Re: BUG #15180: Alter table add column if not exists with uniqueconstraint will add extra duplicate

A possible objection to this design is that right now, you can do

        alter table foo
          add column if not exists f2 text,
          add column if not exists f2 int;

and it will skip the second ADD subcommand because by that point the
column exists.  With this design, both test subcommands would find that
f2 doesn't exist so we'd try to do both ADD subcommands, and the second
one would fail.  That doesn't particularly bother me, because this
command is silly.