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Oracle® Database SQL Language Reference
11g Release 2 (11.2)

Part Number E17118-03
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CREATE MATERIALIZED VIEW LOG

Purpose

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.

Note:

The keyword SNAPSHOT is supported in place of MATERIALIZED VIEW for backward compatibility.

When DML changes are made to master table data, Oracle Database stores rows describing those changes in the materialized view log and then uses the materialized view log to refresh materialized views based on the master table. This process is called incremental or fast refresh. Without a materialized view log, Oracle Database must reexecute the materialized view query to refresh the materialized view. This process is called a complete refresh. Usually, a fast refresh takes less time than a complete refresh.

A materialized view log is located in the master database in the same schema as the master table. A master table can have only one materialized view log defined on it. Oracle Database can use this materialized view log to perform fast refreshes for all fast-refreshable materialized views based on the master table.

To fast refresh a materialized join view, you must create a materialized view log for each of the tables referenced by the materialized view.

There are two types of materialized view logs: timestamp-based materialized view logs and commit SCN-based materialized view logs. Timestamp-based materialized view logs use timestamps and require some setup operations when preparing to refresh the materialized view. Commit SCN-based materialized view logs use commit SCN data rather than timestamps, which removes the need for the setup operations and thus can improve the speed of the materialized view refresh. If you specify the COMMIT SCN clause, then a commit SCN-based materialized view log is created. Otherwise, a time-stamp based materialized view log is created.

Note that only new materialized view logs can take advantage of COMMIT SCN. Existing materialized view logs cannot be altered to add COMMIT SCN unless they are dropped and recreated. Refer to Oracle Database Data Warehousing Guide for more information.

See Also:

Prerequisites

The privileges required to create a materialized view log directly relate to the privileges necessary to create the underlying objects associated with a materialized view log.

In either case, the owner of the materialized view log must have sufficient quota in the tablespace intended to hold the materialized view log or must have the UNLIMITED TABLESPACE system privilege.

See Also:

Oracle Database Data Warehousing Guide for more information about the prerequisites for creating a materialized view log

Syntax

create_materialized_vw_log::=

Description of create_materialized_vw_log.gif follows
Description of the illustration create_materialized_vw_log.gif

(physical_attributes_clause::=, logging_clause::=, parallel_clause::=, table_partitioning_clauses ::= (in CREATE TABLE), new_values_clause::=, mv_log_purge_clause::=.)

physical_attributes_clause::=

Description of physical_attributes_clause.gif follows
Description of the illustration physical_attributes_clause.gif

(storage_clause::=)

logging_clause::=

Description of logging_clause.gif follows
Description of the illustration logging_clause.gif

parallel_clause::=

Description of parallel_clause.gif follows
Description of the illustration parallel_clause.gif

new_values_clause::=

Description of new_values_clause.gif follows
Description of the illustration new_values_clause.gif

mv_log_purge_clause::=

Description of mv_log_purge_clause.gif follows
Description of the illustration mv_log_purge_clause.gif

Semantics

schema

Specify the schema containing the materialized view log master table. If you omit schema, then Oracle Database assumes the master table is contained in your own schema. Oracle Database creates the materialized view log in the schema of its master table. You cannot create a materialized view log for a table in the schema of the user SYS.

table

Specify the name of the master table for which the materialized view log is to be created. Oracle Database encrypts any columns in the materialized view log that are encrypted in the master table, using the same encryption algorithm.

Restriction on Master Tables of Materialized View Logs You cannot create a materialized view log for a temporary table or for a view.

physical_attributes_clause

Use the physical_attributes_clause to define physical and storage characteristics for the materialized view log.

See Also:

physical_attributes_clause and storage_clause for a complete description these clauses, including default values

TABLESPACE Clause

Specify the tablespace in which the materialized view log is to be created. If you omit this clause, then the database creates the materialized view log in the default tablespace of the schema of the materialized view log.

logging_clause

Specify either LOGGING or NOLOGGING to establish the logging characteristics for the materialized view log. The default is the logging characteristic of the tablespace in which the materialized view log resides.

See Also:

logging_clause for a full description of this clause

CACHE | NOCACHE

For data that will be accessed frequently, CACHE specifies that the blocks retrieved for this log are placed at the most recently used end of the least recently used (LRU) list in the buffer cache when a full table scan is performed. This attribute is useful for small lookup tables.

NOCACHE specifies that the blocks are placed at the least recently used end of the LRU list. The default is NOCACHE.

Note:

NOCACHE has no effect on materialized view logs for which you specify KEEP in the storage_clause.

See Also:

CREATE TABLE for information about specifying CACHE or NOCACHE

parallel_clause

The parallel_clause lets you indicate whether parallel operations will be supported for the materialized view log.

For complete information on this clause, refer to parallel_clause in the documentation on CREATE TABLE.

table_partitioning_clauses

Use the table_partitioning_clauses to indicate that the materialized view log is partitioned on specified ranges of values or on a hash function. Partitioning of materialized view logs is the same as partitioning of tables.

See Also:

table_partitioning_clauses in the CREATE TABLE documentation

WITH Clause

Use the WITH clause to indicate whether the materialized view log should record the primary key, rowid, object ID, or a combination of these row identifiers when rows in the master are changed. You can also use this clause to add a sequence to the materialized view log to provide additional ordering information for its records.

This clause also specifies whether the materialized view log records additional columns that might be referenced as filter columns, which are non-primary-key columns referenced by subquery materialized views, or join columns, which are non-primary-key columns that define a join in the subquery WHERE clause.

If you omit this clause, or if you specify the clause without PRIMARY KEY, ROWID, or OBJECT ID, then the database stores primary key values by default. However, the database does not store primary key values implicitly if you specify only OBJECT ID or ROWID at create time. A primary key log, created either explicitly or by default, performs additional checking on the primary key constraint.

OBJECT ID Specify OBJECT ID to indicate that the system-generated or user-defined object identifier of every modified row should be recorded in the materialized view log.

Restriction on OBJECT ID You can specify OBJECT ID only when creating a log on an object table, and you cannot specify it for storage tables.

PRIMARY KEY Specify PRIMARY KEY to indicate that the primary key of all rows changed should be recorded in the materialized view log.

ROWID Specify ROWID to indicate that the rowid of all rows changed should be recorded in the materialized view log.

SEQUENCE Specify SEQUENCE to indicate that a sequence value providing additional ordering information should be recorded in the materialized view log. Sequence numbers are necessary to support fast refresh after some update scenarios.

See Also:

Oracle Database Data Warehousing Guide for more information on the use of sequence numbers in materialized view logs and for examples that use this clause

COMMIT SCN 

Without the COMMIT SCN clause, the materialized view log is based on timestamps and requires some setup operations when preparing to refresh the materialized view. Specify COMMIT SCN to instruct the database to use commit SCN data rather than timestamps. This setting removes the need for the setup operations and thus can improve the speed of the materialized view refresh.

You can create the following types of local materialized views (including both ON COMMIT and ON DEMAND) on base tables with commit SCN-based materialized view logs:

You cannot create remote materialized views on base tables with commit SCN-based materialized view logs.

Restrictions on COMMIT SCN The following restrictions apply to COMMIT SCN:

column Specify the columns whose values you want to be recorded in the materialized view log for all rows that are changed. Typically these columns are filter columns and join columns.

Restrictions on the WITH Clause This clause is subject to the following restrictions:

See Also:

NEW VALUES Clause

The NEW VALUES clause lets you determine whether Oracle Database saves both old and new values for update DML operations in the materialized view log.

INCLUDING Specify INCLUDING to save both new and old values in the log. If this log is for a table on which you have a single-table materialized aggregate view, and if you want the materialized view to be eligible for fast refresh, then you must specify INCLUDING.

EXCLUDING Specify EXCLUDING to disable the recording of new values in the log. This is the default. You can use this clause to avoid the overhead of recording new values. Do not use this clause if you have a fast-refreshable single-table materialized aggregate view defined on the master table.

mv_log_purge_clause

Use this clause to specify the purge time for the materialized view log.

Restriction on mv_log_purge_clause This clause is not valid for materialized view logs on temporary tables.

See Also:

Oracle Database Data Warehousing Guide for more information on purging materialized view logs

Examples

Creating a Materialized View Log: Examples The following statement creates a materialized view log on the oe.customers table that specifies physical and storage characteristics:

CREATE MATERIALIZED VIEW LOG ON customers 
   PCTFREE 5 
   TABLESPACE example 
   STORAGE (INITIAL 10K); 

The materialized view log on customers supports fast refresh for primary key materialized views only.

The following statement creates another version of the materialized view log with the ROWID clause, which enables fast refresh for more types of materialized views:

CREATE MATERIALIZED VIEW LOG ON customers WITH PRIMARY KEY, ROWID; 

This materialized view log on customers makes fast refresh possible for rowid materialized views and for materialized join views. To provide for fast refresh of materialized aggregate views, you must also specify the SEQUENCE and INCLUDING NEW VALUES clauses, as shown in the example that follows.

Specify a Purge Repeat Interval for a Materialized View Log: Example The following statement creates a materialized view log on the oe.orders table. The contents of the log will be purged once every five days, beginning five days after the creation date of the materialized view log:

CREATE MATERIALIZED VIEW LOG ON orders
  PCTFREE 5
  TABLESPACE example
  STORAGE (INITIAL 10K)
  PURGE REPEAT INTERVAL '5' DAY;

Specifying Filter Columns for Materialized View Logs: Example The following statement creates a materialized view log on the sh.sales table and is used in "Creating Materialized Aggregate Views: Example". It specifies as filter columns all of the columns of the table referenced in that materialized view.

CREATE MATERIALIZED VIEW LOG ON sales 
   WITH ROWID, SEQUENCE(amount_sold, time_id, prod_id)
   INCLUDING NEW VALUES; 

Specifying Join Columns for Materialized View Logs: Example The following statement creates a materialized view log on the order_items table of the sample oe schema. The log records primary keys and product_id, which is used as a join column in "Creating a Fast Refreshable Materialized View: Example".

CREATE MATERIALIZED VIEW LOG ON order_items WITH (product_id);

Including New Values in Materialized View Logs: Example The following example creates a materialized view log on the oe.product_information table that specifies INCLUDING NEW VALUES:

CREATE MATERIALIZED VIEW LOG ON product_information 
   WITH ROWID, SEQUENCE (list_price, min_price, category_id), PRIMARY KEY
   INCLUDING NEW VALUES;

You could create the following materialized aggregate view to use the product_information log:

CREATE MATERIALIZED VIEW products_mv 
   REFRESH FAST ON COMMIT
   AS SELECT SUM(list_price - min_price), category_id
         FROM product_information 
         GROUP BY category_id;

This materialized view is eligible for fast refresh because the log defined on its master table includes both old and new values.