REFRESH CLAUSE

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

The refresh option specifies:

1. The refresh method used by Oracle to refresh data in materialized view
2. Whether the view is primary key based or row-id based
3. 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.

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