title: SQL Server Performance: Non-clustered Index + INCLUDE columns vs. Clustered Index - equivalent?
body: <p>Hello SQL Server engine experts; please share with us a bit of your insight...</p>
  
  <p>As I understand it, INCLUDE columns on a non-clustered index allow additional, non-key data to be stored with the index pages.</p>
  
  <p>I am well aware of the performance benefits a clustered index has over a non-clustered index simply due to the 1 fewer step the engine must take in a retrieval in order to arrive at the data on disk.</p>
  
  <p>However, since INCLUDE columns live in a non-clustered index, can the following query be expected to have essentially the same performance across scenarios 1 and 2 since all columns could be retrieved from the index pages in scenario 2 rather than ever resorting to the table data pages?</p>
  
  <p><strong>QUERY</strong></p>
  
  <pre><code>SELECT A, B, C FROM TBL ORDER BY A
  </code></pre>
  
  <p><strong>SCENARIO 1</strong></p>
  
  <pre><code>CREATE CLUSTERED INDEX IX1 ON TBL (A, B, C);
  </code></pre>
  
  <p><strong>SCENARIO 2</strong></p>
  
  <pre><code>CREATED NONCLUSTERED INDEX IX1 ON TBL (A) INCLUDE (B, C);
  </code></pre>