Much of the data dictionary information is kept in the SGA in the dictionary cache,
because Oracle constantly accesses the data dictionary during database operation to
validate user access and to verify the state of schema objects. All information is stored
in memory using the least recently used (LRU) algorithm.

Parsing information is typically kept in the caches. The COMMENTS columns describing
the tables and their columns are not cached unless they are accessed frequently.

缓存数据字典
1. 大多数的数据字典信息被缓存到SGA中的字典缓冲区 ，使用最近最少使用算法
2. 解析信息就是代表性的保存在缓存中