Data Reteieval on Huge Table

Hello Experts

I need your help to resolve a performance tuning issue.

we have a table called 'activity' with row count 17712037. this table will be loaded as a daily basis (load_dt) with hundreds of records.

This table is being used in a function to get the data for specified date ranges, and takes much time (hours) to execute, fetch the results.

we use the date ranges only to the recent 6 months of data.

we considered here two approaches:

1. we created index on column activity.load_dt , which we couldn't achieve best results.
  ( ex: load_dt >= (sysdate-60) and load_dt <= sysdate)