st_table::const_key_parts member is used in determining if
 certain key has a prefix that is compared to constant(s) in
 the query predicates.
 If there's such prefix the index can be used to get the data
 from the remaining suffix columns in sorted order.
 However if a field is compared to another field from a "const"
 table the const_key_parts is not amended.
 This makes the optimizer unable to detect that the key can be 
 used for sorting and adds an extra filesort.
 Fixed by updating const_key_parts after reading in the "const"
 table.