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Materialized Views

Materialized views (Mview) are very much like a regular Oracle table, except that they are based on one or more tables.  They can be simple or complex, read only, or updatable.  Because an Mview is a physical table, changes are managed by updating the effected rows in the Mview when the underlying tables change.  Updating an Mview is called refreshing it.  There are two types of refresh: fast and full.   

In a full refresh, the Mview is truncated and rebuild from the underlying tables.  In a fast refresh, only the changes are updated in the Mview.  A forced refresh tells the Mview to try and execute a fast refresh, and if that fails, execute a full refresh.  In order to execute a fast refresh, the underlying tables must have a materialized view log.  This log records changes to the table so that the Mview can retrieve them. 

Mviews are normally used to aggregate information in a data warehouse database. They can also be used in replicating data from one database to another.  Unlike normal views, INSERTs/UPDATEs/DELETESs of Mview require implementing advanced replication, which is far beyond the scope of this book.  The Mview can be writable, which allows the data to be modified; however, the changes are not propagated back to the underlying tables and are lost when the Mview refreshes.


The above book excerpt is from:

Easy Oracle SQL

Get Started Fast writing SQL Reports with SQL*Plus

ISBN 0-9727513-7-8

John Garmany 

http://www.rampant-books.com/book_2005_1_easy_sql.htm

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