Re: pgsql: postgres_fdw: Inherit the local transaction's access/deferrable

On 2025/06/01 17:34, Etsuro Fujita wrote:
> postgres_fdw: Inherit the local transaction's access/deferrable modes.
>
> Previously, postgres_fdw always 1) opened a remote transaction in READ
> WRITE mode even when the local transaction was READ ONLY, causing a READ
> ONLY transaction using it that references a foreign table mapped to a
> remote view executing a volatile function to write in the remote side,
> and 2) opened the remote transaction in NOT DEFERRABLE mode even when
> the local transaction was DEFERRABLE, causing a SERIALIZABLE READ ONLY
> DEFERRABLE transaction using it to abort due to a serialization failure
> in the remote side.
>
> To avoid these, modify postgres_fdw to open a remote transaction in the
> same access/deferrable modes as the local transaction. This commit also
> modifies it to open a remote subtransaction in the same access mode as
> the local subtransaction.
>
> Although these issues exist since the introduction of postgres_fdw,
> there have been no reports from the field. So it seems fine to just fix
> them in master only.

I'm not sure this change should be considered a bug fix,
since the current behavior of postgres_fdw with a local read-only
transaction isn't clearly documented. Some users might see this
as a behavioral change rather than a fix. Anyway if we go with it,
shouldn't we document the change in the v18 release notes?

Regards,

--
Fujii Masao
NTT DATA Japan Corporation