title: Why is MySQL showing 'using where' in the 'extra' column?
body: <p>I have  a very simple query which really needs to be as fast as possible...</p>
  
  <p>The query is:</p>
  
  <pre><code>select users.userid, users.firstname, users.lastname
         from users_table users   
         where userid IN(1,4,6)
  </code></pre>
  
  <p>userid set as the primary key...</p>
  
  <p>EXPLAIN returns the following:</p>
  
  <pre><code>id  |select_type|table|type |possible_keys|key    |key_len|ref |rows|Extra
  1   |SIMPLE     |users|range|PRIMARY      |PRIMARY|4      |NULL|3   |Using where
  </code></pre>
  
  <p>Now, I was under the impression that 'Using where' is a bad thing?</p>
  
  <p>Can anybody explain why I might be seeing this?</p>