Rebuild Index

In the following scenarios, one can think about rebuilding indexes->

1 If the ratio of deleted to actual leaf entries is more than 20% or
2 If the "blevel">3 (in table dba_indexes)
Here's how you can find it.You are supposed to do
analyze index validate structure ;
and then query the view INDEX_STATS
SELECT name,height,lf_rows,del_lf_rows,(del_lf_rows/lf_rows)*100 FROM INDEX_STATS;

Here's the command to rebuild indexes ->
 
alter index rebuild online;
You will require twice the space because during index rebuilding, the old index will still be there and will be dropped only when the new one has been created successfully.