不知为什么，向目的库插入时速度超慢，在数据库里发现，这些库都有MLOG$东西被锁。

在网上，找到一篇blog，讲的还蛮有道理的，转过来自己收藏：

truncate MLOG$

解决高级复制环境下刷新日志HWM太高导致刷新越来越慢。
Doc ID:?Note:69432.1?Subject:?CAN AN INDEX BE CREATED ON MLOG$ (SNAPSHOT LOG)??Type:?BULLETIN?Status:?PUBLISHED?
Can an index be created on MLOG$ (SNAPSHOT LOG)?
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You have noticed that your fast refreshes are becoming slower and slower and?
less efficient as time goes on. You want to add an index on the MLOG$ snapshot?
log table to speed up the refreshes and to prevent a Full Table Scan on the?
snapshot log.

First of all, the answer is NOT to add an index on the snapshot log table.?
Oracle has to do a full table scan on the snapshot log so that it can read all?
the rows that it needs to refresh for a particular snapshot. Besides, all SQL?
statements generated by the refresh operation is hardcoded in the kernel.?

What may be causing the performance degradation is that your snapshot log's?
High Water Mark (HWM) may be wastefully high. The snapshot log table grows at?
peak times, but never shrinks once the refresh is done. Therefore, during a?
fast refresh of the snapshot, Oracle will read the snapshot log using full?
table scan all the way to the HWM.?

The answer to speeding up the performance of the snapshot refresh is to reset?
the HWM. The only way to do this is to truncate the snapshot log or recreate?
it.?

Once your snapshot log is purged (meaning all snapshots have already refreshed?
against that master table), then issue a truncate on it.

i.e. SQL> truncate table mlog$_EMP;
??????
This will reset the HWM without affecting any of your snapshot's fast refreshes.?

If you choose to RECREATE your snapshot log, you will have to follow up by
performing a COMPLETE refresh on all the affected snapshots.

试验下，再看。

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高级复制中mlog$表的HWM过高导致性能问题

这