有很多种方法可以用来找出哪些SQL语句需要优化，但是很久以来，最简单的方法都是分析保存在V$SQL视图中的缓存的SQL信息。通过V$SQL视图，可以确定具有高消耗时间、CUP和IO读取的SQL语句。

1.查看总消耗时间最多的前10条SQL语句
select *
  from (select v.sql_id,
               v.child_number,
               v.sql_text,
               v.elapsed_time,
               v.cpu_time,
               v.disk_reads,
               rank() over(order by v.elapsed_time desc) elapsed_rank
          from v$sql v) a
 where elapsed_rank <= 10;

2.查看CPU消耗时间最多的前10条SQL语句
select *
  from (select v.sql_id,
               v.child_number,
               v.sql_text,
               v.elapsed_time,
               v.cpu_time,
               v.disk_reads,
               rank() over(order by v.cpu_time desc) elapsed_rank
          from v$sql v) a
 where elapsed_rank <= 10;

3.查看消耗磁盘读取最多的前10条SQL语句
select *
  from (select v.sql_id,
               v.child_number,
               v.sql_text,
               v.elapsed_time,
               v.cpu_time,
               v.disk_reads,
               rank() over(order by v.disk_reads desc) elapsed_rank
          from v$sql v) a
 where elapsed_rank <= 10;