title: Oracle unique constraint and unique index
body: <p>Could someone clarify what is the purpose of having unique index without unique constraint (Oracle)? 
  For example,</p>
  
  <pre><code>create table test22(id int, id1 int, tmp varchar(20));
  create unique index idx_test22 on test22(id);
  insert into test22(id, id1, tmp) values (1, 2, 'aaa'); // ok
  insert into test22(id, id1, tmp) values (1, 2, 'aaa'); // fails, ORA-00001: unique   
    // constraint (TEST.IDX_TEST22) violated
  </code></pre>
  
  <p>So far it looks like there is a constraint. But </p>
  
  <pre><code>create table test33(id int not null primary key, 
  test22_id int not null, 
  foreign key(test22_id) references test22(id) );
  </code></pre>
  
  <p>also fails with <code>"ORA-02270: no matching unique or primary key for this column-list"</code>.
  I'm totally confused by this behaviour. Is there a constraint or not?</p>
  
  <p>There are many articles that explain why it's possible to have a unique constraint without unique index; that is clear and makes perfect sense. However, I don't understand the reason for unique index without constraint.</p>