Thread

Re: [BUG] Incorrect historic snapshot may be serialized to diskduring fast-forwarding

cca5507 <cca5507@qq.com> — 2025-12-30T02:57:46Z

Hi,

> While I agree with your analysis, I'm not sure what actual problems it
> could lead to in practice. Have you had a chance to reproduce this
> behavior by using DDLs instead of a user-catalog table?

I'm not sure if this can be reproduced by using DDLs, but I will try.

> IIUC the problem can occur if a transaction makes catalog changes and writes
> only NEW_CID WAL records without INVALIDATION WAL records.

+1

> However, I'm not sure there are such transactions in practice.

Maybe currently not, but what about future, I'm not sure yet.

> IIUC it would not be a problem in terms of logical decoding even if we
> don't include their XIDs to the snapshot if they change only user-catalog
> tables. It might be more future proof to mark transactions as catalog-changed
> even when fast-forwarding a NEW_CID record, as you proposed, but I'd
> like to confirm the actual problems first.

Our current design of historic snapshot is to track all catalogs even if only a part
of them are useful for logical decoding. So I think this bug breaks our design even
if it's maybe ok in practice.

--
Regards,
ChangAo Chen