Re: MERGE/SPLIT PARTITIONS issues/questions

I don't think 0005 is correct. Similar to the other changes, the
intention should be "do not change the table access method in an
invisible way to the user".

The current patch still drops explicit table AMs for a partition
(CREATE TABLE tp PARTITION OF ... USING otheram) in favor of the
parent AM or even the current session default AM.
Merge of partitions with different AMs still succeed.

I have to agree with Daniel's reasoning earlier:

> Apart from the obviously dangerous ones like RLS and ACL, silently dropping the
> table AM may induce side-effects which are hard for us to even reason about
> since they are external to the core code.

As an example, we have pg_tde which provides an encrypted version of
the heap AM. Silently changing the table AM in our case means that we
remove encryption from the data without notifying the user about it.
We can detect such commands in an event trigger and disable them to
prevent accidents, but I don't think this should be left to extension
authors.

> As I mentioned in [1], I think this is the way to save this feature
> for pg19. I think it's too late to introduce new (and debatable)
> functionality.

My opinion is that things like silently dropping triggers or default
values or constraints can result in similar dangerous accidents.