create table t1 (a int, b int);

create table t2 (a int, b int, c int);

-- successful elimination of D

explain select t1.* from t1 left join 

  (select a, b from t2 group by a, b) D 

  on D.a=t1.a and D.b=t1.b;

--  Field 'a' appears as 'a' and 'a2' and D is joined using 'a2'. It is not eliminated though (a2,b) is also unique

explain select t1.* from t1 left join 

  (select a, b, a as a2 from t2 group by a, b) D 

  on D.a2=t1.a and D.b=t1.b;