尋找bad sql的方法

尋找bad sql的方法

    尋找運行系統裡bad sql是一個古老的話題, 我們要根據自己的實際情況來分析。

    絕不能教條的運用下面介紹的這些方法。

    使用這些SQL語句時，會對系統資料表產生分組操作，當然也增大了系統的負載。

    建議大家在系統啟動了一段時間後，在半夜負載較輕的時間定時(例如:一個月)來查一查。一定要具體問題具體分析。

    下面是我收藏的一些尋找bad sql的方法:

column sql_text format a80;
-- 值得懷疑的SQL 來自http://www.ixora.com.au/
select
  substr(to_char(s.pct, '99.00'), 2) '%'  load,
  s.executions  executes,
  p.sql_text
from
  (
    select
      address,
      disk_reads,
      executions,
      pct,
      rank() over (order by disk_reads desc)  ranking
    from
      (
        select
          address,
          disk_reads,
          executions,
          100 * ratio_to_report(disk_reads) over ()  pct
        from
          sys.v_$sql
        where
          command_type != 47
      )
    where
      disk_reads > 50 * executions
  )  s,
  sys.v_$sqltext  p
where
  s.ranking <= 5 and
  p.address = s.address
order by
  1, s.address, p.piece
/

-- 邏輯讀多的SQL
select * from (select buffer_gets, sql_text
from v$sqlarea
where buffer_gets > 500000
order by buffer_gets desc) where rownum<=30;

-- 執行次數多的SQL
  select sql_text,executions from
  (select sql_text,executions from v$sqlarea order by executions desc)
   where rownum<81;

-- 讀硬碟多的SQL
  select sql_text,disk_reads from
  (select sql_text,disk_reads from v$sqlarea order by disk_reads desc)
   where rownum<21;

-- 排序多的SQL
  select sql_text,sorts from
   (select sql_text,sorts from v$sqlarea order by sorts desc)
    where rownum<21;

--分析的次數太多，執行的次數太少，要用綁變數的方法來寫sql
set pagesize 600;
set linesize 120;
select substr(sql_text,1,80) "sql", count(*), sum(executions) "totexecs"
   from v$sqlarea
   where executions < 5
   group by substr(sql_text,1,80)
   having count(*) > 30
   order by 2;
-- 遊標的觀察
set pages 300;
select sum(a.value), b.name
     from v$sesstat a, v$statname b
     where a.statistic# = b.statistic#
       and b.name = 'opened cursors current'
     group by b.name;

select count(0) from v$open_cursor;

select user_name,sql_text,count(0) from v$open_cursor
group by user_name,sql_text having count(0)>30;

--查看目前使用者&username執行的SQL
select sql_text from v$sqltext_with_newlines where (hash_value,address) in
(select sql_hash_value,sql_address from v$session where username='&username')
order by address,piece;