Index performacne (Clustering factor)

Clustering factor 

Clustering factor should be low and nearly equal  to no of block in the table, if the clustering factor is high then index scan inefficient and it will effect to query performance. always try to make cluster factor in low level , to check the cluster factor use the below query

select ind.index_name,ind.table_name,ind.clustering_factor,tab.num_rows,tab.blocks from all_indexes ind,all_tables tab
where ind.table_name=tab.table_name and ind.table_name ='<table_name>';

if your accessing more than 10% of data from a table where you have high cluster factor index then optimizer will ignore the index and will do the full table scan.