SNAPSHOTis supported in place of
VIEWfor backward compatibility.
Materialized view logs are used for two types of materialized view refreshes: fast refresh and synchronous refresh.
Fast refresh uses a conventional materialized view log. During a fast refresh (also called an incremental refresh), 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.
Synchronous refresh uses a special type of materialized view log called a staging log. During a synchronous refresh, DML changes are first described in the staging log and then applied to the master tables and the materialized views simultaneously. This guarantees that the master table data and materialized view data are in sync throughout the refresh process. This refresh method is useful in data warehousing environments.
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 complete refresh takes more time to complete than a fast refresh or a synchronous 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.
Fast refresh supports 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
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
SCN. Existing materialized view logs cannot be altered to add
SCN unless they are dropped and recreated. Refer to Oracle Database Data Warehousing Guide for more information.
Synchronous refresh supports only timestamp-based staging logs.
CREATE MATERIALIZED VIEW, ALTER MATERIALIZED VIEW, Oracle Database Concepts, Oracle Database Data Warehousing Guide, and Oracle Database Advanced Replication for information on materialized views in general
ALTER MATERIALIZED VIEW LOG for information on modifying a materialized view log
DROP MATERIALIZED VIEW LOG for information on dropping a materialized view log
Oracle Database Utilities for information on using direct loader logs
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.
If you own the master table, then you can create an associated materialized view log if you have the
If you are creating a materialized view log for a table in another user's schema, then you must have the
TABLE system privileges, as well as either the
SELECT object privilege on the master table or the
TABLE system privilege.
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
TABLESPACE system privilege.
See Also:Oracle Database Data Warehousing Guide for more information about the prerequisites for creating a materialized view log
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
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.
You cannot create a materialized view log for a temporary table or for a view.
You cannot create a materialized view log for a master table with a virtual column.
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
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.
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
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
NOCACHEhas no effect on materialized view logs for which you specify
See Also:CREATE TABLE for information about specifying
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
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
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
If you omit this clause, or if you specify the clause without
ID, then the database stores primary key values by default. However, the database does not store primary key values implicitly if you specify only
ROWID at create time. A primary key log, created either explicitly or by default, performs additional checking on the primary key constraint.
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
SCN clause, the materialized view log is based on timestamps and requires some setup operations when preparing to refresh the materialized view. Specify
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
DEMAND) on master tables with commit SCN-based materialized view logs:
Materialized aggregate views, including materialized aggregate views on a single table
Materialized join views
Primary-key-based and rowid-based single table materialized views
ALL materialized views, where each
ALL branch is one of the above materialized view types
You cannot create remote materialized views on master tables with commit SCN-based materialized view logs.
SCN on a table with one or more LOB columns is not supported and causes
Creating a materialized view on master tables with different types of materialized view logs (that is, a master table with timestamp-based materialized view logs and a master table with commit SCN-based materialized view logs) is not supported and causes
If you specify
SCN, then you cannot specify
You can specify only one
SEQUENCE, and one column list for each materialized view log.
Primary key columns are implicitly recorded in the materialized view log. Therefore, you cannot specify any of the following combinations if
column contains one of the primary key columns:
WITH ... PRIMARY KEY ... (column) WITH ... (column) ... PRIMARY KEY WITH (column)
CREATE MATERIALIZED VIEW for information on explicit and implicit inclusion of materialized view log values
Oracle Database Advanced Replication for more information about filter columns and join columns
VALUES clause lets you determine whether Oracle Database saves both old and new values for update DML operations in the materialized view log.
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
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.
Use this clause to specify the purge time for the materialized view log.
SYNCHRONOUS: the materialized view log is purged immediately after refresh. This is the default.
ASYNCHRONOUS: the materialized view log is purged in a separate Oracle Scheduler job after the refresh operation.
INTERVAL set up a scheduled purge that is independent of the materialized view refresh and is initiated during
LOG statement. This is very similar to scheduled refresh syntax in a
WITH datetime expression specifies when the purge starts.
NEXT datetime expression computes the next run time for the purge.
If you specify
INTERVAL, then the next run time will be:
LOG statement with a scheduled purge creates an Oracle Scheduler job to perform log purge. The job calls the
PURGE_LOG procedure to purge the materialized view logs. This process allows you to amortize the purging costs over several materialized view refreshes.
See Also:Oracle Database Data Warehousing Guide for more information on purging materialized view logs
Use this clause to specify the refresh method for which the materialized view log will be used. You can specify only one refresh method for any given master table.
FOR SYNCHRONOUS REFRESH Specify this clause to create a staging log that can be used for synchronous refresh. Use
staging_log_name to specify the name of the staging log to be created. The staging log will be created in the schema in which the master table resides.
After you create the staging log, you cannot perform DML operations directly on the master table. You must use the procedures in the
DBMS_SYNC_REFRESH package to prepare and execute change data operations.
If you specify
REFRESH, then you cannot specify
To be eligible for synchronous refresh, the master table must satisfy the following criteria:
If the master table is a fact table, then it must be partitioned.
The master table must have a key. If the master table is a dimension table, then it must have a primary key defined on it. If the master table is a fact table, then the set of columns that are the foreign keys of the dimension tables joined to the fact table are deemed to be the key.
The master table cannot have a non-NULL Virtual Private Database (VPD) policy or a trigger defined on it.
Oracle Database may allow you to create a staging log on a master table even if all of the preceding criteria are not met. However, the master table will not be eligible for synchronous refresh.
Any existing materialized views on the master table must be refresh-on-demand materialized views. If an existing materialized view is a refresh-on-commit materialized view, then you must change it to a refresh-on-demand materialized view with the alter_mv_refresh clause of
VIEW before you create the staging log.
Oracle Database Data Warehousing Guide for the complete steps for using synchronous refresh
Oracle Database PL/SQL Packages and Types Reference for information on the
FOR FAST REFRESH Specify this clause to create a materialized view log that can be used for fast refresh. The materialized view log will be created in the same schema in which the master table resides. This is the default.
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
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);
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
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.
Creating a Staging Log for Synchronous Refresh: Example The following statement creates a staging log on the
sh.sales fact table. The staging log is named
mystage_log and is stored in the
sh schema. It can be used for synchronous refresh.
CREATE MATERIALIZED VIEW LOG ON sales PCTFREE 5 TABLESPACE example STORAGE (INITIAL 10K) FOR SYNCHRONOUS REFRESH USING mystage_log;