We have been doing a full refresh of a huge table
it takes around 4 hours usually. we truncate the table and recreate the
indexes
Now, we are planning to go for an incremental refresh of the table
and iam thinking that recreating the indexes is not necessary
but iam not sure how iam going to index the newly added rows

Please suggest

By telepathy! Oracle server will understand your motives and
automatically add the new rows to indexes. Try it!

And yes, when you get the chance read this:
http://download-west.oracle.com/docs...schem.htm#3239