title: Why does performance degrade when using a non-indexed field in the SELECT clause?
body: <p>Consider these three queries:</p>
  
  <pre><code>select sampleno from sample
      where markupdate &gt; '1/1/2010'
  
  select sampleno, markupdate from sample
      where markupdate &gt; '1/1/2010'
  
  select sampleno, markuptime from sample
      where markupdate &gt; '1/1/2010'
  </code></pre>
  
  <p><code>sampleno</code> and <code>markupdate</code> are indexed fields (<code>sampleno</code> is the primary key)</p>
  
  <p><code>markuptime</code> is not indexed</p>
  
  <p>Queries 1 and 2 take about 1 second to run (returning 237K rows).  Query 3 is still running after 3 minutes.</p>
  
  <p>Why would the inclusion of a non-indexed field in the SELECT clause cause such a performance degradation?</p>
  
  <p>This is a SQL 6.5 database.</p>