, 0002 were not changed, and 0004 was reassigned to 0003.
> (For now, I focused only on test_decoding, because it is only for evaluation purpose.)
>
> 0004 is what we really wanted to say. is_transactional is added in WAL record, and it stores
> whether the operations is transactional. In order to distinguish the status, rd_createSubid and
> rd_newRelfilelocatorSubid are used. According to the comment, they would be a valid value
> only when the relation was changed within the transaction
> Also, sequences_hash was not needed anymore, so it and related functions were removed.
>
> How do you think?
>

I think it's an a very nice idea, assuming it maintains the current
behavior. It makes a lot of code unnecessary, etc.

regards

--
Tomas Vondra
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