column xn1 format a50
column xn2 format a50
column xn3 format a50
column xv1 new_value xxv1 noprint
column xv2 new_value xxv2 noprint
column xv3 new_value xxv3 noprint
column d1  format a50
column d2  format a50

prompt HIT RATIO: 
prompt
prompt Values Hit Ratio is calculated against:
prompt

      37 opens of replaced files
         8          0

        38 db block gets
         8     416712

        39 consistent gets
         8  124025843


select lpad(name,20,' ')||'  =  '||value xn1, value xv1
from   v$sysstat
where  statistic# = 38
/ 
-- was where  statistic# = 37

select lpad(name,20,' ')||'  =  '||value xn2, value xv2
from   v$sysstat
where  statistic# = 39
/ 

-- was 38
select lpad(name,20,' ')||'  =  '||value xn3, value xv3
from   v$sysstat b 
where  statistic# = 40
/ 
-- was 39

set pages 60 

select 'Logical reads = db block gets + consistent gets ',
        lpad ('Logical Reads = ',24,' ')||to_char(&xxv1+&xxv2) d1
from    dual 
/ 

select 'Hit Ratio = (logical reads - physical reads) / logical reads',
        lpad('Hit Ratio = ',24,' ')||
        round( (((&xxv2+&xxv1) - &xxv3) / (&xxv2+&xxv1))*100,2 )||'%' d2
from    dual 
/ 

prompt If the hit ratio is less than 60%-70%, increase the initialization
prompt parameter DB_BLOCK_BUFFERS.  ** NOTE:  Increasing this parameter will
prompt increase the SGA size.
prompt 

In my 1997 edition of Advanced Oracle Tuning by Aronoff et al (signed by the authors they give a hit ratio tuning, but enough caveats to almost negate it. One interesting thing they point out is how temp segments are not recorded as logical reads, since they are unformatted gets and so don’t need a consistency mechanism. But they are physical reads, so it becomes possible to have a negative hit ratio. Perhaps a clue of too small sort_area_size.

Hopefully, people are finding that the randomness of the topics on this blog continues to keep the blog interesting. There are many great blogs on the Internet with a significant level of detail and research backing each post. And this blog, well, its Miscellaneous Random Oracle Topics, and will sometimes refer back to those other blogs that are backed by detail and research.

Kind regards
Uwe