Re: [PATCH] Hooks at XactCommand level

Hi,

On 2021-07-30 17:49:09 -0400, Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > On 2021-07-30 13:58:51 -0400, Tom Lane wrote:
> >> I've not read this version of the patch, but I see from the cfbot's
> >> results that it's broken postgres_fdw.
>
> > I think this may partially be an issue with the way that postgres_fdw
> > uses the callback than with the patch. It disconnects from the server
> > *regardless* of the XactEvent passed to the callback. That makes it
> > really hard to extend the callback mechanism to further events...
>
> Perhaps. Nonetheless, I thought upthread that adding these events
> as Xact/SubXactCallback events was the wrong design, and I still
> think that. A new hook would be a more sensible way.

I know I've wanted additional events in XactEvent before that'd also be
problematic for pg_fdw, but not make sense as a separate event. E.g. an
event when an xid is assigned.

> > I'm *very* unconvinced it makes sense to implement a feature like this
> > in an extension / that we should expose API for that purpose. To me the
> > top-level transaction state is way too tied to our internals for it to
> > be reasonably dealt with in an extension.
>
> Yeah, that's the other major problem --- the use-case doesn't seem
> very convincing. I'm not even sold on the goal, let alone on trying
> to implement it by hooking into these particular places. I think
> that'll end up being buggy and fragile as well as not very performant.

I'm more favorable than you on the overall goal. Migrations to PG are a
frequent and good thing and as discussed before, lots of PG forks ended
up implementing a version of this. Clearly there's demand.

However, I think a proper implementation would require a substantial
amount of effort. Including things like optimizing the subtransaction
logic so that switching the feature on doesn't lead to xid wraparound
issues. Adding odd hooks doesn't move us towards a real solution imo.

Greetings,

Andres Freund