Substring ignores index

[Matt Ryan]

Not sure if this is a bug or a "feature"

this query hits the don index, 14 char normal index
select * from cbcom where don LIKE 'W919AK%'

this query does not

select * from cbcom where substring(don,1,6)='W919AK'


the problem is I'm trying to do a join on another table

explain
select b.* from afghanistan_dodaac a inner join cbcom b
on a.dodaac = substring_index(b.don,1,6)

no indexes are hit, it runs 30 minutes,

id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE b ALL 15,128,943
1 SIMPLE a ref DODAAC DODAAC 6 func 1 Using where; Using index