create or replace table t (a int, key(a desc)) engine=MyISAM;

create or replace table tm (a int, key(a)) engine=Merge union(t);

insert into t values (1),(5),(3);

# From the underlying table -- OK

select * from t force index for order by (a) order by a;

# From the MERGE table -- not OK

select * from tm force index for order by (a) order by a;

# Cleanup

drop table tm, t;

select * from tm force index for order by (a) order by a;

a

5

3

1