Thanks for ur suggetions.

I've started analyzing the indexes.
Then fired this query:
SELECT name,height,lf_rows,del_lf_rows, USED_SPACE
FROM INDEX_STATS;
It shows following output (for one indx)

NAME HEIGHT LF_ROWS DEL_LF_ROWS USED_SPACE
---------------------------------------------
PK_ORDER_ID 1 22 0 570 and total space of that indx is 10 MB and used space =570, pct_used=8,LF_BLK_LEN=8000,BTREE_SPACE=8000
What does it mean ? Used space is in bytes right ?

Also I've tried this query
alter index PK_ORDER_ID monitoring usage

select index_name,used from v$object_usage where index_name='PK_ORDER_ID'

But it shows nothing.

I am not able to understand what to do from this result. Can u pls guide me about this and tell steps how to proceed further ?

Thanks for your valuable time.