title: Why queries are running slower after converting the column from VARCHAR to DATETIME
body: <p>I have a table with a column <code>record_datetime</code> which used to be a <code>VARCHAR</code>. I tried changing it to DateTime, as it should be, but the queries are taking lot more time [almost 10 times more]. Why would it slow down??</p>
  
  <p>This table has close to 100 million rows and <code>record_datetime</code> is an index. My test query I tried to run is</p>
  
  <pre><code>SELECT * 
  FROM table1 
  WHERE `record_datetime` &gt;= '2012-06-12 00:00:00' 
    AND `Source` = 6 
  ORDER BY `redord_datetime`
  </code></pre>
  
  <p><code>Source</code> is part of the <code>PRIMARY KEY</code>, which is combination of user_id,record_datetime and Source. And <code>record_datetime</code> is the only index.</p>
  
  <p>Here is what EXPLAIN said:</p>
  
  <pre><code>   id   select_type table type possible_keys key  key_len ref   rows      Extra
      1   SIMPLE       t0    ALL    Date       NULL   NULL  NULL 77489783  Using where
  </code></pre>
  
  <p>Thanks</p>