title: When does Oracle index null column values?
body: <p>I used to think that Oracle does not index a row when one of the column values is null.</p>
  
  <p>Some simple experimentation shows this to be not the case. I was able to run some queries unexpectedly accessing only indexes even though some columns were nullable (which of course was a pleasant surprise).</p>
  
  <p>A Google search led to some blogs with conflicting answers: I have read that a row gets indexed unless <em>all</em> indexed columns are null, and also that a row gets indexed unless <em>the leading column value</em> for the index is null.</p>
  
  <p>So, in what cases does a row not enter an index? Is this Oracle version specific?</p>