select 1-(phy.value - lob.value -dir.value)/ ses.value "cache hit ratio"
  from v$sysstat ses, v$sysstat lob, v$sysstat dir, v$sysstat phy
  where ses.name = 'session logical reads'
  and dir.name = 'physical reads direct'
  and lob.name = 'physical reads direct (lob)'
  and phy.name = 'physical reads'