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

On 2025/06/03 19:45, Etsuro Fujita wrote:
> On Mon, Jun 2, 2025 at 12:33 PM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
>> On Mon, Jun 02, 2025 at 12:03:50PM +0900, Fujii Masao wrote:
>>> 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?
>>
>> After going through the thread and the commit, I have to admit that I
>> was surprised to see this applied on HEAD now that we are in feature
>> freeze. This is a behavior change. Perhaps this could be done once
>> v19 happens, still it's rather unclear if the new behavior is better
>> than the previous one.
>
> No, this is a fix, not a feature, as discussed in the thread; as
> mentioned in the commit message, the previous version of postgres_fdw
> could cause surprising behaviors that would never happen in normal
> cases where a read-only and/or deferrable transaction only
> accesses/modifies data on the local server, so this commit fixes those
> behaviors.

I agree this could be considered a fix if the new behavior has been
clearly explained in the documentation from before or based on
standards like SQL/MED. But if that's not the case, it seems more
like a behavior change. In that case, I think it should wait for v19
and be applied only after reaching consensus. Some systems might
rely on the previous behavior.

By the way, if a read-only transaction on the local server is meant
to block all write operations on the remote server, this patch alone
might not be sufficient, for example, that read-only transaction can
invoke a login trigger on the remote server and it could still
perform writes.

Regards,

--
Fujii Masao
NTT DATA Japan Corporation