GROUP BY/DISTINCT pruning optimization must be done before ORDER BY 
optimization because ORDER BY may be removed when GROUP BY/DISTINCT
sorts as a side effect, e.g. in 
  SELECT DISTINCT <non-key-col>,<pk> FROM t1
  ORDER BY <non-key-col> DISTINCT
must be removed before ORDER BY as if done the other way around
it will remove both.


mysql-test/r/distinct.result:
  Test for BUG#21456.
mysql-test/t/distinct.test:
  Test for BUG#21456.
sql/sql_select.cc:
  Bug #21456: SELECT DISTINCT(x) produces incorrect results when using order by
  
  GROUP BY/DISTINCT pruning optimization must be done before ORDER BY 
  optimization because ORDER BY may be removed when GROUP BY/DISTINCT
  sorts as a side effect.