Re: MERGE/SPLIT PARTITIONS issues/questions

Hello

v3 looks good to me, I only have two nitpick comments:

1. MergePartitionsMoveRows now has a stale comment ("We also verify
check constraints againsy these rows")

2. I am unsure of the usefulness of some of the error hints, for example:
+ errhint("Set the replica identity of the new partition
explicitly after the operation."));

The hint is true, the user has to set up replica identity after the
merge if he needs it, but the operation can't be executed as-is, so
the user first have to solve the current situation. I also don't have
a better idea how to explain this without an overly long error hint,
so maybe it's good as is.