hi!
I have been looking though git log, noticed this commit, and did small tests.
This is what I found:
```
reshke=# create table z(i int) partition by range(i);
CREATE TABLE
reshke=# create table z_1 partition of z for values from (0)to(1);
CREATE TABLE
reshke=# create table z_2 partition of z for values from (1)to(2);
CREATE TABLE
reshke=# alter table only z merge partitions (z_1,z_2) into z_12;
ALTER TABLE
reshke=#
```
IMO "alter table only ... merge partitions" does not make perfect
sense and should be rejected rather than executed. WDYT?
