Re: Simplify code building the LR conflict messages

I wrote:
> So I think this area desperately needs significant editorial
> attention, as well as some fundamental rethinking of just what
> information we should show. Perhaps using errcontext would help,
> but I'm not sure. I think a large part of the problem stems from
> trying to cram multiple error conditions into one ereport ... is it
> possible to avoid that?

After a little bit of thought, here's a sketch of a straw-man idea.

1. The longstanding output for unique constraint violations is like

ERROR: duplicate key value violates unique constraint "foo_f1_f2_key"
DETAIL: Key (f1, f2)=(1, 2) already exists.

We could do worse than to use exactly that output (perhaps even
sharing code) and add errcontext like

CONTEXT: replicated INSERT of row with replica identity (f1, f2)=(1, 2) in table "foo"

We should drop the full-row output for the reason I gave previously,
and I do not see the value of the XID printout either. This would
also serve (after s/INSERT/UPDATE/) for unique violations during
replicated updates.

2. For no-matching-row in UPDATE or DELETE, perhaps

ERROR: row to be updated[deleted] does not exist
CONTEXT: replicated UPDATE[DELETE] of row with replica identity (f1, f2)=(1, 2) in table "foo"

3. I don't understand what delete_origin_differs means or why
it's an error condition, so I won't dare to propose new text
for that. But the new text should make the reason clear,
and I think the same errcontext still works.

4. We need to make this a separate ereport for each problematic row.
Without having looked at the code, I suspect the reason it works the
way it does now is that the process will exit after ereport(ERROR).
I don't want to completely redesign how ereport works, but maybe
we could change replication apply so that the per-row reports are
emitted as ereport(LOG), and then when we get to a place where we
want to quit, end with

ERROR: stopping replication because of previously-logged errors

which would make the consequences clearer anyway.

regards, tom lane