Sequence numbers are necessary to support fast refresh after some update scenarios. If the master site or master materialized view site is remote, then the SELECT object privilege must be granted to the user at the master site or master materialized view site to which the user at the materialized view site connects through a database link. 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. Master Site and Master Materialized View Site Mechanisms. In addition to the UNION operation, this statement contains the following subqueries: A subquery referencing the product_information table and the product_descriptions table. 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. A refresh of an updatable materialized view first pushes the deferred transactions at the materialized view site to its master site or master materialized view site. If you do not rescope the REF column, then the materialized view retains the REF scope of the master. Figure 3-13 Refresh Groups Can Contain Objects from Multiple Materialized View Groups. Materialized views are a really useful performance feature, allowing you to pre-calcuate joins and aggregations, which can make applications and reports feel more responsive. A materialized view refresh is an efficient batch operation that makes a materialized view reflect a more current state of its master table or master 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. When a REF column at a materialized view site points to a remote master, the REFs are considered dangling. The ability to create materialized views that are based on other materialized views enables you to create multitier materialized views. All columns that are used in the query must be added to the materialized view log. For example, materialized views based on masters that applications update often might require frequent refreshes. You cannot create a materialized view log for a table in the schema of the user SYS. The following sections provide examples of situations where a subquery is useful. SQL - Materialized View in Oracle. For example, suppose a materialized view named orders_lev1 is based on the oe.orders master table, and a materialized view named orders_lev2 is based on orders_lev1. Advanced Replication includes an optimization for null refresh. If you use deployment templates to build your materialized views, then you can define column subsets on updatable materialized views. You can create materialized view logs on object tables. The update for the credit_limit at the orc1.example.com site is recorded in both the deferred transaction queue and the materialized view log a this master site. If you choose this option, then you must ensure the following: The type is in the same schema at both the materialized view site and the master site. Restriction on Master Tables of Materialized View Logs, Creating a Materialized View Log: Examples, Specifying Filter Columns for Materialized View Logs: Example, Specifying Join Columns for Materialized View Logs: Example, Including New Values in Materialized View Logs: Example, Description of the illustration create_materialized_vw_log.gif, Description of the illustration physical_attributes_clause.gif, Description of the illustration logging_clause.gif, Description of the illustration parallel_clause.gif, Description of the illustration new_values_clause.gif, "Creating a Materialized View Log: Examples", "Specifying Filter Columns for Materialized View Logs: Example", "Specifying Join Columns for Materialized View Logs: Example", "Including New Values in Materialized View Logs: Example", "Creating Materialized Aggregate Views: Example", "Creating a Fast Refreshable Materialized View: Example". The process of setting up a materialized view is sometimes called materialization. A materialized view that is acting as the master for another materialized view is called a master materialized view. Network connectivity must be maintained while performing a refresh. Figure 3-2 Comparison of Simple and Complex Materialized Views. Refresher: the user who refreshes the materialized view. Table 3-2 shows the required privileges when the refresher of the materialized view is not the owner. Clauses that do not comply with the requirements detailed in "Restrictions for Materialized Views with Subqueries", Oracle Database Data Warehousing Guide for information about materialized views with aggregate functions and joins, Oracle Database SQL Language Reference for more information about the CONNECT BY clause, set operations, the DISTINCT keyword, and aggregate functions. This section assumes a basic understanding of the information in that book. 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. Reduce Resource Requirements: If the materialized view is located on a laptop, then hard disks are generally significantly smaller than the hard disks on a corporate server. Oracle returns errors if the materialized view violates any restrictions for subquery materialized views. Materialized View Logs. Each table can be joined only once within the subquery. Object ID: The materialized view records changes to the master object table or master object materialized view based on the object identifier of the affected row objects. In addition, all materialized views that are related by foreign key constraints should be refreshed together or in the same refresh group. You must drop these materialized views manually. The change is applied to the oe.customers_region materialized view at ca.us. The following materialized view is created with a WHERE clause containing a subquery: This type of materialized view is called a subquery materialized view. The following types of materialized views cannot be masters for updatable materialized views: However, these types of materialized views can be masters for read-only materialized views. Is a separate, secondary materialized view, Is refreshed automatically when you refresh the materialized view containing the nested table, Is dropped automatically when you drop the materialized view containing the nested table, Inherits the primary key constraint of the master's storage table. The frequency of this refresh can be configured to run on-demand or at regular time intervals. ROWID materialized views can be used for materialized views based on master tables that do not have a primary key, or for materialized views that do not include all primary key columns of the master tables. You can find the object identifier for a type by querying the DBA_TYPES data dictionary view. One materialized view log can support multiple materialized views on its master table or master materialized view. Without a materialized views log, Oracle Database must re-execute the materialized view query to refresh the materialized views. Use the table_partitioning_clauses to indicate that the materialized view log is partitioned on specified ranges of values or on a hash function. 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. If a fast refresh is attempted for such a materialized view after its master materialized view has performed a complete refresh, then Oracle returns the following error: To perform a 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. For example, if you have group owners hr and ac, then you might name the employees materialized view object as employees_hr and employees_ac, respectively. Notice that one of the restrictions for subqueries states that each table can be in only one EXISTS expression. If you do not specify this clause, Oracle automatically names the storage table. Online redefinition of tables is another possible way to reorganize master tables, but online redefinition is not allowed on master tables with materialized view logs, master materialized views, and materialized views. Oracle 9i came with two packages, dbms_mview.explain_mview and dbms_mview.explain_rewrite that could be used to diagnose why a materialized view wasn't being used for query rewrite. See the Oracle Database PL/SQL Packages and Types Reference for more information. A materialized view log is located in the master database in the same schema as the master table. In addition to the materialized view mechanisms described in the previous section, several other mechanisms organize the materialized views at the materialized view site. Some reasons to use data subsetting are to: Reduce Network Traffic: In a column-subsetted materialized view, only changes that satisfy the WHERE clause of the materialized view's defining query are propagated to the materialized view site, thereby reducing the amount of data transferred and reducing network traffic. 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. Figure 3-11 Master Site and Master Materialized View Site Objects. It was a pleasure to finally meet you. For the parent table of a nested table to be fast refreshed, both the parent table and the nested table's storage table must have a materialized view log. A materialized view log is located in the master database in the same schema as the master table. DBlink speed depends on network latency, so network bottlenecks, bandwidth and network speed can play a huge part. If you are creating a materialized view log for a table in another user's schema, then you must have the CREATE ANY TABLE and COMMENT ANY TABLE system privileges, as well as either the SELECT object privilege on the master table or the SELECT ANY TABLE system privilege. If the creator of a materialized view is not the owner, certain privileges must be granted to the creator and to the owner to create a materialized view. If you own the master table, then you can create an associated materialized view log if you have the CREATE TABLE privilege. You will create a materialized view, add the CACHE clause to the definition of an analytic view and query the analytic view with and without the materialized view. This is a quick post regarding materialized views refresh. Materialized view log is a table associated with the master table of a materialized view. A subquery referencing the product_information table and the inventories table. Online redefinition is allowed on master tables that do not have materialized view logs. To rescope a materialized view, you can either use the SCOPE FOR clause in the CREATE MATERIALIZED VIEW statement, or you can use the ALTER MATERIALIZED VIEW statement after creating the materialized view. This requirement includes LOB columns with NOT NULL constraints. Specifically, a materialized view with a UNION operation is complex if any one of these conditions is true: Any query within the UNION is complex. An object materialized view inherits the object identifier (OID) specifications of its master. Oracle can find the object referenced directly using the rowid contained in the REF, without the need to fetch the rowid from the OID index. This privilege can be granted either explicitly or through a role. Some of these objects are optional and are used only as needed to support the created materialized view environment. 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. 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. When you drop a master materialized view, Oracle does not automatically drop the materialized views based on it. Figure 3-10 Materialized View Replication Objects. Scripting on this page enhances content navigation, but does not change the content in any way. Typically, you would rescope the REF column to the local object materialized view instead of the original remote object table. This process is called a complete refresh. Oracle Database SQL Language Reference for more information about user-defined types and Oracle-supplied types. A scoped REF requires less storage space and provides more efficient access than an unscoped REF. If you specify force refresh, then you might not receive any errors because, when a force refresh is requested, Oracle automatically performs a complete refresh if it cannot perform a fast refresh. This chapter, and this Oracle Database Advanced Replication manual in general, discusses materialized views for use in a replication environment. Query performance against the view would not be as good as the query performance against the complex materialized view in Method A. A materialized view log is located in the master database in the same schema as the master table. The outermost SELECT list columns do not match for the queries in the UNION. Consider a replication environment with the following characteristics: Master site orc1.example.com has the oe.customers table. Refresh-on-commit materialized views are those created using the ON COMMIT REFRESH clause in the CREATE MATERIALIZED VIEW statement. Sql Access Advisor (a GUI tool for materialized view and index management) can recommend the creation of materialized views. For example, using the cust_address_typ user-defined data type described in the previous section, suppose a customers_sub master table is created at master site orc1.example.com: You can create the following read-only materialized view at a remote materialized view site: Notice that the postal_code attribute is specified in the cust_address column object. That is, an object materialized view is composed of row objects. If you reorganize or truncate your master table, then your rowid materialized view must be COMPLETE refreshed because the rowids of the master table have changed. Now lets take a look at how to create and manage a Updatable Materialized View. Oracle offers several types of materialized views to meet the needs of many different replication (and nonreplication) situations. The following statement creates the hr.employees materialized view for the United Kingdom office. In Oracle 19c the cache type is always MATERIALIZED. 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. The innermost SELECT list has no bearing on whether a materialized view is complex. The three mechanisms displayed in Figure 3-11 are required at a master site and at a master materialized view site to support fast refreshing of materialized views. Doing so enables you to define different subqueries for your materialized view definitions in each materialized view group, and allows each user to access only his or her subset of the data. This parameter can override the setting for the parameter num. The user fast refreshes the materialized view. A nested table's storage table must have a primary key. See the Oracle Database Administrator's Guide for more information about online redefinition of tables. A primary key must exist for each table at each nested level. Here, the defining query in the CREATE MATERIALIZED VIEW statement on the customers table has a subquery with a one to many relationship. Now, using multitier materialized views, another level of materialized views at the local level can be based on the materialized views at the national level. You can use this clause to avoid the overhead of recording new values. This behavior is best illustrated through an example. When a materialized view is created, additional mechanisms are created at the materialized view site to support the materialized view. For example, refreshing a refresh group with 100 materialized views is faster than refreshing five refresh groups with 20 materialized views each. The country_id must be logged in the materialized view log for the hr.locations master materialized view at reg_uk.example.com. You might 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. These actions can occur indirectly when they are performed on the materialized view that contains the nested table. The following are examples of simple date expressions that you can use to specify an interval: An interval of seven days is specifies as: Scheduled materialized view refreshes might not always be the appropriate solution for your environment. The type has exactly the same attributes in exactly the same order at both the materialized view site and the master site. To keep a materialized view's data relatively current with the data of its master, the materialized view must be refreshed periodically. This process is called incremental or fast refresh. It specifies as filter columns all of the columns of the table referenced in that materialized view. This process is called incremental or fast refresh. For example, consider a company with international, national, and local offices. Special care on view log. For example, a combination materialized view log can track both the primary key and the rowid of the affected row are recorded. 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: The materialized view log on customers supports fast refresh for primary key materialized views only. If you are creating a materialized view based on a master that has a scoped REF column, then you can rescope the REF to a different object table or object materialized view at the materialized view site. In SQL, dereferencing a dangling REF returns a NULL. An Oracle object that occupies a single column in a table is called a column object. As illustrated in Figure 3-12, a materialized view group has the same name as the master group on which the materialized view group is based. For example, you can create a materialized view base on the categories_tab master by using the following SQL statement: In this case, the categories_relmv materialized view must be read-only, and the rows in this materialized view function in the same way as rows in a relational table. The following sections describe each type of materialized view and also describe some environments for which they are best suited. 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. You must specify a primary key for the storage table if you plan to create materialized views based on its parent table. A materialized view is a transactionally (read) consistent reflection of its master as the data existed at a specific point in time (that is, at creation or when a refresh occurs). In Figure 3-8, two level 2 materialized views are based on one level 1 materialized view. 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. In this case, Oracle uses the updatable materialized view log to remove or overwrite the row in the materialized view. Specifically, a materialized view is considered complex when the defining query of the materialized view contains: For example, the following statement creates a complex materialized view: An INTERSECT, MINUS, or UNION ALL set operation. View resides in this case, the replication environment multilevel WHERE EXISTS clause see, the customers with a network! 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