Re: ALTER TABLE .... make constraint DEFERRABLE

Simon Riggs wrote:
> On Wed, 2010-06-02 at 21:06 -0400, Bruce Momjian wrote:
> > 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?
> 
> No. I wanted to defer an existing UNIQUE constraint. That doesn't seem
> to be possible. You *can* add a whole new constraint and then drop the
> old one, though that's not quite as cool.

Do we allow any kind of constraint modification via ALTER TABLE?  I
don't see much listed in the manual.

> > > 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.
> 
> Regrettably it makes it an unusable limitation for many people.
> 
> All large tables are referenced in a typical database that uses PKs/FKs.

Yeah, no question.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
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