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NESTED_TABLE_SET_REFS hint 是 相当于数据泵的操作 expdp 、impdp的操作
INSERT /*+NESTED_TABLE_SET_REFS+*/ 

一个Oracle未公开的hint提示出现了我的面前，再看这个session对应的后台应用，原来是一个export操作,以前还真没注意过，到metalink上查到了如下的解释：

This hint specifies that nested tables should be retrieved as refs.It is used for exporting, importing and loading. It should never be issued by 
user queries, and in fact is unlikely to make a difference if tried.

This is also consistent with seeing large amounts of queries with this hint, and that those queries are not reused. If this is a problem for you, try to export at another time of day. If the exports are really long-running or the system load is constant, try to find out what you are using exports for and suggest alternatives (eg. backup and recovery purposes, etc). If this is a DSS system, you probably have a smaller shared pool and use import/export heavily. If so, it might be a good idea to use a bigger shared pool when doing this activity.

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