如何查找消耗资源较大的SQL

对于优化来讲，查找消耗资源较大的SQL至关重要，下面介绍几个之前用到的SQL。
1.从V$SQLAREA中查询最占用资源的查询。
select b.username username,a.disk_reads reads,    
a.executions exec,a.disk_reads/decode(a.executions,0,1,a.executions) rds_exec_ratio,    
a.sql_text Statement    
from v$sqlarea a,dba_users b    
where a.parsing_user_id=b.user_id    
and a.disk_reads > 100000    
order by a.disk_reads desc;    
用buffer_gets列来替换disk_reads列可以得到占用最多内存的sql语句的相关信息。
V$SQL是内存共享SQL区域中已经解析的SQL语句。
2.列出使用频率最高的5个查询：
select sql_text,executions    
from (select sql_text,executions,    
rank() over    
(order by executions desc) exec_rank    
from v$sql)    
where exec_rank <=5;    
3.消耗磁盘读取最多的sql top5：
select disk_reads,sql_text    
from (select sql_text,disk_reads,    
dense_rank() over    
(order by disk_reads desc) disk_reads_rank    
from v$sql)    
where disk_reads_rank <=5;    
找出需要大量缓冲读取（逻辑读）操作的查询：
select buffer_gets,sql_text    
from (select sql_text,buffer_gets,    
dense_rank() over    
(order by buffer_gets desc) buffer_gets_rank    
from v$sql)    
where buffer_gets_rank<=5;