Re: BUG #19380: Transition table in AFTER INSERT trigger misses rows from MERGE when used with INSERT in a CTE

On Tue, 20 Jan 2026 at 17:21, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Dean Rasheed <dean.a.rasheed@gmail.com> writes:
>
> > The problem is that MERGE really needs to use the same
> > AfterTriggersTableData structs as INSERT, UPDATE, and DELETE, so that
> > any captured tuples get added to the same tuplestores.
>
> Yeah, I had just come to the same conclusion when I saw your email.
> Using CMD_MERGE as the AfterTriggersTableData lookup key could be
> correct if MERGE were supposed to have separate transition tables,
> but AFAICT from the spec it isn't.

Yeah, I think that's right. The spec is clear that the effects of a
MERGE action should be the same as the effects of an
INSERT/UPDATE/DELETE command, and that includes triggers. However, I
don't think the spec says anything about what should happen if there
are multiple INSERT/UPDATE/DELETE/MERGE sub-commands within a single
top-level command, because including INSERT/UPDATE/DELETE/MERGE
statements inside a WITH statement is our own extension of the
standard.

So I think that we were free to choose whether to execute
statement-level triggers once per sub-command, or once for the entire
WITH query. Since we have chosen the latter, it follows that each
MERGE sub-command must add cumulatively to the set of rows in the
INSERT/UPDATE/DELETE transition tables, just as each
INSERT/UPDATE/DELETE sub-command does.

> > Attached is a rough patch doing that.
>
> I haven't read this in detail, but it seems like one issue to think
> about is whether it's okay to add fields to struct
> TransitionCaptureState in released branches?  Although it's nominally
> an ABI break, I can't think of a reason why any extension would be
> manufacturing its own TransitionCaptureState structs rather than
> calling MakeTransitionCaptureState(), nor should an extension be
> touching the stated-to-be-private tcs_private field.  So it seems
> like we should be able to get away with it.

Yeah, I think that's probably right. FWIW, I grepped for
TransitionCaptureState in a clone of PGXN, and didn't find any uses.

Regards,
Dean