update& unique index

SQL> create table t( x int,y int);

Table created.

SQL> insert into t values(1,1);

1 row created.

SQL> insert into t values(1,2);

1 row created.

SQL> create unique index u_t on t(x,y);

Index created.

SQL> commit;

Commit complete.

SQL> insert into t values(1,3);

1 row created.

SQL> insert into t values(1,3);
insert into t values(1,3)
*
ERROR at line 1:
ORA-00001: unique constraint (THE9.U_T) violated

SQL> update t set y=2 where x=1;
update t set y=2 where x=1
*
ERROR at line 1:
ORA-00001: unique constraint (THE9.U_T) violated

SQL> update t set y=2 where x=1 and y=2;

1 row updated.

SQL> rollback;

Rollback complete.

SQL> select * from t;

         X          Y
---------- ----------
         1          1
         1          2

SQL> update t set x=1 where y=2;

1 row updated.

SQL>