Materialized Views and MV Logs

This is a very important topic for Interviews. So don't skip this!

A materialized view, or Snapshot as they were previously known, is a table segment whose contents are periodically refreshed based on a query, either against a local or remote table. Using materialized views against remote tables is the simplest way to achieve replication of data between sites.

Basic syntax :

CREATE MATERIALIZED VIEW view-name
BUILD [IMMEDIATE | DEFERRED]
REFRESH [FAST | COMPLETE | FORCE ]
ON [COMMIT | DEMAND ]
[[ENABLE | DISABLE] QUERY REWRITE]
[ON PREBUILT TABLE]
AS
SELECT ...;

The BUILD clause options are shown below.

• IMMEDIATE : The materialized view is populated immediately.
• DEFERRED : The materialized view is populated on the first requested refresh.

The following refresh types are available.

• FAST : A fast refresh is attempted. If materialized view logs are not present against the source tables in advance, the creation fails.
• COMPLETE : The table segment supporting the materialized view is truncated and repopulated completely using the associated query.
• FORCE : A fast refresh is attempted. If one is not possible a complete refresh is performed.

A refresh can be triggered in one of two ways.

• ON COMMIT : The refresh is triggered by a committed data change in one of the dependent tables.
• ON DEMAND : The refresh is initiated by a manual request or a scheduled task.

The QUERY REWRITE clause tells the optimizer if the materialized view should be consider for query rewrite operations. An example of the query rewrite functionality is shown below.

The ON PREBUILT TABLE clause tells the database to use an existing table segment, which must have the same name as the materialized view and support the same column structure as the query.

Basic things to check : 

• User has CREATE MATERIALIZED VIEW privilege
• If creating MV on a remote database, does user have CREATE DATABASE LINK privilege ? 

Connect to the materialized view owner and create the database link and the materialized view itself.

CONNECT scott/tiger@db2

CREATE DATABASE LINK DB1.WORLD CONNECT TO scott IDENTIFIED BY tiger USING 'DB1.WORLD';

CREATE MATERIALIZED VIEW emp_mv
BUILD IMMEDIATE 
REFRESH FORCE
ON DEMAND
AS
SELECT * FROM emp@db1.world;

Remember to gather stats after building the materialized view.

BEGIN
  DBMS_STATS.gather_table_stats(
    ownname => 'SCOTT',
    tabname => 'EMP_MV');
END;
/

Create Materialized View Logs

Since a complete refresh involves truncating the materialized view segment and re-populating it using the related query, it can be quite time consuming and involve a considerable amount of network traffic when performed against a remote table. To reduce the replication costs, materialized view logs can be created to capture all changes to the base table since the last refresh. This information allows a fast refresh, which only needs to apply the changes rather than a complete refresh of the materialized view.

To take advantage of the of the fast refresh, connect to the master instance and create the materialized view log.

CONNECT scott/tiger@db1

CREATE MATERIALIZED VIEW LOG ON scott.emp
TABLESPACE users
WITH PRIMARY KEY
INCLUDING NEW VALUES;

Refresh 

A materialized view can be manually refreshed using the DBMS_MVIEW package.

EXEC DBMS_MVIEW.refresh('EMP_MV');

Rather than using a refresh group, you can schedule DBMS_MVIEW.REFRESH called using the Oracle Scheduler

Application :

Materialized views can be used to improve the performance of a variety of queries, including those performing aggregations and transformations of the data. This allows the work to be done once and used repeatedly by multiple sessions, reducing the total load on the server.

Create a materialized view to perform the aggregation in advance, making sure you specify the ENABLE QUERY REWRITE clause.

CREATE MATERIALIZED VIEW emp_aggr_mv
BUILD IMMEDIATE 
REFRESH FORCE
ON DEMAND
ENABLE QUERY REWRITE 
AS
SELECT deptno, SUM(sal) AS sal_by_dept
FROM   emp
GROUP BY deptno;

EXEC DBMS_STATS.gather_table_stats(USER, 'EMP_AGGR_MV');