title: sql server avg_user_impact
body: <p>In the DMV <code>sys.dm_db_missing_index_group_stats</code>, there is a field named <code>avg_user_impact</code>. Per BOL, this shows the </p>
  
  <blockquote>
    <p>Average percentage benefit that user queries could experience if this missing index group was implemented. The value means that the query cost would on average drop by this percentage if this missing index group was implemented.</p>
  </blockquote>
  
  <p>When I run a query to find the missing indexes that would have the greatest impact on performance, I see a few that claim the <code>avg_user_impact</code> will be in the thousands. </p>
  
  <p>I built an index using one of these recommendations, and it was never used according to the DMV <code>sys.dm_db_index_usage_stats</code>. As soon as I disabled the index, it immediately showed up again as a missing index. </p>
  
  <p>Is it possible that once the <code>avg_user_impact</code> field shows over 100, it's in error? Or is this an example of how a recommended index must still be tested for usefulness? What am I doing wrong or misunderstanding?</p>