oracle的sql调优一个例子

查询某个session造成lmode为3的TM锁的表：

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 select owner,object_name,subobject_name
 from dba_objects
 where object_id in (
     select object_id
     from v$locked_object
     where locked_mode=3 and session_id=798);

上面这条sql需要执行很长时间，而子查询select object_id from v$locked_object where locked_mode=3 and session_id=798执行很快，父查询select owner,object_name,subobject_name from dba_objects where object_id in （'aaa','bbbb','cccc'）也执行很快。
后来做了调优，具体如下：

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 with a as (
     select object_id
     from v$locked_object
     where locked_mode=3 and session_id =798)
 select b.owner,b.object_name,b.subobject_name
 from dba_objects b,a
 where b.object_id=a.object_id;

至于原因，还不清楚。

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转载于:http://blog.itpub.net/31441616/viewspace-2153265/