Oracle Materialized View

A materialized view in Oracle 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 table’s data.

A materialized view can query tables, views, and other materialized views.
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.

Syntax

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

Methods

BUILD

IMMEDIATE : The materialized view is populated immediately.This option is default one.
DEFERRED : The materialized view is populated on the first requested refresh.

Refresh Types

• FAST: A fast refresh is attempted only there is a change in the base table. If materialized view logs are not present against the source tables in advance, the creation fails. To maintain the history of change in the base table, it is known as materialized view log.It is named as MLOG$_<base_table>. Materialized view log will be located in the source database in the same schema as the master table. Refresh fast will perform refresh according to the changes that occurred in the master table.
• 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.

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.

GRANT CREATE MATERIALIZED VIEW TO INDIA;
Grant succeeded.

GRANT CREATE DATABASE LINK TO INDIA;
Grant succeeded.

Creating Database link to point remote database 

SQL> CREATE DATABASE LINK PROD_LINK CONNECT TO ph IDENTIFIED BY ph USING 'orcl';

Database link created.

COMPLETE REFRESH

Creating Materialized View

SQL> CREATE MATERIALIZED VIEW dept_mv BUILD IMMEDIATE REFRESH COMPLETE AS 
SELECT * FROM ph.emp@PROD_LINK;

Materialized view created.

SQL> select * from emp;

        NO NAME                     SALARY
---------- -------------------- ----------
         1 apple                     20000
         2 orange                    23000
         3 mango                     32000

Table Data present in Remote database

SQL> insert into emp values (6,'ice',30000);

1 row created.

SQL> commit;

Commit complete.

SQL> select rowid,no,name,salary from dept_mv;

ROWID                      NO NAME                     SALARY
------------------ ---------- -------------------- ----------
AAAR36AABAAAbURAAA          1 apple                     20000
AAAR36AABAAAbURAAB          2 orange                    23000
AAAR36AABAAAbURAAC          3 mango                     32000

Manually using complete refresh using the DBMS_VIEW package

SQL>  execute DBMS_MVIEW.REFRESH( LIST => 'dept_mv', METHOD => 'C' );

PL/SQL procedure successfully completed.

SQL> select rowid,no,name,salary from dept_mv;

ROWID                      NO NAME                     SALARY
------------------ ---------- -------------------- ----------
AAAR36AABAAAbURAAD          1 apple                     20000
AAAR36AABAAAbURAAE          2 orange                    23000
AAAR36AABAAAbURAAF          3 mango                     32000
AAAR36AABAAAbURAAG          6 ice                       30000

DEFERRED

Initially drop the existing materialized view

SQL> drop MATERIALIZED VIEW dept_mv;

Materialized view dropped.

Materialized view creation using DEFERRED option

SQL> CREATE MATERIALIZED VIEW dept_mv BUILD DEFERRED REFRESH COMPLETE AS 
SELECT * FROM ph.emp@PROD_LINK;

Materialized view created.


SQL> execute DBMS_MVIEW.REFRESH( LIST => 'dept_mv', METHOD => 'C' );

PL/SQL procedure successfully completed.

QL> select rowid,no,name,salary from dept_mv;

ROWID                      NO NAME                     SALARY
------------------ ---------- -------------------- ----------
AAAR39AABAAAbURAAA          1 apple                     20000
AAAR39AABAAAbURAAB          2 orange                    23000
AAAR39AABAAAbURAAC          3 mango                     32000
AAAR39AABAAAbURAAD          6 ice                       30000

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