unfortunately...

I don't like implementation of referential integrity constraints
in contrib/spi/refint.* and implementation of ... unique indices:
backend checks constraints when tuple is changed.

Oracle checks constraints after entire _statement_ is done.
Do you remember this:

vac=> create table x (a int unique);
NOTICE:  CREATE TABLE/UNIQUE will create implicit index x_a_key for table x
CREATE
vac=> insert into x values (1);
INSERT 143852 1
vac=> insert into x values (2);
INSERT 143853 1
vac=> update x set a = a + 1;
ERROR:  Cannot insert a duplicate key into a unique index

This is simplest case. Keeping in mind triggers
I would like to see Oracle behaviour some day and so
I decided to do not implement FOREIGN keys in 6.4 -:((

Sorry.

Vadim