Re: BUG #16088: Multiple foreigh keys created on "ALTER TABLE" command

PG Bug reporting form <noreply@postgresql.org> writes:
> If you run this command multiple times:

> ALTER TABLE myschema.mytable ADD IF NOT EXISTS mycolumn uuid REFERENCES
> myschema.mytable2(userid);

> It adds the column once, but adds a duplicate FOREIGN KEY each time.

Yeah, this is a well known problem :-(.  The IF NOT EXISTS conditionalizes
addition of the column, but not the quasi-independent addition of the
foreign key constraint.  There's work afoot to make this behave more
intuitively, but it won't land before v13 at the earliest.

            regards, tom lane