title: ORDER BY datetime makes the query very slow
body: <p>I am trying to pull data from multiple tables and when I user ORDER BY a datetime field it return the results after at least 10 seconds but if I do the same query without ORDER BY then it return the results for under 2 seconds.</p>
  
  <p>This is my current query</p>
  
  <pre><code>SELECT
  ph.call_subject AS callSubject,
  ac.account_name AS accountName,
  DATE_FORMAT(ph.trigger_on, "%c/%e/%Y %h:%i %p") AS triggerOn,
  ind.name AS industry,
  cc.call_code_name AS callCode
  FROM phone_calls AS ph
  INNER JOIN accounts AS ac ON ph.account_id = ac.account_id
  INNER JOIN industries AS ind ON ind.industry_id = ac.industry_id
  INNER JOIN call_codes AS cc ON ph.call_code_id = cc.call_code_id
  WHERE ac.status = 1 AND ph.status = 1 AND ph.owner_id = 1 AND ac.do_not_call = 0
  AND ph.trigger_on BETWEEN '2012-11-19 00:00:00' AND '2013-03-19 23:59:59'
  ORDER BY ph.trigger_on ASC LIMIT 0,1000
  </code></pre>
  
  <p>the following fields are all of the type INT(11) UNSIGNED</p>
  
  <pre><code>ph.account_id
  ac.account_id
  ind.industry_id
  ac.industry_id
  ph.call_code_id
  cc.call_code_id
  ph.owner_id
  </code></pre>
  
  <p>The following fields are all of the type tinyint(1)</p>
  
  <pre><code>ac.status 
  ph.status
  ac.do_not_call
  </code></pre>
  
  <p>this field is a datetime type</p>
  
  <pre><code>ph.trigger_on
  </code></pre>
  
  <p>Please note that has accounts 300K records and phone_calls has 5 million records.
  What can I do to preform this ORDER BY faster? note that all of my where clause fields, all my ON clause and ph.trigger_on are indexed. and I am using InnoDB storage engine not MyIsam.</p>
  
  <p>Thanks</p>