title: rownum very slow after a particular value
body: <p>I am running a query in Oracle SQL developer which looks something like this : </p>
  
  <p>select * from dummy_table where col1 &lt; 10 and  col2 &lt; 20 and col3 &lt; 40 
  and <strong>rownum &lt;= x</strong></p>
  
  <p>The query takes around 3 seconds and returns x rows if the value of x is &lt;= 12.</p>
  
  <p>But if replace x by anything greater than 12, the query takes more than 7 seconds and returns only 12 results ( in other words, there are only 12 rows satisfying the where clause ) . </p>
  
  <p>Why is rownum behaving like this ? I was expecting this query to take almost same time if value of x is changed from 12 to 13. </p>
  
  <p><strong>Edit :</strong> Another thing which I noticed is that there is a composite index on col1,col2 and col3. If I remove the index ( or disable it using a hint ) , the query runs quite fast.</p>