Materialized Views

Materialized View Types

Oracle offers several types of materialized views to meet the needs of many different replication (and non-replication) situations.

• Primary Key Materialized Views
• Subquery Materialized Views
• ROWID Materialized Views
• Object Materialized Views
• Complex Materialized Views

Primary Key Materialized Views

Primary key materialized views are the default type of materialized views in Oracle. They are updatable if the materialized view was created as part of a materialized view group and FOR UPDATE was specified when defining the materialized view. An updatable materialized view must belong to a materialized view group that has the same name as the replication group at its master site or master materialized view site. In addition, an updatable materialized view must reside in a different database than the master replication group.

The following statement creates the primary key materialized view on the table emp located on a remote database.

SQL> CREATE MATERIALIZED VIEW mv_emp_pk

BUILD DEFFERED

REFRESH FAST

START WITH SYSDATE NEXT SYSDATE + 1/48

WITH PRIMARY KEY

AS SELECT * FROM emp@remote_db;

Changes are propagated according to the row-level changes that have occurred, as identified by the primary key value of the row (not the ROWID).

The following is an example of a SQL statement for creating an updatable, primary key materialized view:

SQL> CREATE MATERIALIZED VIEW offshore.customers

FOR UPDATE

AS SELECT * FROM onsite.customers@orcl;

Primary key M-views allow materialized view master tables to be reorganized without affecting the eligibility of the materialized view for fast refresh.

Subquery Materialized Views

Materialized views may contain a subquery so that we can create a subset of rows at the remote materialized view site. A subquery is a query imbedded within the primary query, so that we have more than one SELECT statement in the CREATE MATERIALIZED VIEW statement. This subquery may be as simple as a basic WHERE clause or as complex as a multilevel WHERE EXISTS clause. Primary key materialized views that contain a selected class of subqueries can still be incrementally (or fast) refreshed, if each master referenced has a materialized view log. A fast refresh uses materialized view logs to update only the rows that have changed since the last refresh.

The following statement creates a subquery materialized view based on the emp and dept tables located on the remote database:

SQL> CREATE MATERIALIZED VIEW mv_empdept

DISABLE QUERY REWRITE

AS SELECT * FROM emp@remote_db e

WHERE EXISTS

(SELECT * FROM dept@remote_db d WHERE e.dept_no = d.dept_no);

ROWID Materialized Views

For backward compatibility, Oracle supports ROWID materialized views in addition to the default primary key materialized views. A ROWID materialized view is based on the physical row identifiers (rowids) of the rows in a master. ROWID materialized views should be used only for materialized views based on master tables from an Oracle7 database, and should not be used from Oracle8 or higher.

The following statement creates the rowid materialized view on table emp located on a remote database:

SQL> CREATE MATERIALIZED VIEW mv_emp_rowid

REFRESH WITH ROWID

ENABLE QUERY REWRITE

AS SELECT * FROM emp@remote_db;

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

Object Materialized Views

Object materialized view is based on an object table and is created using the OF type clause. An object materialized view is structured in the same way as an object table. That is, an object materialized view is composed of row objects, and each row object is identified by an object identifier (OID) column.

SQL> CREATE MATERIALIZED VIEW customer OF cust_objtype

AS SELECT CustNo FROM scott.Customer@orcl;

SQL> CREATE MATERIALIZED VIEW LOG ON categories_tab WITH OBJECT ID;

An object materialized view inherits the object identifier (OID) specifications of its master. If the master has a primary key-based OID, then the OIDs of row objects in the materialized view are primary key-based. If the master has a system generated OID, then the OIDs of row objects in the materialized view are system generated. Also, the OID of each row in the object materialized view matches the OID of the same row in the master, and the OIDs are preserved during refresh of the materialized view. Consequently, REFs to the rows in the object table remain valid at the materialized view site.

Complex Materialized Views

To be fast refreshed, the defining query for a materialized view must observe certain restrictions. If we require a materialized view whose defining query is more general and cannot observe the restrictions, then the materialized view is complex and cannot be fast refreshed.

A materialized view is considered complex when the defining query of the materialized view contains:

i) A CONNECT BY clause
For example, the following statement creates a complex materialized view:
SQL> CREATE MATERIALIZED VIEW hr.emp_hierarchy
AS SELECT LPAD(' ', 4*(LEVEL-1))||email USERNAME
FROM hr.employees@orcl START WITH manager_id IS NULL
CONNECT BY PRIOR employee_id = manager_id;

ii) An INTERSECT, MINUS, or UNION ALL set operation

For example, the following statement creates a complex materialized view because it has a UNION ALL set operation:
SQL> CREATE MATERIALIZED VIEW hr.mview_employees AS
SELECT employees.employee_id, employees.email
FROM hr.employees@orcl
UNION ALL
SELECT new_employees.employee_id, new_employees.email
FROM hr.new_employees@orcl;

iii) In some cases, the DISTINCT or UNIQUE keyword, although it is possible to have the DISTINCT or UNIQUE keyword in the defining query and still have a simple materialized view

For example, the following statement creates a complex materialized view:
SQL> CREATE MATERIALIZED VIEW hr.employee_depts
AS SELECT DISTINCT department_id FROM hr.employees@orcl
ORDER BY department_id;

iv) An aggregate function

For example, the following statement creates a complex materialized view:
SQL> CREATE MATERIALIZED VIEW hr.average_sal
AS SELECT AVG(salary) "Average" FROM hr.employees@orcl;

v) Joins other than those in a subquery

For example, the following statement creates a complex materialized view:
SQL> CREATE MATERIALIZED VIEW hr.emp_join_dep AS
SELECT last_name
FROM hr.employees@orc1.world e, hr.departments@orcl d
WHERE e.department_id = d.department_id;

vi) In some cases, a UNION operation. Specifically, a materialized view with a UNION operation is complex if any one of these conditions is true:

o Any query within the UNION is complex. The previous bullet items specify when a query makes a materialized view complex.

o The outermost SELECT list columns do not match for the queries in the UNION. In the following example, the first query only has order_total in the outermost SELECT list while the second query has customer_id in the outermost SELECT list. Therefore, the materialized view is complex.

SQL> CREATE MATERIALIZED VIEW oe.orders AS

SELECT order_total FROM oe.orders@orcl o

WHERE EXISTS (SELECT cust_first_name, cust_last_name

FROM oe.customers@orcl c

WHERE o.customer_id = c.customer_id AND c.credit_limit > 50)

UNION

SELECT customer_id FROM oe.orders@orcl o

WHERE EXISTS (SELECT cust_first_name, cust_last_name

FROM oe.customers@orcl c

WHERE o.customer_id = c.customer_id AND c.account_mgr_id = 30);

o The innermost SELECT list has no bearing on whether a materialized view is complex. In the previous example, the innermost SELECT list is cust_first_name and cust_last_name for both queries in the UNION.

Note: If possible, we should avoid using complex materialized views because they cannot be fast refreshed, which may degrade network performance.

Materialized views in Oracle

Oracle materialized views were first introduced in Oracle8.

Materialized views are schema objects that can be used to summarize, precompute, replicate and distribute data.

In mview, the query result is cached as a concrete table that may be updated from the original base tables from time to time. This enables much more efficient access, at the cost of some data being potentially out-of-date. It is most useful in datawarehousing scenarios, where frequent queries of the actual base tables can be extremely expensive.

Oracle uses materialized views (also known as snapshots in prior releases) to replicate data to non-master sites in a replication environment and to cache expensive queries in a datawarehouse environment.

A materialized view 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 is a replica of a target master from a single point in time. We can define a materialized view on a base/master table (at a master site), partitioned table, view, synonym or a master materialized view (at a materialized view site). Whereas in multi master replication tables are continuously updated by other master sites, materialized views are updated from one or more masters through individual batch updates, known as a refreshes, from a single master site or master materialized view site.

A materialized view provides indirect access to table data by storing the results of a query in a separate schema object. Unlike an ordinary view, which does not take up any storage space or contain any data, Mview stores data, whereas view stores only query.

The existence of a materialized view is transparent to SQL, but when used for query rewrites will improve the performance of SQL execution. An updatable materialized view lets you insert, update, and delete.

We can define indexes on a materialized view.

A materialized view can be stored in the same database as it's base table(s) or in a different database. Materialized views stored in the same database as their base tables can improve query performance through query rewrites. Query rewrites are particularly useful in a datawarehouse environment.

A materialized view can query tables, views and other materialized views. Collectively these are called master tables (a replication term) or detail tables (a datawarehouse term).

For replication purposes, materialized views allow us to maintain copies of remote data on local node. These copies are read-only. If we want to update the local copies, we have to use the Advanced Replication feature. We can select data from a materialized view as we would from a table or view.

For datawarehousing purposes, mviews commonly created are aggregate views, single-table aggregate views and join views.

In replication environments, mviews commonly created are primary key, rowid and subquery materialized views.

Whenever you create a materialized view, regardless of it's type, always specify the schema name of the table owner in the query for the materialized view.

Prerequisites:

To create mviews, the user should have any one of

CREATE MATERIALIZED VIEW or CREATE ANY MATERIALIZED VIEW privileges.

SQL> GRANT CREATE MATERIALIZED VIEW TO user-name;

And

SQL> GRANT QUERY REWRITE TO user-name;

And following init parameters should be set

query_rewrite_enabled = true (default)

query_rewrite_integrity = enforced|trusted|stale_tolerated

The background processes responsible for these materialized view refreshes are the coordinated job queue (CJQ) processes.

job_queue_processes=n

Syntax:

CREATE MATERIALIZED VIEW mview-name

[partitioning-options]

[storage-parameters]

[TABLESPACE tablespace-name]

[OF object-type]

[FOR UPDATE]

[BUILD IMMEDIATE|BUILD DEFFERED|ON PREBUILT TABLE]

[REFRESH [FAST|COMPLETE|FORCE|NEVER]

 [ON DEMAND|COMMIT]

 [START WITH date]

 [NEXT date]

 [WITH PRIMARY KEY|ROWID]]

[DISABLE|ENABLE QUERY REWRITE]

AS select-query;

Refresh Types

Oracle can refresh a materialized view using either a fast, complete or force refresh.

The refresh option specifies:

a. Refresh method used by Oracle to refresh data in materialized view. FORCE is the default option.

b. Whether the view is primary key based or row-id based. PRIMARY KEY is the default option.

c. Time and interval at which the view is to be refreshed.

Complete Refresh

To perform COMPLETE refresh of a materialized view, the server that manages the materialized view executes the materialized view's defining query, which essentially recreates the materialized view. To refresh the materialized view, the result set of the query replaces the existing materialized view data. Oracle can perform a complete refresh for any materialized view. Depending on the amount of data that satisfies the defining query, a complete refresh can take a substantially longer amount of time to perform than a fast refresh.

Note: If a materialized view is complete refreshed, then set it's PCTFREE to 0 and PCTUSED to 99 for maximum efficiency.

The complete refresh re-creates the entire materialized view. If we request a complete refresh, Oracle performs a complete refresh even if a fast refresh is possible.

SQL> CREATE MATERIALIZED VIEW mv_emp

 REFRESH COMPLETE

 START WITH SYSDATE NEXT SYSDATE + 1

 WITH PRIMARY KEY

 AS SELECT * FROM emp@remote_db;

To refresh this mview,

SQL> EXEC DBMS_MVIEW.REFRESH('mv_emp', 'C');

From Oracle 10g, complete refresh of single materialized view can do delete instead of truncate. To force the refresh to do truncate instead of delete, parameter ATOMIC_REFRESH must be set to false.

ATOMIC_REFRESH = FALSE, mview will be truncated and whole data will be inserted. The refresh will go faster, and no undo will be generated.

ATOMIC_REFRESH = TRUE (default), mview will be deleted and whole data will be inserted. Undo will be generated. We will have access at all times even while it is being refreshed.

SQL> EXEC DBMS_MVIEW.REFRESH('mv_emp', 'C', atomic_refresh=>FALSE);

If we perform complete refresh of a master materialized view, then the next refresh performed on any materialized views based on this master materialized view must be a complete refresh. If a fast refresh is attempted for such a materialized view after it's master materialized view has performed a complete refresh, then Oracle returns the following error:

ORA-12034 mview log is younger than last refresh

Fast Refresh

To perform FAST refresh, the master that manages the materialized view 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 because the participating server and network replicate a smaller amount of data.

We can perform fast refreshes of materialized views only when the master table or master materialized view has a materialized view log. Also, for fast refreshes to be faster than complete refreshes, each join column in the CREATE MATERIALIZED VIEW statement must have an index on it.

SQL> CREATE MATERIALIZED VIEW mv_emp

 BUILD IMMEDIATE

 REFRESH FAST

 START WITH SYSDATE NEXT SYSDATE + 2

 WITH PRIMARY KEY

 ENABLE QUERY REWRITE

 AS SELECT * FROM emp@remote_db;

A materialized view log is a schema object that records changes to a master table's data so that a materialized view defined on the master table can be refreshed incrementally.

We should create a materialized view log for the master tables if we specify the REFRESH FAST clause.

SQL> CREATE MATERIALIZED VIEW LOG ON emp;

To refresh this mview,

SQL> EXEC DBMS_MVIEW.REFRESH('mv_emp', 'F');

After a direct path load on a master table or master materialized view using SQL*Loader, a fast refresh does not apply the changes that occurred during the direct path load. Also, fast refresh does not apply changes that result from other types of bulk load operations on masters. Examples of these operations include some INSERT statements with an APPEND hint and some INSERT ... SELECT * FROM statements.

Note:

->> Fast refreshable materialized views can be created based on master tables and master materialized views only.

->> Materialized views based on a synonym or a view must be complete refreshed.

->> Materialized views are not eligible for fast refresh if the defined subquery contains an analytic function.

Force Refresh

To perform FORCE refresh of a materialized view, the server that manages the materialized view attempts to perform a fast refresh. If fast refresh is not possible, then Oracle performs complete refresh. Use the force setting when you want a materialized view to refresh if fast refresh is not possible.

If you do not specify a refresh method, FORCE is the default.

SQL> CREATE MATERIALIZED VIEW mv_emp

 REFRESH FORCE

 START WITH SYSDATE NEXT SYSDATE + 3

 WITH PRIMARY KEY

 DISABLE QUERY REWRITE

 AS SELECT * FROM emp@remote_db;

To refresh this mview,

SQL> EXEC DBMS_MVIEW.REFRESH(LIST =>'mv_emp', METHOD =>'?');

(or)

SQL> EXEC DBMS_MVIEW.REFRESH(LIST =>'mv_emp');

Partition Change Tracking (PCT) Refresh

In Oracle9i, a new mechanism called Partition Change Tracking (PCT) has been introduced. This mechanism keeps track of the base table partitions that have been updated since the materialized view was last refreshed. It tracks partition maintenance operations (PMOPs), like add and truncate partition as well as DML changes to the base data, on partition basis. This allows Oracle to identify fresh data in the materialized view.

Partition Change Tracking (PCT) refresh refers to MV refresh using only the changed partitions of the base tables of an MV. This refresh method is possible only if the base tables are partitioned and changes to base tables are tracked on a partition basis.

To refresh these type of mviews (from Oracle 10g),

SQL> EXEC DBMS_MVIEW.REFRESH('mv_emp', 'P');

From Oracle 9i, it was supporting, range and range-hash partitioning.

From Oracle 10g, it was supporting, list partitioning also.

Enhanced Partition Change Tracking (EPCT) Refresh refers to PCT based refresh applied to MVs containing columns that are partition-join dependent on the partitioning column of the base table.

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;

In the above example, the first copy of the materialized view is made at SYSDATE (immediately) and the interval at which the refresh has to be performed is every two days.

SQL> CREATE MATERIALIZED VIEW mv_emp_pk

 REFRESH COMPLETE

 START WITH SYSDATE NEXT SYSDATE + 2/(24*60)

 WITH ROWID

 AS SELECT * FROM emp@remote_db;

In this example, the interval is two minutes. For every two minutes, fast refresh will happen.

SQL> CREATE MATERIALIZED VIEW mv_emp_pk

 REFRESH FORCE

 START WITH SYSDATE NEXT SYSDATE + 30/(24*60*60)

 WITH PRIMARY KEY

 AS SELECT * FROM emp@remote_db;

In this example, the interval is 30 seconds.

SQL> CREATE MATERIALIZED VIEW mv_emp_f

 REFRESH FAST ON COMMIT

 WITH PRIMARY KEY

 AS SELECT * FROM emp@remote_db;

SQL> CREATE MATERIALIZED VIEW mv_emp_c

 REFRESH COMPLETE ON DEMAND

 WITH ROWID

 AS SELECT * FROM emp@remote_db;

SQL> EXECUTE DBMS_MVIEW.REFRESH('mv_emp_c','C');

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. PRIMARY KEY is the default option. To use the PRIMARY KEY clause we should have defined PRIMARY KEY on the master table or else you should use ROWID based materialized views.

How to know when was the last refresh happened on materialized views:

SQL> select MVIEW_NAME, to_char(LAST_REFRESH_DATE,'YYYY-MM-DD HH24:MI:SS') from dba_mviews;

(or)

SQL> select MVIEW_NAME, to_char(LAST_REFRESH_DATE,'YYYY-MM-DD HH24:MI:SS') from dba_mview_analysis;

(or)

SQL> select NAME, to_char(LAST_REFRESH,'YYYY-MM-DD HH24:MI:SS')

from dba_mview_refresh_times;

Read-Only, Updatable and Writeable Materialized Views

A materialized view can be either read-only, updatable or writeable. Users cannot perform data manipulation language (DML) statements on read-only materialized views, but they can perform DML on updatable and writeable materialized views.

1. Read only

• Cannot be updated and complex materialized views are supported.

2. Updateable

• Can be updated even when disconnected from the master site.
• Are refreshed on demand.
• Consumes fewer resources.
• Requires Advanced Replication option to be installed.

3. Writeable

• Created with the for update clause.
• Changes are lost when view is refreshed.
• Requires Advanced Replication option to be installed.

Note: For read-only, updatable, and writeable materialized views, the defining query of the materialized view must reference all of the primary key columns in the master.

Read-Only Materialized Views

We can make a materialized view read-only during creation by omitting the FOR UPDATE clause or disabling the equivalent option in the Replication Management tool. Read-only materialized views use many of the same mechanisms as updatable materialized views, except that they do not need to belong to a materialized view group.

In addition, using read-only materialized views eliminates the possibility of a materialized view introducing data conflicts at the master site or master materialized view site, although this convenience means that updates cannot be made at the remote materialized view site.

The following is an example of a read-only materialized view:

SQL> CREATE MATERIALIZED VIEW hr.emp

AS SELECT * FROM hr.emp@orcl;

Updatable Materialized Views

We can make a materialized view updatable during creation by including the FOR UPDATE clause or enabling the equivalent option in the Replication Management tool. For changes made to an updatable materialized view to be pushed back to the master during refresh, the updatable materialized view must belong to a materialized view group.

Updatable materialized views enable us to decrease the load on master sites because users can make changes to the data at the materialized view site.

The following is an example of an updatable materialized view:

SQL> CREATE MATERIALIZED VIEW hr.dept

FOR UPDATE

AS SELECT * FROM hr.departments@orcl;

The following statement creates a materialized view group:

BEGIN

 DBMS_REPCAT.CREATE_MVIEW_REPGROUP (

 gname => 'hr_repg',

 master => 'orcl',

 propagation_mode => 'ASYNCHRONOUS');

END;

/

The following statement adds the hr.departments materialized view to the materialized view group, making the materialized view updatable:

BEGIN

 DBMS_REPCAT.CREATE_MVIEW_REPOBJECT (

 gname => 'hr_repg',

 sname => 'hr',

 oname => 'departments',

 type => 'SNAPSHOT',

 min_communication => TRUE);

END;

/

We can also use the Replication Management tool to create a materialized view group and add a materialized view to it.

Note:

1. Do not use column aliases when we are creating an updatable materialized view. Column aliases cause an error when we attempt to add the materialized view to a materialized view group using the CREATE_MVIEW_REPOBJECT procedure.
2. An updatable materialized view based on a master table or master materialized view that has defined column default values does not automatically use the master's default values.
3. Updatable materialized views do not support the DELETE CASCADE constraint.

The following types of materialized views cannot be masters for updatable materialized views:

• ROWID materialized views
• Complex materialized views
• Read-only materialized views

However, these types of materialized views can be masters for read-only materialized views.

Additional Restrictions for Updatable Materialized Views Based on Materialized Views, those must:

• Belong to a materialized view group that has the same name as the materialized view group at it's master materialized view site.
• Reside in a different database than the materialized view group at it's master materialized view site.
• Be based on another updatable materialized view or other updatable materialized views, not on a read-only materialized view.
• Be based on a materialized view in a materialized view group that is owned by PUBLIC at the master materialized view site.

Writeable Materialized Views

A writeable materialized view is one that is created using the FOR UPDATE clause but is not part of a materialized view group. Users can perform DML operations on a writeable materialized view, but if we refresh the materialized view, then these changes are not pushed back to the master and the changes are lost in the materialized view itself. Writeable materialized views are typically allowed wherever fast-refreshable read-only materialized views are allowed.

Note: writeable materialized views are rarely used.

Materialized Views Types

Uses of Materialized Views

We can use materialized views to achieve one or more of the following goals:

• Less network loads
• Create mass deployment environment
• Enable data subsetting
• Enable disconnected computing

Less Network Loads

We can use materialized views to reduce network loads & to distribute your corporate database to regional sites. Instead of the entire company accessing a single database server, user load is distributed across multiple database servers. Through the use of multitier materialized views, we can create materialized views based on other materialized views, which enables us to distribute user load to an even greater extent because clients can access materialized view sites instead of master sites. To decrease the amount of data that is replicated, a materialized view can be a subset of a master table or master materialized view.

While multimaster replication also distributes a database among multiple sites, the networking requirements for multimaster replication are greater than those for replicating with materialized views because of the transaction by transaction nature of multimaster replication. Further, the ability of multimaster replication to provide real-time or near real-time replication may result in greater network traffic, and might require a dedicated network link.

Materialized views are updated through an efficient batch process from a single master site or master materialized view site. They have lower network requirements and dependencies than multimaster replication because of the point in time nature of materialized view replication. Whereas multimaster replication requires constant communication over the network, materialized view replication requires only periodic refreshes.

In addition to not requiring a dedicated network connection, replicating data with materialized views increases data availability by providing local access to the target data. These benefits, combined with mass deployment and data subsetting (both of which also reduce network loads), greatly enhance the performance and reliability of your replicated database.

Create mass deployment environment

Deployment templates allow us to precreate a materialized view environment locally. We can then use deployment templates to quickly and easily deploy materialized view environments to support sales force automation and other mass deployment environments. Parameters allow us to create custom data sets for individual users without changing the deployment template. This technology enables you to roll out a database infrastructure to hundreds or thousands of users.

Enable data subsetting

Materialized views allow us to replicate data based on column and row-level subsetting, while multimaster replication requires replication of the entire table. Data subsetting enables us to replicate information that pertains only to a particular site. For example, if we have a regional sales office, then we might replicate only the data that is needed in that region, thereby cutting down on unnecessary network traffic.

Enable disconnected computing

Materialized views do not require a dedicated network connection. Though we have the option of automating the refresh process by scheduling a job, we can manually refresh your materialized view on-demand, which is an ideal solution for sales applications running on a laptop. For example, a developer can integrate the replication management API for refresh on-demand into the sales application. When the salesperson has completed the day's orders, the salesperson simply dials up the network and uses the integrated mechanism to refresh the database, thus transferring the orders to the main office.

Note:

• Both the master site and the materialized view site must have compatibility level (COMPATIBLE initialization parameter) 9.0.1 or higher to replicate user-defined types and any objects on which they are based.
• We cannot create refresh-on-commit materialized views based on a master with user-defined types. Refresh-on-commit materialized views are those created using the ON COMMIT REFRESH clause in the CREATE MATERIALIZED VIEW statement.
• Advanced Replication does not support type inheritance.

Materialized View Log

Updatable Materialized View Log

Materialized View Groups

A materialized view group in a replication system maintains a partial or complete copy of the objects at the target replication group at it's master site or master materialized view site. Materialized view groups cannot span the boundaries of the replication group at the master site or master materialized view site.

Group A at the materialized view site contains only some of the objects in the corresponding Group A at the master site. Group B at the materialized view site contains all objects in Group B at the master site. Under no circumstances, however, could Group B at the materialized view site contain objects from Group A at the master site. A materialized view group has the same name as the master group on which the materialized view group is based. For example, a materialized view group based on a personnel master group is also named personnel.

In addition to maintaining organizational consistency between materialized view sites and their master sites or master materialized view sites, materialized view groups are required for supporting updatable materialized views. If a materialized view does not belong to a materialized view group, then it must be a read-only or writeable materialized view.

Refresh Groups

Managing MVs is much easier in Oracle 10g with the introduction of the powerful new tuning advisors that can tell us a lot about the design of the MVs. Tuning recommendations that can generate a complete script that can be implemented quickly, saving significant time and effort. The ability to force rewriting or abort the query can be very helpful in decision-support systems where resources must be conserved, and where a query that is not rewritten should not be allowed to run amuck inside the database.

Related Views

DBA_MVIEWS

DBA_MVIEW_LOGS

DBA_MVIEW_KEYS

DBA_REGISTERED_MVIEWS

DBA_REGISTERED_MVIEW_GROUPS

DBA_MVIEW_REFRESH_TIMES

DBA_MVIEW_ANALYSIS

Related Package/Procedures

DBMS_MVIEW package

• REFRESH
• REFRESH_ALL
• REFRESH_ALL_MVIEWS
• REFRESH_DEPENDENT
• REGISTER_MVIEW
• UNREGISTER_MVIEW
• PURGE_LOG

DBMS_REPCAT package

DBMS_REFRESH package

Materialized View Log

A materialized view log is required on a master if we want to fast refresh materialized views based on the master. When we create a materialized view log for a master table or master materialized view, Oracle creates an underlying table as the materialized view log. A Mview log can hold the primary keys, rowids, or object ids of rows, or both, that have been updated in the master table or master materialized view. A materialized view log can also contain other columns to support fast refreshes of materialized views with subqueries.

CREATE MATERIALIZED VIEW LOG ON [schema.]table_name

[... storage options ...]

[... table_partitioning_clause ...]

[WITH {OBJECT | PRIMARY KEY | ROWID | SEQUENCE | column_list}]

[{INCLUDING | EXCLUDING} NEW VALUES];

SQL> CREATE MATERIALIZED VIEW LOG ON emp;

The name of a materialized view log's table is MLOG$_master_name. The materialized view log is created in the same schema as the target master. One materialized view log can support multiple materialized views on its master table or master materialized view.

When changes are made to the master table or master materialized view using DML, an internal trigger records information about the affected rows in the materialized view log. This information includes the values of the primary key, rowid, or object id, or both, as well as the values of the other columns logged in the materialized view log. This is an internal AFTER ROW trigger that is automatically activated when we create a materialized view log for the target master table or master materialized view. It inserts a row into the materialized view log whenever an INSERT, UPDATE, or DELETE statement modifies the table's data. This trigger is always the last trigger to fire.

SQL> CREATE MATERIALIZED VIEW LOG ON emp WITH SEQUENCE, ROWID INCLUDING NEW VALUES;

Following are the types of materialized view logs:

• Primary Key: The materialized view records changes to the master table or master materialized view based on the primary key of the affected rows.
• Row ID: The materialized view records changes to the master table or master materialized view based on the ROWID of the affected rows.
• Object ID: The materialized view records changes to the master object table or master object materialized view based on the object identifier (ID) of the affected row objects.
• Combination: The materialized view records changes to the master table or master materialized view based any combination of the three options. It is possible to record changes based on the primary key, the ROWID and the object ID of the affected rows. Such a materialized view log supports primary key, ROWID and object materialized views, which is helpful for environments that have all three types of materialized views based on a master.

A combination materialized view log works in the same manner as a materialized view log that tracks only one type of value, except that more than one type of value is recorded. For example, a combination materialized view log can track both the primary key and the rowid of the affected row are recorded.

Though the difference between materialized view logs based on primary keys and rowids is small (one records affected rows using the primary key, while the other records affected rows using the physical rowid), the practical impact is large. Using rowid materialized views and materialized view logs makes reorganizing and truncating your master tables difficult because it prevents your ROWID materialized views from being fast refreshed. If we reorganize or truncate your master table, then your rowid materialized view must be COMPLETE refreshed because the rowids of the master table have changed.

SQL> CREATE MATERIALIZED VIEW LOG ON cust

TABLESPACE mv_logs STORAGE(INITIAL 1M NEXT 1M) WITH ROWID;

Updatable Materialized View Log

An updatable materialized view log (USLOG$_materialized_view_name), in Oracle database, is used to determine which rows must be overwritten or removed from a materialized view during a fast refresh. A read-only materialized view does not create this log, and Oracle does not use this log during a complete refresh because, the entire materialized view is replaced.

If there is a conflict between an updatable M-view and a master, then, during a refresh, the conflict may result in an entry in the updatable materialized view log that is not in the materialized view log at the master site or master materialized view site. In this case, Oracle uses the updatable materialized view log to remove or overwrite the row in the materialized view.

The updatable materialized view log is also used when we fast refresh a writeable materialized view, as illustrated in the following scenario:

1. A user inserts a row into a writeable materialized view that has a remote master. Because the materialized view is writeable and not updatable, the transaction is not stored in the deferred transaction queue at the materialized view site.
2. Oracle logs information about this insert in the updatable materialized view log.
3. The user fast refreshes the materialized view.
4. Oracle uses the information in the updatable materialized view log and deletes the inserted row. A materialized view must be an exact copy of the master when the fast refresh is complete. Therefore, Oracle must delete the inserted row.

Internal Trigger for the Updatable Materialized View Log

Like the internal trigger at the master site or master materialized view site, an internal trigger at the materialized view site records DML changes applied to an updatable materialized view in the USLOG$_materialized_view_name log. A read-only materialized view does not create this trigger.

Restriction on import of MViews and MView Logs to different Schema

Materialized views and materialized view logs are exported with the schema name explicitly given in the DDL statements. Therefore, materialized views and materialized view logs cannot be imported into a schema that is different than the schema from which they were exported. If we attempt to use the FROM USER and TO USER import options to import an export dumpfile that contains materialized views or materialized view logs, then an error will be written to the import log file and the items will not be imported.

M-Views Refresh Groups

A refresh group can contain materialized views from more than one materialized view group to maintain transactional (read) consistency across replication group boundaries.

To preserve referential integrity and transactional (read) consistency among multiple materialized views, Oracle has the ability to refresh individual materialized views as part of a refresh group. After refreshing all of the materialized views in a refresh group, the data of all materialized views in the group correspond to the same transactionally consistent point in time.

While you may want to define a single refresh group for each materialized view group, it may be more efficient to use one large refresh group that contains objects from multiple materialized view groups. Such a configuration reduces the amount of overhead needed to refresh your materialized views. A refresh group can contain up to 400 materialized views.

One configuration that we want to avoid is using multiple refresh groups to refresh the contents of a single materialized view group. Using multiple refresh groups to refresh the contents of a single materialized view group may introduce inconsistencies in the materialized view data, which may cause referential integrity problems at the materialized view site. Only use this type of configuration when we have in-depth knowledge of the database environment and can prevent any referential integrity problems.

Refresh Group Size

There are a few trade-offs to consider when you are deciding on the size of your refresh groups. Oracle is optimized for large refresh groups. So, large refresh groups refresh faster than an equal number of materialized views in small refresh groups, assuming that the materialized views in the groups are similar. For example, refreshing a refresh group with 100 materialized views is faster than refreshing five refresh groups with 20 materialized views each. Also, large refresh groups enable you to refresh a greater number of materialized views with only one call to the replication management API.

During the refresh of a refresh group, each materialized view in the group is locked at the materialized view site for the amount of time required to refresh all of the materialized views in the refresh group. This locking is required to prevent users from updating the materialized views during the refresh operation, because updates may make the data inconsistent. Therefore, having smaller refresh groups means that the materialized views are locked for less time when you perform a refresh.

Network connectivity must be maintained while performing a refresh. If the connectivity is lost or interrupted during the refresh, then all changes are rolled back so that the database remains consistent. Therefore, in cases where the network connectivity is difficult to maintain, consider using smaller refresh groups.

Advanced Replication includes an optimization for null refresh. That is, if there were no changes to the master tables or master materialized views since the last refresh for a particular materialized view, then almost no extra time is required for the materialized view during materialized view group refresh. However, for materialized views in a database prior to release 8.1, consider separating materialized views of master tables that are not updated often into a separate refresh group of their own. Doing so shortens the refresh time required for other materialized view groups that contain materialized views of master tables that are updated frequently.

On-Demand Refresh

Scheduled materialized view refreshes may not always be the appropriate solution for your environment. For example, immediately following a bulk data load into a master table, dependent materialized views no longer represent the master table's data. Rather than wait for the next scheduled automatic group refreshes, you can manually refresh dependent materialized view groups to immediately propagate the new rows of the master table to associated materialized views.

You may also want to refresh your materialized views on-demand when your materialized views are integrated with a sales force automation system located on a disconnected laptop.

The following example illustrates an on-demand refresh of the hr_refg refresh group: SQL> EXECUTE DBMS_REFRESH.REFRESH('hr_refg');