/*
Hit ratios below 90-95% are usually a sign of poor indexing, 
BUT Distortion of the hit ratio numbers is possible
*/
select (1-(sum(decode(name, 'physical reads', value,0))/ 	
	 (sum(decode(name, 'db block gets', value,0)) + 	
	 (sum(decode(name, 'consistent gets', value,0)))))) * 100
	 "Read Hit Ratio" 
from   v$sysstat;