When using index for group by and range access the server isolates    
 a set of ranges based on the conditions over the key parts of the
 index used. Then it uses only the ranges over the GROUP BY fields to
 jump from one group to another. Since the GROUP BY fields may form a
 prefix over the index, we may use only a prefix of the ranges produced
 by the range optimizer.
 Each range contains a notion on whether it includes its border values.
 The problem is that when using a range prefix, the last range is open
 because it assumes that there is a range on the next keypart. Thus when
 we use a prefix range as it is, it excludes all border values.
 The solution is when ignoring the suffix of the range conditions 
 (to jump over the GROUP BY prefix only) the server must change the 
 remaining intervals so they always contain their borders, e.g. 
 if the whole range was :
 (1,-inf) <= (<group_by_col>,<min_max_arg_col>) < (1, 3) we must make
 (1) <= (<group_by_col>) <= (1) because (a,b) < (c1,c2) means :
 a < c1 OR (a = c1 AND b < c2).