所谓视图实际上是不存储物理信息的（同表相区别，表存储世纪的数据和表的索引信息等），试图仅仅存储一个select语句而已，而物化视图就要视图也存储实际的数据。
物化视图看成是，一个定时运行的计算JOB+一个存计算结果的表，物化视图 实质上就是表，只不过会定时刷新。
A materialized view provides indirect access to table data by storing the results of a
query in a separate schema object. Unlike an ordinary view, which does not take up
any storage space or contain any data, a materialized view contains the rows
resulting from a query against one or more base tables or views. A materialized
viewcan be stored in the same database as its base tables or in a different database.
Materialized views stored in the same database as their base tables can improve
query performance through query rewrites. Query rewrites are particularly useful
in a data warehouse environment.
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