9 Copyright © 2005, Oracle. All rights reserved. Materialized Views.

9Copyright © 2005, Oracle. All rights reserved.

Materialized Views

9-2 Copyright © 2005, Oracle. All rights reserved.

Objectives

After completing this lesson, you should be able to do the following:

• Describe how summaries can be used to improve performance

• Differentiate materialized view types

• Explain materialized view integrity

• Create a materialized view

• List globalization implications for materialized views


The Need for Summary Management

• How can you improve query response time?– Use indexes.– Partition your data.– Implement parallel execution.

• What about precomputing query results?

• Create summaries: Materialized views– Automatically rewrite SQL applications.– Automatically refresh data.


Using Summaries to Improve Performance

• Special types of aggregate views

• Improve query execution time by precalculating expensive joins and aggregation operations before execution and storing results in a database table

• Created using a schema object called a materialized view


Using Summaries

• Original query by user:

• DBA creates summary table:

• New query by user using summary table:SELECT * FROM cust_sales_sum;

SELECT c.cust_id, SUM(amount_sold)FROM sales s, customers cWHERE s.cust_id = c.cust_idGROUP BY c.cust_id;

CREATE TABLE cust_sales_sum ASSELECT c.cust_id, SUM(amount_sold) AS amount FROM sales s, customers cWHERE s.cust_id = c.cust_idGROUP BY c.cust_id;


Using Materialized Views forSummary Management

• DBA creates materialized view:

• User issues original query:

• Query is rewritten by the Oracle server:SELECT * FROM cust_sales_mv;

CREATE MATERIALIZED VIEW cust_sales_mvENABLE QUERY REWRITE ASSELECT c.cust_id, SUM(amount_sold) FROM sales s, customers cWHERE s.cust_id = c.cust_idGROUP BY c.cust_id;

SELECT c.cust_id, SUM(amount_sold)FROM sales s, customers cWHERE s.cust_id = c.cust_idGROUP BY c.cust_id;


Using Materialized Views for Summary Management

DBA creates materialized view.

End user queries tables and views.

Oracle server rewrites SQL query to use materialized view.

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Summary Management Components

• Mechanisms to define materialized views and dimensions

• Refresh mechanism to ensure that materialized views contain the latest data

• Query rewrite capability to transparently rewrite a query to use a materialized view

• SQL Access Advisor: Recommends materialized views and indexes to be created

• DBMS_ADVISOR.TUNE_MVIEW procedure: Shows you how to make your materialized view fast refreshable and use general query rewrite


Using Summary Management

1. Use the SQL Access Advisor to determine how you will use materialized views.

2. Create materialized views and design how queries will be rewritten.

3. Use DBMS_ADVISOR.TUNE_MVIEW to obtain an optimized materialized view as necessary.


How Many Materialized Views?

• Query rewrite chooses which materialized view to use.

• One materialized view per query:– Ideal for query performance– Consumes too much disk space– Not recommended

• One materialized view for multiple queries:– One materialized view can be used to satisfy

multiple queries– Less disk space needed– Less time needed to maintain materialized views


One Materialized View for Multiple Queries

CREATE MATERIALIZED VIEW cust_sales_mv2ENABLE QUERY REWRITE ASSELECT cust_last_name, channel_id, SUM(amount_sold) FROM sales s, customers cWHERE s.cust_id = c.cust_idGROUP BY c.cust_last_name, s.channel_id;

SELECT cust_last_name, channel_id, SUM(amount_sold)FROM sales s, customers cWHERE s.cust_id = c.cust_idGROUP BY c.cust_last_name, s.channel_id;

SELECT *FROM cust_sales_mv2;

Query rewrite




SELECT cust_last_name, SUM(amount_sold)FROM sales s, customers cWHERE s.cust_id = c.cust_idGROUP BY cust_last_name;

SELECT cust_last_name, SUM(amount)FROM cust_sales_mv2GROUP BY cust_id;Query rewrite




SELECT channel_id, SUM(amount_sold)FROM salesGROUP BY channel_id;

SELECT channel_id, SUM(amount)FROM cust_sales_mv2GROUP BY channel_id;

Query rewrite


Determining Which Materialized View to Create

• One materialized view can be used to satisfy multiple queries.

• Multiple materialized views can satisfy the same query.

• A balance between performance and space usage must be found.

• Which materialized view should you create?– Analyze your workload.– Use the SQL Access Advisor.– Use DBMS_MVIEW.EXPLAIN_REWRITE to see why a

materialized view is used or ignored.


Types of Materialized Views

• Materialized views with aggregates:

• Materialized views containing only joins:

CREATE MATERIALIZED VIEW cust_sales_mv ASENABLE QUERY REWRITE ASSELECT c.cust_id, s.channel_id, SUM(amount_sold) FROM sales s, customers cWHERE s.cust_id = c.cust_idGROUP BY c.cust_id, s.channel_id;

CREATE MATERIALIZED VIEW sales_products_mv AS ENABLE QUERY REWRITE ASSELECT s.time_id, p.prod_nameFROM sales s, products pWHERE s.prod_id = p.prod_id(+);


The Need for Nested Materialized Views

• Typical data warehouse need: Create aggregate materialized views using different grouping columns on a single join.

• Maintaining the materialized views is time consuming because the underlying join is performed multiple times.

SALES_TIME_PROD_MV

PRODUCTSSALES

TIMES

SALES_PROD_TIME_MV

Same tables joinedDifferent grouping columns

PRODUCTSSALES

TIMES


Using Nested Materialized Views

• Definition is based on another materialized view.

• Definition can also reference normal tables.

PRODUCTSSALES

TIMES

SALES_PROD_TIME_JOIN

SALES_TIME_PROD_MV

SALES_PROD_TIME_MV

Tables are joined one time.


Nested Materialized Views: Restrictions

A nested materialized view cannot be a parent and a grandparent as shown:

SALES TIMES

SALES_TIMES_JOIN

PRODUCTS

SALES_TIME_PROD_SUM

Parent of PRODUCTS

Grandparent of PRODUCTS


Materialized View: Example

CREATE MATERIALIZED VIEW cust_sales_mv

PCTFREE 0 TABLESPACE example

STORAGE (INITIAL 1M NEXT 1M PCTINCREASE 0)

BUILD DEFERRED

REFRESH COMPLETE

ENABLE QUERY REWRITE

AS SELECT c.cust_id, s.channel_id, SUM(amount_sold) FROM sales s, customers c WHERE s.cust_id = c.cust_id GROUP BY c.cust_id, s.channel_id ORDER BY c.cust_id, s.channel_id;

Name

Storage options

When to build it

How to refresh the data

Use this for query rewrite

Detailquery

Detail tables

MV keys


Materialized Views Storage

• When a materialized view is created, the following objects are created:– A container table to store the materialized view

rows– The materialized view itself– One index for materialized views with aggregates

only

• The OBJECT_TYPE column of DBA_OBJECTS contains MATERIALIZED VIEW for the object.

• The container table has the same name as the materialized view object.

• A container table can be prebuilt.


Estimating Materialized View Size

> VARIABLE num_rows NUMBER;

> VARIABLE mv_size NUMBER;

EXEC DBMS_MVIEW.ESTIMATE_MVIEW_SIZE( -'simple_store', -'SELECT c.cust_state_province, ' ||-' SUM(amount_sold) ' ||-'FROM sales s, customers c ' ||-'WHERE s.cust_id= c.cust_id ' ||-'GROUP BY c.cust_state_province', -:num_rows, :mv_size);

Estimated rowsEstimated storage in bytes

Statement ID for EXPLAIN PLAN


Specifying Build Methods

• Two build methods are available when creating the materialized view:– BUILD DEFERRED: Created but not populated– BUILD IMMEDIATE: Created and populated

• The BUILD_MODE column in DBA_MVIEWS contains the method used.


Specifying Refresh Options

• Specify how the materialized view should be refreshed from the detail tables:– COMPLETE– FAST – FORCE– NEVER

• The REFRESH_METHOD column in DBA_MVIEWS contains the option value.


Specifying Refresh Execution Modes

• Two refresh execution modes:– ON DEMAND: Manual– ON COMMIT: Refresh done at transaction commit;

only possible for fast-refreshable materialized views. In case of failure, subsequent refreshes are manual.

• Schedule: At regular intervals

• The REFRESH_MODE column in DBA_MVIEWS contains the refresh execution mode value.


Using Column Aliases in Materialized Views

CREATE MATERIALIZED VIEW sales_mvENABLE QUERY REWRITE ASSELECT s.time_id sales_tid, c.time_id costs_tidFROM sales s, products p, costs cWHERE s.prod_id = p.prod_id AND c.prod_id = p.prod_id AND p.prod_name IN (SELECT prod_name FROM products);

SELECT s.time_id, c.time_idFROM sales s, products p, costs cWHERE s.prod_id = p.prod_id AND c.prod_id = p.prod_id AND p.prod_name IN (SELECT prod_name FROM products);

Query rewrite


CREATE MATERIALIZED VIEW sales_mv(sales_tid, costs_tid)ENABLE QUERY REWRITE ASSELECT s.time_id, c.time_idFROM sales s, products p, costs cWHERE s.prod_id = p.prod_id AND c.prod_id = p.prod_id AND p.prod_name IN (SELECT prod_name FROM products);

SELECT s.time_id, c.time_idFROM sales s, products p, costs cWHERE s.prod_id = p.prod_id AND c.prod_id = p.prod_id AND p.prod_name IN (SELECT prod_name FROM products);

Using Materialized View Column Alias Lists

Query rewrite


Registering User-Defined Materialized View Tables

• Register existing materialized view tables with ON PREBUILT TABLE.

• The registered materialized view can be used for query rewrites.

• It can be maintained by refresh methods.• Requirements:

– Table and materialized view have the same name.– Column aliases in detail query must correspond.– Column data types must match; can use the WITH

REDUCED PRECISION clause.– Table columns not referenced in the defining query

are unmanaged columns.

• Table remains after the materialized view is dropped.


Partitioning and Materialized Views

• Partitioning the fact tables:– Improves the opportunity of fast refreshing the

materialized view– May enable Partition Change Tracking (PCT) refresh

on the materialized view

• Partitioning the materialized view:– Partition pruning can be used for query rewrite.


Partitioned Materialized View: Example

CREATE MATERIALIZED VIEW part_sales_mv

PARALLEL PARTITION BY LIST (gid)

(PARTITION g1 VALUES (0),

PARTITION g2 VALUES (1),

PARTITION g3 VALUES (3))

BUILD IMMEDIATE REFRESH COMPLETE

ENABLE QUERY REWRITE AS

SELECT prod_id,cust_id,time_id,

GROUPING_ID(prod_id,cust_id,time_id) AS gid,

sum(amount_sold) AS sum_sales

FROM sales

GROUP BY GROUPING SETS ((prod_id,cust_id,time_id),

(prod_id,cust_id),(prod_id));


Using Enterprise Manager toCreate Materialized Views

cr_mv_gen.gif


Privileges Required toCreate Materialized Views

• Must be granted directly, not through roles

• To create a materialized view in your schema:– CREATE MATERIALIZED VIEW, and– CREATE [ANY] TABLE, and– SELECT privilege on each detail table not owned

• To create a materialized view in another schema:– CREATE ANY MATERIALIZED VIEW, and– Materialized view owner must have

CREATE [ANY] TABLE, and

SELECT privilege on each detail table not owned


Additional Privileges Required toCreate Materialized Views

• To create a materialized view refreshed at commit time:– ON COMMIT REFRESH object privilege on each detail

table not owned, or– ON COMMIT REFRESH system privilege

• To enable query rewrite:– Detail table owner must have QUERY REWRITE

system privilege– If you do not own detail tables: GLOBAL QUERY

REWRITE system privilege or QUERY REWRITE on each detail table not owned

• SELECT WITH GRANT OPTION if materialized view is defined on a prebuilt table


Globalization and Materialized Views

• Materialized views use the settings in effect during materialized view creation.

• Always specify globalization parameters to ensure that correct results are returned.

WHERE varchar_col = TO_DATE('01-FEB-02')

TO_DATE('01-FEB-2002', 'DD-MON-YYYY', 'NLS_DATE_LANGUAGE = AMERICAN')


Globalization Parameters Significant for Materialized Views

NLS_LANGUAGE

NLS_TIMESTAMP_TZ_FORMATNLS_ISO_CURRENCY

NLS_TIMESTAMP_FORMATNLS_DUAL_CURRENCY

NLS_TIME_TZ_FORMATNLS_DATE_LANGUAGE

NLS_TIME_FORMATNLS_DATE_FORMAT

NLS_NUMERIC_CHARACTERSNLS_CALENDAR

NLS_TERRITORYNLS_CURRENCY

NLS_SORTNLS_COMP


Adding Comments to Materialized Views

• Adding a comment for an existing materialized view:

• Viewing comments:

COMMENT ON MATERIALIZED VIEW cust_sales_mvIS 'sales materialized view';

SELECT mview_name, commentsFROM user_mview_comments WHERE mview_name = 'CUST_SALES_MV';


Altering Materialized Views

ALTER MATERIALIZED VIEW cust_sales_mv DISABLE QUERY REWRITE;

• Changing the refresh option and refresh mode:

• Recompiling the materialized view:

• Enabling or disabling its use for query rewrite:

ALTER MATERIALIZED VIEW cust_sales_mv REFRESH FAST ON COMMIT;

ALTER MATERIALIZED VIEW cust_sales_mv COMPILE;


Altering Materialized Views

• Allocating an extent:

• Modifying the logging attribute:

ALTER MATERIALIZED VIEW cust_sales_mv ALLOCATE EXTENT;

ALTER MATERIALIZED VIEW cust_sales_mv NOLOGGING;


Maintaining Partitions of a Materialized View

ALTER MATERIALIZED VIEW sales_mv TRUNCATE PARTITION year_1995;

ALTER MATERIALIZED VIEW sales_mvDROP PARTITION year_1994;

ALTER MATERIALIZED VIEW fact_mvEXCHANGE PARTITION year_2001 WITH TABLE sales_2001;


Dropping Materialized Views

• You must be the owner or you must have the DROP ANY MATERIALIZED VIEW system privilege.

• You must have privileges to drop underlying objects.

• Drop a materialized view using the DROP MATERIALIZED VIEW command:

• If the materialized view was created on a prebuilt container table, the table is retained.

DROP MATERIALIZED VIEW cust_sales_mv;


Viewing Staleness Information

• Materialized view may diverge from the detail tables.

• Query the STALENESS column in DBA_MVIEWS:– FRESH– STALE– UNUSABLE– UNKNOWN– UNDEFINED


Materialized View Integrity

• QUERY_REWRITE_INTEGRITY: Initialization parameter controls materialized view integrity.

• Parameter values:– ENFORCED– TRUSTED– STALE_TOLERATED


Invalidating Materialized Views

• Automatic invalidation during dependency changes

• Automatic revalidation during:– Refresh– Query rewrite

• STATUS column in DBA_OBJECTS: – INVALID– VALID

• COMPILE_STATE in DBA_MVIEWS: – VALID– NEEDS_COMPILE

• Manual revalidation with: ALTER MATERIALIZED VIEW COMPILE


Summary

In this lesson, you should have learned how to:

• Use summaries in a data warehouse environment

• Differentiate types of materialized views

• Create a materialized view

• Use the QUERY_REWRITE_INTEGRITY parameter


Practice 9: Overview

This practice covers the following topics:

• Creating materialized views on prebuilt tables

• Estimating the number of rows of a potential materialized view

• Creating new materialized views

• Viewing objects created when materialized views are created

9 Copyright © 2005, Oracle. All rights reserved. Materialized Views.

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