Checking The Cache Hit Ratio
SELECT (P1.value + P2.value - P3.value) / (P1.value + P2.value) FROM v$sysstat P1, v$sysstat P2, v$sysstat P3 WHERE P1.name = 'db block gets' AND P2.name = 'consistent gets' AND P3.name = 'physical reads';
http://www.dbspecialists.com/files/presentations/buffercache.html
SELECT name, physical_reads, db_block_gets, consistent_gets, 1 - (physical_reads / (db_block_gets + consistent_gets)) "Hit Ratio" FROM V$BUFFER_POOL_STATISTICS;
http://docs.oracle.com/cd/E16655_01/server.121/e15857/tune_buffer_cache.htm#TGDBA545