title: Why isn't index likely to do much if it's DATETIME or DATETIME2 because they include the time portion?
body: <p>Comments to <a href="https://stackoverflow.com/questions/4222095/how-to-decrease-response-time-of-a-simple-select-querry">question "How to decrease response time of a simple select query?"</a> tell:  </p>
  
  <ul>
  <li><p>"What is the data type on LaunchDate? An index isn't likely to do much if it's DATETIME or DATETIME2 because they include the time portion – OMG Ponies"</p></li>
  <li><p>"@OMG - Why wouldn't a Clustered Index on a DateTime column improve performance? The query is a range scan which would allow for a fast range index lookup as all data would be in sequential blocks? Semi-related...msdn.microsoft.com/en-us/library/ms177416.aspx – Calgary Coder"</p></li>
  <li><p>"Calgary Coder: <strong>DATETIME/2 includes time -- an index, clustered or non-clustered, would be good for dates with duplicate times but not ranges</strong>. – OMG Ponies"</p></li>
  </ul>
  
  <p>I created a test table with clustered index on <code>DATETIME</code> type column <code>LaunchDate</code> and observe index seeks for queries similar to cited in above question:</p>
  
  <pre><code>SELECT COUNT(primaryKeyColumn) 
  FROM   MarketPlan 
  WHERE  LaunchDate &gt; @date
  </code></pre>
  
  <p>instead  of table or index scans.  </p>
  
  <p>Why wouldn't a clustered index on a <code>DateTime</code> column improve performance?<br>
  Why isn't index likely to do much if it's <code>DATETIME</code> or <code>DATETIME2</code> because they include the time portion?</p>
  
  <p>I'd appreciate a script illustrating that indexing of <code>DATETIME</code> column does not improve performance.  </p>
  
  <p>Update: Also, Did OMG imply that index on <code>DATE</code> type column would be helpful but not <code>DATETIME</code> and <code>DATETIME2</code>?  </p>