事件：当运行DROP MATERIALIZED VIEW 时 会话hang住。

用下面命令生成跟踪文件（通过任意会话）：

我们可以在系统文件中看到下面的等待事件：

当我们试图删除这个物化视图时由于DBMS_JOB引起了这个等待事件，所以在删除这个JOB后我们就可以删除该物化视图

原因：Due to the JI contention

解决办法：
1.   killled正在运行的job，检查Check DBMS_JOBS_RUNNING 视图.

2.  刷新job并离线，移除job

3.  完成删除，看不到相关job和物化视图。

附等待事件介绍：出处：http://www.askmaclean.com/archives/enq-ji-contention.html

Sessions waiting on this event are waiting on locks held during materialized view operations (such as refresh, alter) to prevent concurrent operations on the same materialized view.

Solutions

A materialized view cannot be fast refreshed more than once in a given period because it is serialized during the commit phase. Ensure that only one session at a time is performing the refreshes. If there is more than one session, the first session will work normally but the subsequent sessions will wait on “enq: JI – contention”.

Waits on this event can also be caused by on-commit time logic within the materialized view. Normally when a session updates record 1 and commits and then another session updates record 2 and commits, they do not have to wait for each other. However, when using an on commit-time fast refreshable materialized view on top of the table, we do have to wait when two sessions do totally unrelated transactions concurrently against the same table. This is not a problem when the table is modified infrequently or only by a single session, but it can be a big problem when applied to a table that performs a lot of modifications concurrently. Be sure to use on commit-time fast refreshable materialized views for implementing business rules only on tables that are not concurrently accessed or infrequently changed

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