title: mysql single column condition query on a multi-column index
body: <p>suppose a table has only one index : <code>idx_goods  (is_deleted,price)</code><br />
  and is_deleted column is either 0 or 1</p>
  <p>my query looks like this : <code>where price &lt; 10</code>, which of the follwing behavior is this case:<br />
  <strong>1</strong> mysql do a full scan on the secondary index for price match<br />
  <strong>2</strong> mysql partially scan the secondary index for price match where it starts with <code>is_deleted=0</code> in the secondary index, reach <code>price=10</code>, then jump to <code>is_deleted=1</code> in the secondary index and continue there<br />
  <strong>3</strong> mysql ignore the secondary index and scan the base table for price match, <strong>in other words, condition field that's not in the query's index key prefix is not matched against secondary index but against base table, event though the condition field is part of index key</strong></p>