Data Replication with Materialized Views

ISYS 650

What is a Materialized View ?• A materialized view (MV) is a database object

that stores the results of a query at a single point in time. Unlike a view, materialized view is not virtual.

• Materialized views are sometimes referred to as snapshots.

• Materialized views may be stored locally or remotely in other site.– Remote materialized view (RMV)

Why use materialized views?• Support applications that do not require current

data or require data valid at a specific point in time (snapshot data).

• Increase query performance since it contains results of a query.

• Remote materialized views are an efficient way to replicate data at different sites compared to fully consistent distributed data.– Do not require dedicated network connection and

network load is reduced. – Efficiently support remote users

Materialized View Management

• Define materialized view• Refresh materialized view

– Data of a materialized view may be out-of-date and require to be refreshed.

• Drop materialized view

Types of Materialized Views• 1. Read-Only Materialized Views

– Records in the MV cannot be changed by users.– Eliminates the possibility of a materialized view

introducing data conflicts with the master (base tables).

• 2. Updatable Materialized Views – users can make changes to the data at the

materialized view site. – Changes made to an updatable materialized view are

pushed back to the master during refresh.– Oracle only allow RMV to be updatable.

Refresh Methods

• 1. Complete Refresh– essentially re-creates the materialized view

• 2. Fast Refresh (Differential Refresh)– To perform a fast refresh, first identifies the changes

that occurred in the master since the most recent refresh of the materialized view and then applies these changes to the materialized view.

– Fast refreshes are more efficient than complete refreshes when there are few changes to the master or the view is refreshed frequently.

Materialized View Log for Fast Refresh

• A materialized view log is required on a master if you want to perform a fast refresh on materialized views based on the master. The log is used to record changes to the master.

• The log is designed to record changes to the master since the last refresh, and net changes since the last refresh can be identified.

Fast Refresh of a MV

Oracle’s Implementation of MV

• 1. Primary Key Materialized Views– The default type of materialized view.– Primary key materialized views allow materialized view

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

• 2. ROWID Materialized Views– A ROWID materialized view is based on the physical row

identifiers (rowids) of the rows in a master.– Ex. Select ROWID, CID, Cnama From Customer;– For views based on master tables that do not have a

primary key, or for views that do not include all primary key columns of the master tables.

Define a Read-Only, Primary Key Materialized Views

• http://psoug.org/reference/materialized_views.html

• CREATE MATERIALIZED VIEW mv_Customer AS SELECT * FROM Customer;

• Note: Compare view and materialized view

Multiple Tables MVCREATE MATERIALIZED VIEW "STUDENTUNITSAS select sid,sname,sum(units) as TotalUnits from (student natural join (registration natural join course)) group by sid,sname;

Initiating an On-Demand Refresh

• On-Demand Refresh– Immediately refresh dependent materialized view

to propagate the new rows of the master table to associated materialized views.

– Example:– execute DBMS_MVIEW.REFRESH( 'MV‘)

– Note: ‘MV’ is the view’s name.

Requirements for Fast Refresh

• 1. The base table must have a primary key constraint.

• 2. Must create an update log.

Define a MV for Fast Refresh

CREATE MATERIALIZED VIEW mv_Customer

REFRESH FAST

AS SELECT * FROM Customer;

Note: Must create a log first.

CREATE MATERIALIZED VIEW LOG for Fast Refresh

• Use the CREATE MATERIALIZED VIEW LOG statement to create a materialized view log, which is a table associated with the master table of a materialized view.

• A master table can have only one log.– The log’s name is: MLOG$_TableName– http://www.sqlsnippets.com/en/topic-12878.html

• Example: – create materialized view log on tableName WITH

PRIMARY KEY ; – create materialized view log on Faculty WITH

PRIMARY KEY ;

Log’s Structure with Primary Key

Log’s Structure with ROWID

Demo

• Make a few changes to the base table.• See the log records.• See the MV records.• Issue a refresh command:

– execute DBMS_MVIEW.REFRESH( 'MV‘)

• The Log records will be deleted automatically after the refresh.

Fast Refresh On Commit

CREATE MATERIALIZED VIEW mv_Customer

REFRESH FAST On Commit

AS SELECT * FROM Customer;

Note: Must create a log first.Note: MV is refreshed after each update.

Other Way to Initiate Refresh

• Scheduled Refresh– An interval of one hour is specifies as:

• SYSDATE + 1/24 – An interval of seven days is specifies as:

• SYSDATE + 7

• Example:• CREATE MATERIALIZED VIEW MVFaculty2• REFRESH FAST START WITH SYSDATE NEXT SYSDATE +

1/4096 • AS SELECT * FROM faculty;

Grant Privileges to Other User

• 1. Select the table or view in the Object Browser

• 2. Click the Grants tab and grant the privileges to other user (grantee).

• The Grantee can access the data from his/her database by adding the granter’s name:

• Example: If user HR grants the Select privilege:– Select * From HR.Customer;

MV Based on Granted Table

• Login HR and grant Employees table privilege to dchao. Logout HR

• Login dchao and create an MV based on HR.Employees. Logout dchao

• Login HR and make a few changes. Logout HR• Login dchao and list records in the MV.• Issue a refresh command.

Complex & Simple Materialized View

• Simple Materialized View– Each row in the materialized view can be mapped

back to a single row in a source table

• Complex Materialized View – Each row in the materialized view can not be

mapped back to a single row in a source table.

Simple vs Complex MV

If you refresh rarely and want faster query performance, then use Method A (complex materialized view).If you refresh regularly and can sacrifice query performance, then use Method B (simple materialized view).

Materialized View Concepts and Architecture

• http://download.oracle.com/docs/cd/B10501_01/server.920/a96567/repmview.htm#30769

• http://download.oracle.com/docs/cd/B19306_01/server.102/b14200/statements_6002.htm

• http://www.sqlsnippets.com/en/topic-12890.html

Define a ROWID Materialized Views

• CREATE MATERIALIZED VIEW orders • REFRESH WITH ROWID • AS SELECT * FROM orders;

• Create log with ROWID:– create materialized view log on tableName WITH

ROWID;

Differential Refresh• Three kinds of update:

– Insertion– Deletion– Modification:

• Deletion of the before-image, and insertion of the after-image.

• Differential refresh:Let Deletions is the set of the all deleted records, and

Insertions is the set of all new records. New MV = Old MV – Deletions + Insertions