查找低劣的SQL方法

查找運行系統裏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。建議大家在系統啓動了一段時間後，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查找bad sql的方法:

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;

    -- 讀硬盤多的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，要用綁變量的方法來寫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;

    --查看當前用戶&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;

1