materialized view in Oracle

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)

REFRESH CLAUSE



[refresh [fast|complete|force]
 [on demand | commit]
 [start with date] [next date]
 [with {primary key|rowid}]]



The refresh option specifies:


The refresh method used by Oracle to refresh data in materialized view
 
Whether the view is primary key based or row-id based
 
The time and interval at which the view is to be refreshed
 



Refresh
Method - FAST Clause



The
FAST refreshes use the materialized view logs (as seen above) to send the rows
that have changed from master tables to the materialized view.



You
should create a materialized view log for the master tables if you specify the REFRESH FAST clause. 



SQL> CREATE MATERIALIZED VIEW LOG ON emp;

Materialized view log created.

 

Materialized
views are not eligible for fast refresh if the defined subquery contains an
analytic function.



Refresh
Method - COMPLETE Clause



The
complete refresh re-creates the entire materialized view. If you request a
complete refresh, Oracle performs a complete refresh even if a fast refresh is
possible.



Refresh
Method - FORCE Clause



When
you specify a FORCE
clause, Oracle will perform a fast refresh if one is possible or a complete
refresh otherwise. If you do not specify a refresh method (FAST, COMPLETE, or FORCE), FORCE is
the default.



PRIMARY
KEY and ROWID Clause



WITH PRIMARY
KEY
is used to create a primary key materialized view i.e. the materialized view is
based on the primary key of the master table instead of ROWID (for ROWID
clause). PRIMARY KEY is the default option. To use the PRIMARY KEY clause you
should have defined PRIMARY KEY on the master table or else you should use
ROWID based materialized views.



Primary
key materialized views allow materialized view master tables to be reorganized
without affecting the eligibility of the materialized view for fast refresh. 



Rowid
materialized views should have a single master table and cannot contain any of
the following:

Distinct or aggregate functions
 
GROUP BY Subqueries , Joins & Set operations
 





Timing the refresh



The
START WITH clause tells the database when to perform the first replication from
the master table to the local base table. It should evaluate to a future point
in time. The NEXT clause specifies the interval between refreshes



SQL>  CREATE MATERIALIZED VIEW mv_emp_pk
   REFRESH FAST
 START WITH SYSDATE
 NEXT  SYSDATE + 2
 WITH PRIMARY KEY
 AS SELECT * FROM emp@remote_db;

Materialized view created.


In the above example, the first copy of the
materialized view is made at SYSDATE and the interval at which the refresh has
to be performed is every two days.



Summary



Materialized Views thus offer us flexibility of
basing a view on Primary key or ROWID, specifying refresh methods and
specifying time of automatic refreshes.