本文探讨了Exadata环境中出现的一种等待事件——快速全索引扫描卸载到存储单元的过程。这种事件与智能表扫描类似，但针对的是索引而非表。文中提到，由于Exadata在执行全表扫描方面非常高效，此类事件并不常见。文章进一步解释了只有全速扫描（而非范围或全扫描）的索引才有资格进行智能扫描，这降低了该事件发生的概率。尽管如此，如果某个查询在此等待事件上花费过多时间，则可能需要对该查询进行调优，以更有效地利用索引。
This is an Exadata wait event that occurs when fast full index scans are offloaded to the storage cells. This event is related to cell smart table scan except that the object is an index. This event does not show up very frequently in Exadata probably because it is very good at performing full table scans and index reads may actually be less efficient. Also, only full fast scans (not range or full scans) of indexes are eligible for smart scans, which reduces the likelihood of seeing this event.
Solutions
Offloading the index scans to the storage cells within an Exadata machine is probably a good thing, so there may be no tuning required. However, if you have a query that waits extensively on this wait event, it may mean that query tuning is required so it utilizes indexes more effectively. Also review the Objects tab in Ignite to understand which is causing the most wait time.
本文转自maclean_007 51CTO博客，原文链接:http://blog.51cto.com/maclean/1277895
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