The optimizer needs to evaluate whether predicates are better
 evaluated using an index. IN is one such predicate.
 To qualify an IN predicate must involve a field of the index
 on the left and constant arguments on the right.
 However whether an expression is a constant can be determined only
 by knowing the preceding tables in the join order. 
 Assuming that only IN predicates with expressions on the right that
 are constant for the whole query qualify limits the scope of 
 possible optimizations of the IN predicate (more specifically it
 doesn't allow the "Range checked for each record" optimization for
 such an IN predicate.
 Fixed by not pre-determining the optimizability of the IN predicate
 in the case when all right IN operands are not SQL constant expressions


mysql-test/r/func_in.result:
  BUG#20420: optimizer reports wrong keys on left join with IN
   - test case
mysql-test/t/func_in.test:
  BUG#20420: optimizer reports wrong keys on left join with IN
   - test case
sql/item_cmpfunc.h:
  BUG#20420: optimizer reports wrong keys on left join with IN
   - cannot decide on the optimizability of a function without
     knowing the tables before it in the join order