oracle搜集表的信息，并不是每张表都搜集,一下若干种情况，会被搜集到:
统计信息是空的表，会被搜集
达到一定比例的表，会被搜集
在搜集表信息的时候，加上CASCADE关键字之后，就会搜集此表相关的索引等信息.
Statistics Preferences:Overview
Using Statistics Preferences
- PUBLISH:Used to decide whether to publish the statistics to the dictionary or to store them in a pending area before.
- publish the statistics to the dictionary means : 就是将统计信息的数据存储到数据字典里;
- STALE_PERCENT:Used to determine the threshold level at whcih an object is considered to have stale statistics.The value is a percentage of rows modified since the last statistics gathering.
- INCREMENTAL:Used to gather global statistics on partitioned tables in an incremental way.
Setting Global Preferences with Enterprise Manager
