title: Meaning of "Select tables optimized away" in MySQL Explain plan
body: <p>What is the meaning of <code>Select tables optimized away</code> in MySQL Explain plan?    </p>
  
  <pre><code>explain select count(comment_count) from wp_posts;
  
  +----+-------------+---------------------------+-----------------------------+
  | id | select_type | table,type,possible_keys, | Extra                       |
  |    |             | key,key_len,ref,rows      |                             |
  +----+-------------+---------------------------+-----------------------------+
  | 1  | SIMPLE      | all NULLs                 | Select tables optimized away| 
  +----+-------------+---------------------------+-----------------------------+
  1 row in set (0.00 sec)
  </code></pre>
  
  <p>Note: <code>explain plan</code> output edited for legibility.</p>