title: SELECT statement not using possible_keys
body: <p>I have a table from a legacy system which does not have a primary key. It records transactional data for issuing materials in a factory.</p>
  
  <p>For simplicities sake, lets say each row contains job_number, part_number, quantity &amp; date_issued.</p>
  
  <p>I added an index to the date issued column. When I run an EXPLAIN SELECT * FROM issued_parts WHERE date_issued > '20100101', it shows this:</p>
  
  <pre>
  +----+-------------+----------------+------+-------------------+------+---------+------+---------+-------------+
  | id | select_type | table          | type | possible_keys     | key  | key_len | ref  | rows    | Extra       |
  +----+-------------+----------------+------+-------------------+------+---------+------+---------+-------------+
  |  1 | SIMPLE      | issued_parts   | ALL  | date_issued_alloc | NULL | NULL    | NULL | 9724620 | Using where |
  +----+-------------+----------------+------+-------------------+------+---------+------+---------+-------------+
  </pre>
  
  <p>So it sees the key, but it doesn't use it?
  Can someone explain why?</p>