Re: Is this a bug? Deleting a column deletes the constraint.

Bryce Nesbitt <bryce1(at)obviously(dot)com> writes:
> I got bit by this to be sure, but is it a bug? I guess I'd hoped for a
> warning at the critical step (see "poof" below):

> create table tester (one int, two int, three int);
> alter table only tester add constraint no_dupes unique (one, two, three);
> ...
> alter table tester drop column old_three;
> \d tester; -- poof, the constraint is
> gone with no warning

I don't think the renaming is relevant: you get the same with

regression=# create table tester (one int, two int, three int, unique (one,two,three));
NOTICE: CREATE TABLE / UNIQUE will create implicit index "tester_one_key" for table "tester"
CREATE TABLE
regression=# alter table tester drop column three;
ALTER TABLE
regression=# \d tester
Table "public.tester"
Column | Type | Modifiers
--------+---------+-----------
one | integer |
two | integer |

regression=#

It does seem like this is wrong, in view of SQL92's statement about
ALTER TABLE DROP COLUMN:

4) If RESTRICT is specified, then C shall not be referenced in
the <query expression> of any view descriptor or in the <search
condition> of any constraint descriptor other than a table con-
straint descriptor that contains references to no other column
and that is included in the table descriptor of T.

IOW we should only allow unique constraints to be auto-dropped if
they reference just the one single column. Ick.

regards, tom lane