Connected to Oracle Database 10g Enterprise Edition Release  10 . 1 . 0 . 5 . 0  
Connected as test

SQL> create table contractors (
   2     contractor_id integer not null primary key,
   3     contractor_id_root integer not null references contractors (contractor_id),
   4     contractor_name varchar2( 4000 ) not null,
   5     inn char( 12 ),
   6     kpp varchar2( 4000 ),
   7     date_from date not null,
   8     date_to date not null,
   9     check (date_from < date_to));

Table created

SQL> create materialized view actual_contractors
   2     refresh complete on commit as
   3   select * from contractors where date_to = to_date ('01.01.3000', 'dd.mm.yyyy');

Materialized view created

SQL> create unique index actual_contractors_ui on actual_contractors (inn, kpp);

Index created

SQL> insert into contractors values ( 1 ,  1 , 'Петров', '000000000000', '000', sysdate- 10 , sysdate- 5 );

 1  row inserted

SQL> insert into contractors values ( 2 ,  1 , 'Васечкин', '000000000000', '000', sysdate- 5 , to_date ('01.01.3000', 'dd.mm.yyyy'));

 1  row inserted

SQL> commit;

Commit complete

SQL> insert into contractors values ( 3 ,  3 , 'Пупкин', '000000000000', '000', sysdate- 100 , to_date ('01.01.3000', 'dd.mm.yyyy'));

 1  row inserted

SQL> commit;

ORA- 12008 : error in materialized view refresh path
ORA- 00001 : unique constraint (TEST.ACTUAL_CONTRACTORS_UI) violated