index advisor

hi,

can anyone please tell me how a secondary index can be deemed as effective?

i did trace for a user and the EXPLAIN shows:

Now after running index advisor and doing another EXPLAIN including the recommended secondary index, the new EXPLAIN shows:

Since the "Estimated Costs" (high-lighted in yellow) did not actually improve, this means that the recommended secondary index is not effective. Is my assumption correct? If not, how do I know if a secondary index is effective?

Thank you.