A materialized view is a database object that contains the results of a query. They are local copies of data located remotely, or are used to create summary tables based on aggregations of a tables data. Materialized views, which store data based on remote tables are also, know as snapshots.

A materialized view can query tables, views, and other materialized views. Collectively these are called master tables (a replication term) or detail tables (a data warehouse term).

For replication purposes, materialized views allow you to maintain copies of remote data on your local node. These copies are read-only. If you want to update the local copies, you have to use the Advanced Replication feature. You can select data from a materialized view as you would from a table or view.

For data warehousing purposes, the materialized views commonly created are aggregate views, single-table aggregate views, and join views.

In this article, we shall see how to create a Materialized View and discuss Refresh Option of the view.

In replication environments, the materialized views commonly created are primary key, rowid, and subquery materialized views.

Primary Key Materialized Views

The following statement creates the primary-key materialized view on the table emp located on a remote database.

SQL7gt; 	CREATE MATERIALIZED VIEW mv_emp_pk
   	REFRESH FAST START WITH SYSDATE 
	NEXT  SYSDATE + 1/48
	WITH PRIMARY KEY 
	AS SELECT * FROM emp@remote_db;

Materialized view created.

Note: When you create a materialized view using the FAST option you will need to create a view log on the master tables(s) as shown below:

SQL> CREATE MATERIALIZED VIEW LOG ON emp;
Materialized view log created.

Rowid Materialized Views

The following statement creates the rowid materialized view on table emp located on a remote database:

SQL> 	CREATE MATERIALIZED VIEW mv_emp_rowid 
	REFRESH WITH ROWID 
   	AS SELECT * FROM emp@remote_db; 

Materialized view log created.

Subquery Materialized Views

The following statement creates a subquery materialized view based on the emp and dept tables located on the remote database:

SQL> CREATE MATERIALIZED VIEW  mv_empdept
AS SELECT * FROM emp@remote_db e
WHERE EXISTS
     (SELECT * FROM dept@remote_db d
     WHERE e.dept_no = d.dept_no)

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