Data replication with materialized views
Sponsored Links
This presentation is the property of its rightful owner.
1 / 27

Data Replication with Materialized Views PowerPoint PPT Presentation


  • 91 Views
  • Uploaded on
  • Presentation posted in: General

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.

Download Presentation

Data Replication with Materialized Views

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


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    • Data of a materialized view may be out-of-date and require to be refreshed.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

CREATE MATERIALIZED VIEW "STUDENTUNITS

AS

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


  • Login