Re: logical decoding and replication of sequences, take 2

On 6/23/23 15:18, Ashutosh Bapat wrote:
> ...
>
> I reviewed 0001 and related parts of 0004 and 0008 in detail.
>
> I have only one major change request, about
> typedef struct xl_seq_rec
> {
> RelFileLocator locator;
> + bool created; /* creates a new relfilenode (CREATE/ALTER) */
>
> I am not sure what are the repercussions of adding a member to an existing WAL
> record. I didn't see any code which handles the old WAL format which doesn't
> contain the "created" flag. IIUC, the logical decoding may come across
> a WAL record written in the old format after upgrade and restart. Is
> that not possible?
>

I don't understand why would adding a new field to xl_seq_rec be an
issue, considering it's done in a new major version. Sure, if you
generate WAL with old build, and start with a patched version, that
would break things. But that's true for many other patches, and it's
irrelevant for releases.

> But I don't think it's necessary. We can add a
> decoding routine for RM_SMGR_ID. The decoding routine will add relfilelocator
> in XLOG_SMGR_CREATE record to txn->sequences hash. Rest of the logic will work
> as is. Of course we will add non-sequence relfilelocators as well but that
> should be fine. Creating a new relfilelocator shouldn't be a frequent
> operation. If at all we are worried about that, we can add only the
> relfilenodes associated with sequences to the hash table.
>

Hmmmm, that might work. I feel a bit uneasy about having to keep all
relfilenodes, not just sequences ...

regards

--
Tomas Vondra
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