thanks for your advises.

so you are saying that the db_file_multiblock_read_count is too high so that Oracle think FTS is cheaper so that it do full table scan instead of using index. By changing the db_file_multiblock_read_count =16, do you think Oracle will try to use indexes somehow. The thing is that before the changes I made today, sometime Oracle use index and sometime it do FTS and it's very inconsistent.