Re: MERGE/SPLIT PARTITIONS issues/questions

> Copying the value of generated column "as is" can produce data that differs from
> what the generated expression would compute if any merged partition's generation
> expression differs from the partitioned table's.

I think this would be probably fine, as we can get the same effect by
replacing a function used by the expression, a preexisting condition
for many existing cases. But I do agree that requiring the same
expression is a better approach.

Also, not directly related to this patch, but now that I looked into
this, I can still use tableoids for check constraints with a text
cast:

CREATE TABLE t (i int) PARTITION BY RANGE (i);
CREATE TABLE tp_0_1 PARTITION OF t FOR VALUES FROM (0) TO (1);
CREATE TABLE tp_1_2 PARTITION OF t FOR VALUES FROM (1) TO (2);
ALTER TABLE t ADD CONSTRAINT cc CHECK (tableoid::regclass::text <> 'tp_0_2');
INSERT INTO t VALUES (0),(1);
ALTER TABLE t MERGE PARTITIONS (tp_0_1, tp_1_2) INTO tp_0_2; --
SUCCESS, but should ERROR instead?

And another question I realized while looking at differences to other
rewrite operators: currently merge/split doesn't fire a rewrite event
trigger, but shouldn't it?

For the replication changes: shouldn't we also restrict schema
changes? `TABLES IN SCHEMA` can still be problematic if the parent and
the specific partitions are in different schemas, they either get
published or unpublished.

+ <command>ALTER TABLE ... MERGE PARTITIONS</command> is a schema
change and
+ is not itself replicated to logical replication subscribers; to
reflect it
+ on a subscriber, run the equivalent command there, or drop and recreate
+ the affected partitions and refresh the subscription.

I think this still results in my original (3) data loss scenario, so I
don't think it's a good idea to recommend it.

For example if we MERGE + UPDATE/INSERT on the publisher, the
subscriber worker error-loops on the merged partition not existing. We
replay the MERGE locally on the subscriber, the worker continues
before we have a chance to run REFRESH PUBLICATION and discards the
UPDATE/INSERT.