title: In SQL, How does using DISTINCT affect performance?
body: <p>I am attempting to select a distinct list where duplicates are created over several fields. For example, </p>
  
  <pre><code>SELECT tablename.field1Date, 
         tablename.field2Number, 
         tablename.field3Text 
  FROM tablename;
  </code></pre>
  
  <p>Would select duplicating records over the date, number and text fields respectively.</p>
  
  <p>Now, when I select distinct records to provide what I am looking for, the performance seems to decrease dramatically. </p>
  
  <pre><code>SELECT DISTINCT tablename.field1Date,
                  tablename.field2Number, 
                  tablename.field3Text 
  FROM tablename;
  </code></pre>
  
  <p>Is there any known reasons for this? I must admit I am using MS Access 2003 which may be the issue. </p>