title: Should you index on week number?
body: <p>I have several large MySQL transactional tables that include a transaction date stored in datetime format. </p>
  
  <p>The tables are often queried using the YEARWEEK() function. For example:</p>
  
  <pre><code>SELECT * WHERE YEARWEEK(workdate, 3) = '201402'
  </code></pre>
  
  <p>There is already an index on the datetime field (i.e. 'workdate'). Should an additional index be added to optimize the performance of queries like the sample above?</p>