WL#2474 "Multi Range Read: Change the default MRR implementation to implement new MRR interface"
WL#2475 "Batched range read functions for MyISAM/InnoDb"
        "Index condition pushdown for MyISAM/InnoDB"
Igor's fix from sp1r-igor@olga.mysql.com-20080330055902-07614:
  There could be observed the following problems:
  1. EXPLAIN did not mention pushdown conditions from on expressions in the 
  'extra' column.  As a result if a query had no where conditions pushed 
  down to a table, but had on conditions pushed to this table the 'extra' 
  column in the EXPLAIN for the table missed 'using where'.
  2. Conditions for ref access were not eliminated from on expressions 
  though such conditions were eliminated from the where condition.