title: The MYSQL "explain query" shows "key" NULL
body: <p>I have made mysql explain the following query:</p>
  
  <pre><code>SELECT carid,pic0,bio,url,site,applet 
  FROM cronjob_reloaded 
  WHERE 
  carid LIKE '%bmw%' 
  OR 
  carid LIKE '%mer%' 
  OR 
  age BETWEEN '5' AND '10' 
  OR 
  category IN ('used') 
  ORDER BY CASE 
      WHEN carid LIKE '%bmw%' OR carid LIKE '%mer%' THEN 1 
      WHEN age BETWEEN '5' AND '10' THEN 2 
      ELSE 3 
  END 
  </code></pre>
  
  <p>And here is the explain result:</p>
  
  <pre><code>EXPLAIN SELECT carid, pic0, bio, url, site, applet
  FROM cronjob_reloaded
  WHERE carid LIKE '%bmw%'
  OR carid LIKE '%mer%'
  OR carid IS NOT NULL
  AND age
  BETWEEN '5'
  AND '10'
  </code></pre>
  
  <p>What I do not understand it this:</p>
  
  <p><img src="https://i.stack.imgur.com/NFHgo.jpg" alt="enter image description here"></p>
  
  <ol>
  <li><p>Why is the key NULL?</p></li>
  <li><p>Can I make this query faster? It takes  0.0035 sec - is this slow or fast for a 1000 rows table?</p></li>
  </ol>
  
  <p>In my table carid is the primary key of the table.</p>