Materialized Views in Oracle
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.
Basic Syntax-
The full syntax description for the CREATE MATERIALIZED VIEW command is available in the documentation. Here we will only concern ourselves with the basics.
--Normal
CREATE MATERIALIZED VIEW view-name
BUILD [IMMEDIATE | DEFERRED]
REFRESH [FAST | COMPLETE | FORCE ]
ON [COMMIT | DEMAND]
[[ENABLE | DISABLE] QUERY REWRITE]
AS
SELECT ...;
-- Pre-Built
CREATE MATERIALIZED VIEW view-name ON PREBUILT TABLE
REFRESH [FAST | COMPLETE | FORCE ]
ON [COMMIT | DEMAND]
[[ENABLE | DISABLE] QUERY REWRITE]
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 considered. 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.
Check Privileges -
Check the user who will own the materialized views has the correct privileges. At minimum they will require the CREATE MATERIALIZED VIEW privilege. If they are creating materialized views using database links, you may want to grant them CREATE DATABASE LINK privilege also.
GRANT CREATE MATERIALIZED VIEW TO username;
GRANT CREATE DATABASE LINK TO username;
Create Materialized View-
Connect to the materialized view owner and create the database link and the materialized view itself.
CREATE MATERIALIZED VIEW A2.employee_salary_summary
BUILD IMMEDIATE REFRESH FAST ON
COMMIT
AS
SELECT employee_id, SUM(salary) as total_salary
FROM A2.employees
GROUP BY employee_id;
Create Materialized View Logs-
Since a complete refresh involves truncating the materialized view segment and re-populating it is 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
CREATE MATERIALIZED VIEW LOG ON employees;
Refresh Materialized Views-
If a materialized view is configured to refresh on commit, you should never need to manually refresh it, unless a rebuild is necessary. Remember, refreshing on commit is a very intensive operation for volatile base tables. It makes sense to use fast refreshes where possible.
For on demand refreshes, you can choose to manually refresh the materialized view or refresh it as part of a refresh group.
The following code creates a refresh group defined to refresh every minute and assigns a
materialized view to it.
Information about refresh groups and the materialize views in a refresh group can be queried from the DBA_RGROUP and DBA_RCHILD views respectively.
A materialized view can be manually refreshed using the DBMS_MVIEW package.
Rather than using a refresh group, you can schedule DBMS_MVIEW.REFRESH called using the Oracle Scheduler.
Cleaning Up-
To clean up we must remove all objects.
DROP MATERIALIZED VIEW your_materialized_view_name;
DROP DATABASE LINK your_database_link_name;
Considerations-
Before using materialized views and materialized view logs, consider the following:
Populating a materialized view adds load to both servers involved. The source server is queried to capture the data, which is inserted into the destination server. Be sure the additional load does not adversely affect your primary system.
Although materialized view logs improve the performance of materialized view refreshes, they do increase the work needed to perform DML on the base table. Check the additional work does not adversely affect performance on the primary system.
If regular refreshes are not performed, materialized view logs can grow very large, potentially reducing the performance of their maintenance and blowing tablespace limits.
Depending on the Oracle version and the complexity of the associated query, fast refreshes may not be possible.
When using materialized views to improve performance of transformations and aggregations
the QUERY_REWRITE_INTEGRITY and QUERY_REWRITE_ENABLED parameters must be set, or the server will not be able to automatically take advantages of query rewrites. These parameters may be set in the pfile or spfile file if they are needed permanently. Later releases have them enabled by
--BY Tarif Ahmed
