DATE/DATETIME values are out of the currently supported
 4 basic value types (INT,STRING,REAL and DECIMAL).
 So expressions (not fields) of compile type DATE/DATETIME are 
 generally considered as STRING values. This is not so
 when they are compared : then they are compared as 
 INTEGER values.
 But the rule for comparison as INTEGERS must be checked
 explicitly each time when a comparison is to be performed.
 filesort is one such place. However there the check was 
 not done and hence the expressions (not fields) of type 
 DATE/DATETIME were sorted by their string representation.
 Fixed to compare them as INTEGER values for filesort.


mysql-test/r/order_by.result:
  Bug #26672: test case
mysql-test/t/order_by.test:
  Bug #26672: test case
sql/filesort.cc:
  Bug #26672: sort dates/times as integers