title: Estimate Rows vs Actual Rows, what is the impact on performance?
body: <p>I have a query that performs very quickly but in production when server loads are high its performance is underwhelming.  I have a suspicion that it might be the Estimated Rows being much lower than the Actual Rows in the execution plan.  I know that server statistics are not stale.</p>
  
  <p><a href="https://i.stack.imgur.com/CEStH.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/CEStH.png" alt="Sample SQL Sentry Plan"></a></p>
  
  <p>I am now optimizing a new query and I worry that it will have the same problem in production.  The number of rows returned and the CPU and Reads are well within the designated thresholds my data admins require.  As you can see in the above SQL Sentry plan there are a few temp tables that estimate a single row but return 100 times as many rows.  </p>
  
  <p>My question is this, even when the number of rows are few, does a difference in rows by such a large percentage cause bottlenecks on the server's performance?  Secondary question, if the problem isn't a bad cached plan or stale stats, what other issues would cause a plan to show such a discrepancy?</p>