当ORACLE数据库系统启动后，数据字典总是可用，它驻留在SYSTEM表空间中。数据字典包含视图集，在许多情况下，每一视图集有三种视图包含有类似信息，彼此以前缀相区别，前缀为USER、ALL和DBA。-When after the start of the ORACLE database system , the data dictionary is always available, it resides in the SYSTEM tablespace . The data dictionary contains a set of views , in many cases , each view set of three views containing similar information , and another as a prefix to distinguish the prefix for the USER , ALL , and DBA .