Clarification, please

In Oracle, deferrable primary keys are enforced by non-unique indexes.
That seems logical, because index should tolerate duplicate values for
the duration of transaction:

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application
Testing options

SQL> create table test1
2 (col1 integer,
3 constraint test1_pk primary key(col1) deferrable);

Table created.

Elapsed: 00:00:00.35
SQL> select uniqueness from user_indexes where index_name='TEST1_PK';

UNIQUENES
---------
NONUNIQUE

PostgreSQL 9.0, however, creates a unique index:

scott=# create table test1
scott-# (col1 integer,
scott(# constraint test1_pk primary key(col1) deferrable);
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index
"test1_pk" for table "test1"
CREATE TABLE
Time: 67.263 ms
scott=# select indexdef from pg_indexes where indexname='test1_pk';
indexdef
----------------------------------------------------------
CREATE UNIQUE INDEX test1_pk ON test1 USING btree (col1)
(1 row)

When the constraint is deferred in the transaction block, however, it
tolerates duplicate values until the end of transaction:

scott=# begin;
BEGIN
Time: 0.201 ms
scott=# set constraints test1_pk deferred;
SET CONSTRAINTS
Time: 0.651 ms
scott=# insert into test1 values(1);
INSERT 0 1
Time: 1.223 ms
scott=# insert into test1 values(1);
INSERT 0 1
Time: 0.390 ms
scott=# rollback;
ROLLBACK
Time: 0.254 ms
scott=#

No errors here. How is it possible to insert the same value twice into a
UNIQUE index? What's going on here?

--

Mladen Gogala
Sr. Oracle DBA
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New York, NY 10036
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