Re: ALTER TABLE .... make constraint DEFERRABLE

Simon Riggs wrote:
>
> Deferrable unique constraints seem an interesting feature, though I have
> either some questions or some issues, not sure which.
>
> I don't seem to be able to find any way to do an ALTER TABLE that adds
> this new capability to an existing table.

I was able to do it:

test=> create table test (x int unique DEFERRABLE INITIALLY DEFERRED);
NOTICE: CREATE TABLE / UNIQUE will create implicit index "test_x_key"
for table "test"
CREATE TABLE

test=> alter table test add column y int;
ALTER TABLE

test=> alter table test add unique (y) DEFERRABLE INITIALLY DEFERRED;
NOTICE: ALTER TABLE / ADD UNIQUE will create implicit index
"test_y_key" for table "test"
ALTER TABLE

Is that what you were asking?

> There is no way to add a constraint via a CREATE TABLE AS SELECT, so
> that means there is no way to use the feature at all in that case.

Uh, CREATE TABLE AS SELECT seems to be very limited, but I have not
heard any complaints about it before.

> Also, foreign keys can't be defined that refer to a deferrable primary
> key. That isn't mentioned at all in the manual with regard to the
> DEFERRABLE clause, though it is mentioned in the FK section. You get
> this error message
> ERROR: cannot use a deferrable unique constraint for referenced table
>
> The use case for this feature looks a little narrow at present. Can we
> do something about usability?

Not sure why that was a limitation.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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