How might I induce mysql 5.6.x to perform better here

[Eric Goff]

I have a table like so:

foo:
a int
b int
c int

With 163M rows
I have a partition on "a"
I have an index on "b"

If I run a query like so:
select b,sum(c) where a=4 group by 1 ;

I get a full scan of the partition.

If (using JDBC) I do repeated scans against ranges of "b" i.e.
select b,sum(c) where a=4 where b between 1 and 100 group by 1 ;
select b,sum(c) where a=4 where b between 101 and 200 group by 1 ;
etc..
to max value of "b".

combining the sums at the end I get much better performance.
If I run the queries concurrently, I get even better performance.

Is there any way to coax simple queries to take the route I took
using JDBC ?

Thank you,
goffster