- Queries in the query cache are identified by the individual 
   characters in the query statement, the current database and 
   the current environment expressed as a set of system variable
   flags.
 - Since the set of environment flags didn't properly describe the
   current environment unexpected results were returned from the 
   query cache.
 - Query cache is now cleared when the variable ft_boolean_syntax is 
   updated.
 - An identification flag for the variable default_week_format is 
   added to the query cache record.
 
Thanks to Martin Friebe who has supplied significant parts of this patch.