title: Insert into if not exists not inserting
body: <pre><code>INSERT INTO my_table (field_1, field_2)
  SELECT val_1, val_2     
  FROM my_table
  WHERE NOT EXISTS (SELECT field_1
                          FROM my_table
                          WHERE field_2 = val_2)
  LIMIT 1
  </code></pre>
  
  <p>I can not use unique index on field_2 field.
  I'm trying to insert if not exists a tuple with field2 = val_2.
  Without the "where" clause, this insert.
  With the "where" clause <strong>EVEN WHEN EMPTY TABLE</strong>, it won't insert.</p>
  
  <p>Any help on that? </p>