for a DW environment a lower buffer hit ratio is common.

Actually considering buffer cache hit ratio's as measure for tuning is flawed.

COnnor Mcdonald & Jonathan lewis have given a classic example to prove this. With his method, you can actually get any hit ratio YOU WANT!!

http://oracledba.co.uk/tips/choose.htm

In these days where people are still mistakenly
worrying about hit ratios, here is a simple routine to
bump up your hit ratio to any value desired. Thanks to
Jonathan Lewis for some of the strategy used.
create or replace
procedure choose_a_hit_ratio(p_ratio number default 99,p_show_only boolean default false) is
v_phy number;
v_db number;
v_con number;
v_count number;
v_additional_congets number;
v_hit number;
procedure show_hit is
begin
select p.value, d.value, c.value
into v_phy, v_db, v_con
from
( select value from v$sysstat where name = 'physical reads' ) p,
( select value from v$sysstat where name = 'db block gets' ) d,
( select value from v$sysstat where name = 'consistent gets' ) c;
v_hit := 1-(v_phy/(v_db+v_con));
dbms_output.put_line('Current ratio is: '||round(v_hit*100,5));
end;
begin
--
-- First we work out the ratio in the normal fashion
--
show_hit;
if p_ratio/100 < v_hit or p_ratio > 99.9999999 then
dbms_output.put_line('Sorry - I cannot help you');
return;
end if;
--
-- Flipping the formula we can work out how many more consistent gets
-- we need to increase the hit ratio
--
v_additional_congets := trunc(v_phy/(1-p_ratio/100)-v_db - v_con);
dbms_output.put_line('Another '||v_additional_congets||' consistent gets needed...');
if p_show_only then return; end if;
--
-- Create a simple table to hold 200 rows in a single block
--
begin
execute immediate 'drop table dummy';
exception
when others then null;
end;
execute immediate 'create table dummy (n primary key) organization index as '||
'select rownum n from all_objects where rownum <= 200';
--
-- Turn off any new 9i connect-by features to ensure we still do lots of
-- logical IO
--
begin
execute immediate 'alter session set "_old_connect_by_enabled" = true';
exception
when others then null;
end;
--
-- Grind away until we do all those additional gets
--
execute immediate '
select count(*)
from (
select n
from dummy
connect by n > prior n
start with n = 1 )
where rownum < :v_additional_congets' into v_count using v_additional_congets;
show_hit;
end;
/
And some output to keep the hit ratio fanatics happy!
SQL> exec choose_a_hit_ratio(85,true);
Current ratio is: 82.30833
Another 29385 consistent gets needed...
PL/SQL procedure successfully completed.
SQL> exec choose_a_hit_ratio(85);
Current ratio is: 82.30833
Another 29385 consistent gets needed...
Current ratio is: 86.24548
PL/SQL procedure successfully completed.
SQL> exec choose_a_hit_ratio(90,true);
Current ratio is: 86.24731
Another 79053 consistent gets needed...
PL/SQL procedure successfully completed.
SQL> exec choose_a_hit_ratio(90);
Current ratio is: 86.24731
Another 79053 consistent gets needed...
Current ratio is: 90.5702
PL/SQL procedure successfully completed.
SQL> exec choose_a_hit_ratio(98,true);
Current ratio is: 90.5709
Another 1141299 consistent gets needed...
PL/SQL procedure successfully completed.
SQL> exec choose_a_hit_ratio(98);
Current ratio is: 90.5709
Another 1141299 consistent gets needed...
Current ratio is: 98.02386
PL/SQL procedure successfully completed.

[Updated on: Thu, 09 March 2006 12:43]

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