Key compressed indexes are a way to index a low cardinality column without talking up as much space storing the repetitive values. Please view the Oracle Manuals for a full description on implementing compressed indexes.  

Benefits

• Fewer index leaf blocks need to be read since more rows fit into fewer blocks
• Insert statements are faster since fewer leaf blocks need to be read to determine where the new row is inserted
• Faster full index scans due to fewer leaf blocks
• Space savings