Oracle stores a view’s definition in the data dictionary as the text of the query that
defines the view. When you reference a view in a SQL statement, Oracle:
1. Merges the statement that references the view with the query that defines the view
2. Parses the merged statement in a shared SQL area
3. Executes the statement
Oracle parses a statement that references a view in a new shared SQL area only if no
existing shared SQL area contains a similar statement. Therefore, you get the benefit of
reduced memory use associated with shared SQL when you use views.

视图机制
当在SQL语句中引用了视图时 , Oracle做了以下工作 :
1. 将引用了视图的语句与视图的定义语句整合为一个语句
2. 在共享 SQL 区解析整合后的语句
3. 执行语句