The index on jdate will restrict the rows from j1tst and the index on j2tst.jttype will support the join.
Group by with columns from both tables can prevent the use of indexes and change the plan.
Try explaining the query below:

select jdate,jtname,sum(jtsum) from j1tst v1
  inner join j2tst v2 on (v1.jttype=v2.jttype)
  where jdate between '2010-01-01' and '2010-01-10'
  group by jdate,jtname;