Evening Ann,

> I think Oracle has cascading delete ... but not cascading
> update.

You are correct. Oracle doesn't (at least at 10g anyway) have a
cascading update, only a cascading delete. It does have deferred
constraints as well.

What Oracle does to make a primary or unique constraint deferred is base
it on a non-unique index. That allows the ability to have duplicates in
the constrained column(s).

For foreign Key constraints, the index is already non-unique, so it
makes no difference.

As for performance, running Oracle with deferred constraints does add a
slight overhead - looking up a PK in a non-unique index is done by an
index range scan rather than by a unique lookup.

Cheers,
Norman.