title: Oracle Index with multiple Columns querying on single column
body: <p>In a table in our Oracle installation we have a table with an index on two of the columns (X and Y).  If I do a query on the table with a where clause only touching column X, will Oracle be able to use the index?</p>
  
  <p>For example:</p>
  
  <p>Table Y:
   Col_A,
   Col_B,
   Col_C,</p>
  
  <p>Index exists on (Col_A, Col_B)</p>
  
  <pre><code>SELECT * FROM Table_Y WHERE Col_A = 'STACKOVERFLOW';
  </code></pre>
  
  <p>Will the index be used, or will a table scan be done?</p>