查找bad sql的方法
查找运行系统里bad sql是一个古老的话题, 我们要根据自己的实际情况来分析。
绝不能教条的运用下面介绍的这些方法。
使用这些SQL语句时，会对系统表产生分组操作，当然也增大了系统的负载。
建议大家在系统启动了一段时间后，在半夜负载较轻的时间定时(例如:一个月)来查一查。一定要具体问题具体分析。
下面是我收藏的一些查找bad sql的方法:
column sql_text format a80;
-- 值得怀疑的SQL 来自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
/
-- 逻辑读多的SQL
select * from (select buffer_gets, sql_text
from v$sqlarea
where buffer_gets > 500000
order by buffer_gets desc) where rownum<=30;
-- 执行次数多的SQL
select sql_text,executions from
(select sql_text,executions from v$sqlarea order by executions desc)
where rownum<81;
-- 读硬盘多的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;
--分析的次数太多，执行的次数太少，要用绑变量的方法来写sql
set pagesize 600;
set linesize 120;
select substr(sql_text,1,80) "sql", count(*), sum(executions) "totexecs"
from v$sqlarea
where executions < 5
group by substr(sql_text,1,80)
having count(*) > 30
order by 2;
-- 游标的观察
set pages 300;
select sum(a.value), b.name
from v$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$open_cursor
group by user_name,sql_text having count(0)>30;
--查看当前用户&username执行的SQL
select sql_text from v$sqltext_with_newlines where (hash_value,address) in
(select sql_hash_value,sql_address from v$session where username='&username')
order by address,piece;
