[20171109]缓存命中率神话.txt

--//在oracle版本的早期,缓存命中率是一个很重要的优化指标,实际上这个根本不重要.
--//一般OLTP系统即使出现严重的性能问题,这个数值也很高,实际上一个简单的情况就能说明问题,
--//比如走hash join的计划,不小心走了nested loop,可能导致逻辑读上升.缓存命令率很高,但是数据库
--//未必运行在最佳性能.

--//这个也是我学习oracle早期一个不好理解的问题,^_^.

--//https://connor-mcdonald.com/2017/11/07/buffer-cache-hit-ratio-blast-from-the-past/给出一个例子,能很好的说明问题.

SYS@book> @ &r/ver1
PORT_STRING                    VERSION        BANNER
------------------------------ -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx            11.2.0.4.0     Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

SYS@book> grant select on v_$sysstat to scott ;
Grant succeeded.

--//注:我修改源代码,加入AUTHID CURRENT_USER .不然报如下错误:
SCOTT@book> exec choose_a_hit_ratio(92);
Current ratio is: 90.72333
Another 18142 consistent gets needed...
BEGIN choose_a_hit_ratio(92); END;

*
ERROR at line 1:
ORA-01031: insufficient privileges
ORA-06512: at "SCOTT.CHOOSE_A_HIT_RATIO", line 72
ORA-06512: at line 1

--//代码如下:
create or replace
procedure choose_a_hit_ratio(p_ratio number default 99,p_show_only boolean default false) AUTHID CURRENT_USER
  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;
/

--//执行如下:

SCOTT@book> set serveroutput on
SCOTT@book> exec choose_a_hit_ratio(85,true);
Current ratio is: 90.71867
Sorry - I cannot help you
PL/SQL procedure successfully completed.

SCOTT@book> exec choose_a_hit_ratio(92,true);
Current ratio is: 90.72316
Another 18144 consistent gets needed...
PL/SQL procedure successfully completed.

SCOTT@book> exec choose_a_hit_ratio(92);
Current ratio is: 90.86547
Another 16374 consistent gets needed...
Current ratio is: 92.06869
PL/SQL procedure successfully completed.

SCOTT@book> exec choose_a_hit_ratio(98);
Current ratio is: 92.33213
Another 443318 consistent gets needed...
Current ratio is: 98.01083
PL/SQL procedure successfully completed.

SCOTT@book> exec choose_a_hit_ratio(99,true);
Current ratio is: 98.0109
Another 598700 consistent gets needed...
PL/SQL procedure successfully completed.

SCOTT@book> exec choose_a_hit_ratio(99);
Current ratio is: 98.01091
Another 598698 consistent gets needed...
Current ratio is: 99.00114
PL/SQL procedure successfully completed.