Converting Nested loop to hashjoin for not is distinct from case

Hi,

          I have a  join query with is not distinct from criteria (select * from table1 join table2 on a is not  distinct from b). on examining,  it was the  nested loop join that makes the query slow.  For join with ' is not distinct from ' qual can't the planner choose  hash-join over nested loop  or will there be any problem by doing so. Kindly enlighten me about the same.

Thanks
Pradeep