strange "using where" in simple query
See the documentation.


  Using index
  
  The column information is retrieved from the table using only
  information in the index tree without having to do an additional seek
  to read the actual row. This strategy can be used when the query uses
  only columns that are part of a single index.
  
  If the Extra column also says Using where, it means the index is being
  used to perform lookups of key values. Without Using where, the
  optimizer may be reading the index to avoid reading data rows but not
  using it for lookups. For example, if the index is a covering index
  for the query, the optimizer may scan it without using it for lookups.


In short, your query is working as intended and is not performing an additional disk seek.