The following statement creates the hr.employees materialized view for the United Kingdom office. Oracle can perform a complete refresh for any materialized view. For example, if the hr_repg materialized view group owned by hr is based on the associated master group at the orc1.example.com master site, then the hr_repg materialized view group owned by personnel must also be based on the associated master group at orc1.example.com, assuming that the hr and personnel owned groups are at the same materialized view site. "Materialized View Groups" for more information about materialized view groups, "Materialized Views with Subqueries" for more information about materialized views with subqueries, "Refresh Types" for more information about fast refresh, "Materialized View Log" for more information about materialized view logs, Oracle Database SQL Language Reference for more information about subqueries. This subquery is one to many because one product can have multiple product descriptions (for different languages). If the subquery is many to many or one to many, join columns that are not part of a primary key must be included in the materialized view log of the master. To keep a materialized view's data relatively current with the data of its master, the materialized view must be refreshed periodically. I created the following example and query the log table. An object materialized view is structured in the same way as an object table. Additional indexes can be created by Oracle at the remote materialized view site to support fast refreshing of materialized views with subqueries. Collection columns are columns based on varray and nested table data types. 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. Look at the relationships in Figure 3-3, and notice that the customers and orders tables are related through the customer_id column. There are 4 different options that we will try in order to achieve a parallel MV refresh. Owner: the user who owns the materialized view. ROWID Specify ROWID to indicate that the rowid of all rows changed should be recorded in the materialized view log. If new customers are identified that have an order total greater than $20,000, then the new data will be propagated to the materialized view site during the subsequent refresh process. 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. However, you can use row subsetting on the parent table of a nested table and doing so can result in a subset of the nested tables in the materialized view. See "Logging Columns in the Materialized View Log" for more information. A master table can have only one materialized view log defined on it. For example, a materialized view group based on a personnel master group is also named personnel. In this case, you can use the VALIDATE STRUCTURE option of the ANALYZE TABLE statement to determine which rowid hints at each replication site are incorrect. 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. Some materialized view sites might not need to replicate all of the data in master tables, and, in fact, these sites might not have the storage capacity for all of the data. A materialized view log is located in the master database in the same schema as the master table. 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. You create varrays with the CREATE TYPE ... AS VARRAY statement, and you create nested tables with the CREATE TYPE ... AS TABLE statement. Here, the customers table appears in two EXISTS expressions, but the EXISTS expressions are in separate UNION blocks. It was a pleasure to finally meet you. If necessary, you can log REF columns in the materialized view log. Updatable materialized views enable you to decrease the load on master sites because users can make changes to the data at the materialized view site. You can use materialized views to achieve one or more of the following goals: If one of your goals is to reduce network loads, then you can use materialized views to distribute your corporate database to regional sites. The need for a materialized log, and what it contains, depends mainly on the SQL and refresh type that will be used to refresh the materialized view. Subqueries can include AND conditions, but each OR condition can only reference columns contained within one row. 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. They have lower network requirements and dependencies than multimaster replication because of the point in time nature of materialized view replication. Table 3-2 shows the required privileges when the refresher of the materialized view is not the owner. Advanced Replication includes an optimization for null refresh. Oracle automatically registers a materialized view at its master site or master materialized view site when you create the materialized view, and unregisters the materialized view when you drop it. "Materialized View Replication" for an introduction to read-only and updatable materialized views, "Data Type Considerations for Replicated Tables" for information about data type considerations for materialized views. 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. 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 privilege can be granted either explicitly or through a role. The refresh interval must be greater than the length of time necessary to perform a refresh. The type has exactly the same attributes in exactly the same order at both the materialized view site and the master site. Decide how and when to refresh each materialized view to make it more current. If you try to create a materialized view with a name larger than 30 bytes, Oracle returns an error. The following statement is valid because it specifies the entire cust_address column object: If a materialized view is based on an object table and is created using the OF type clause, then the materialized view is called an object materialized view. 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. Users cannot perform data manipulation language (DML) statements on read-only materialized views, but they can perform DML on updatable and writeable materialized views. 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. The process of setting up a materialized view is sometimes called materialization. The following is an example of a CREATE MATERIALIZED VIEW statement that creates a ROWID materialized view: "Materialized View Log" for more information about the differences between a ROWID and primary key materialized view, Oracle Database SQL Language Reference for more information about the WITH ROWID clause in the CREATE MATERIALIZED VIEW statement. This procedure creates the type and adds it to a materialized view group. Oracle supports ROWID materialized views in addition to the default primary key materialized views. Because the materialized view conforms to the conditions for fast … For example, you can drop the objects as a group. SEQUENCE Specify SEQUENCE to indicate that a sequence value providing additional ordering information should be recorded in the materialized view log. ORA-12000: a materialized view log already exists on table if it does not exists initially.. so I was thinking of adding a check to see if it exists before doing the drop, but I cannot find which system table keeps the refrence to it ? Simple Materialized Views with a Joined View: Method B in Figure 3-2 shows two simple materialized views in Database II, as well as a view that performs the join in the materialized view's database. In addition, Oracle recommends that you add a user-defined type to the replication group in which it is used, but doing so is not required. 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. Figure 3-10 Materialized View Replication Objects. 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). Consider a scenario where you have the customers table and orders table in the oe schema, and you want to create a materialized view of the customers table based on data in both the customers table and the orders table. A materialized view log is located in the master database in the same schema as the master table. If the owner of a materialized view at the materialized view site has a private database link to the master site or master materialized view site, then the database link connects to the owner of the master at the master site or master materialized view site. After you have defined your materialized view group with the addition of a group owner, you add your materialized view objects to the target materialized view group by defining the same group owner. The materialized view created by this statement is fast refreshable and updatable. If the refresher of a materialized view also owns the materialized view, this user must have SELECT object privilege on the master and the master's materialized view log or SELECT ANY TABLE system privilege. When all of these items are the same for two or more types, the types have the same hashcode. 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. 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. 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". Primary key constraints on materialized views might or might not be deferrable. A fast refresh of the level 2 materialized view oe.customers_sf pushes the new value for the credit_limit to oe.customers_region materialized view at ca.us. In a single master site environment that has updatable materialized views, quiesce is not required when you perform administration operations on the master site if you: Propagate all of the deferred transactions at the databases containing updatable materialized views before you perform the administration operations to the master group. Manual refresh is an ideal solution when the refresh is performed with a dial-up network connection. When a REF column at a materialized view site points to a remote master, the REFs are considered dangling. Partitioning of materialized view logs is the same as partitioning of tables. Therefore, materialized view logs cannot be imported into a schema that is different than the schema from which they were exported. For example, a row in a master table might be updated 10 times since the last refresh of a materialized view, but the result is still only one update of the corresponding row in the materialized view during the next refresh. 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. The size limit for a materialized view name is 30 bytes. Updatable materialized view is useful if you are looking to replicate the changes happened on the client site to the master site. Oracle Database SQL Language Reference for more information about user-defined types and Oracle-supplied types. 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. This process is called a complete refresh. An object materialized view inherits the object identifier (OID) specifications of its master. The following is an example of a read-only materialized view: You can make a materialized view updatable during creation by including the FOR UPDATE clause or enabling the equivalent option in the Advanced Replication interface in Oracle Enterprise Manager. Otherwise, you cannot specify the SCOPE FOR clause during materialized view creation. 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. In addition, an updatable materialized view must reside in a different database than the master replication group. See "Master Site and Master Materialized View Site Mechanisms". You can also replicate collections. If you do not specify this clause, Oracle automatically names the storage table. 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. Fast refreshes allow you to run refreshes more often, and in some cases you can make use of refreshes triggered on commit of changes to the base tables, but this can represent a significant overhe… Oracle Database can use this materialized view log to perform fast refreshes for all fast-refreshable materialized views based on the master table. The user fast refreshes the materialized view. A fast refresh of the level 2 materialized view oe.customers_sf pulls the credit_limit value of 5000 back down to this materialized view at sf.ca. PCT is also used to support fast refresh after partition maintenance operations on a materialized view's master table. This requirement includes LOB columns with NOT NULL constraints. This process is called incremental or fast refresh. Special care on view log. Use the physical_attributes_clause to define physical and storage characteristics for the materialized view log. Each table can be in only one EXISTS expression. Consider a multinational company that maintains all employee information at headquarters, which is in the in the United States. When creating a refresh group, you can configure the group so that Oracle automatically refreshes the group's materialized views at scheduled intervals. PCT-based refresh on a materialized view is possible only if a number of conditions are satisfied. The update for the credit_limit at the ca.us site is recorded in the materialized view log a this level 1 materialized view site. 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. Invalid rowid hints can cause performance problems. Both master materialized views and materialized views based on materialized views must be primary key materialized views. As described in the previous section, the internal trigger adds change information to the materialized view log whenever a DML transaction has taken place on the target master. Materialized views enable you to replicate data based on column- and row-level subsetting, while multimaster replication requires replication of the entire table. However, if you try to refresh orders_lev2, Oracle returns an error because orders_lev1 no longer exists. For example, the following statement creates a complex materialized view because it has a UNION ALL set operation: In some cases, an aggregate function, although it is possible to have an aggregate function in the defining query and still have a simple materialized view, In some cases, joins other than those in a subquery, although it is possible to have joins in the defining query and still have a simple materialized view. For example, the following statement deletes rows from the materialized view log that has dependency rows in the least recently refreshed materialized view: If you have materialized views based on partitioned master tables, then you might be able to use Partition Change Tracking (PCT) to identify which materialized view rows correspond to a particular partition. It seems that snaptime$$ always has the same time. Therefore, you cannot specify either of the following combinations if column contains one of the primary key columns: 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, "Specifying Filter Columns for Materialized View Logs: Example" and "Specifying Join Columns for Materialized View Logs: Example". 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. 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. You use the BEGIN_TABLE_REORGANIZATION and END_TABLE_REORGANIZATION procedures in the DBMS_MVIEW package to reorganize a master table. That is, this statement creates a materialized view that contains customers who have an order with an order total greater than $20,000: Figure 3-4 Row Subsetting with One to Many Subqueries. Changes made to the master table or master materialized view, as recorded by the materialized view log, are propagated to the materialized view during the refresh process. Figure 3-9 illustrates an example that shows a master materialized view at level 1 (orders_1) and level 2 (orders_2). A materialized view log (snapshot log) is a schema object that records changes to a master table's data so that a materialized view defined on that master table can be refreshed incrementally. The ability to create materialized views that are based on other materialized views enables you to create multitier materialized views. An error is written to the import log file and the items are not imported if you attempt an import using the Data Pump Import utility that specifies the REMAP_SCHEMA import parameter to import an export dump file that contains materialized view logs in the specified schema. A nested table's storage table must have a primary key. You can find the object identifier for a type by querying the DBA_TYPES data dictionary view. You may had to add WITH SEQUENCE to your log creation to cope with certain use as stated in Oracle documentation : Specify SEQUENCE to indicate that a sequence value providing additional ordering information should be recorded in the materialized view log. As described in the following section, you can rescope a REF column to a local materialized view or table at the materialized view site during creation of the materialized view. The updatable materialized view log is also used when you fast refresh a writeable materialized view, as illustrated in the following scenario: A user inserts a row into a writeable materialized view that has a remote master. This subquery is one to many because a product can be in many inventories. Further, the ability of multimaster replication to provide real-time or near real-time replication can result in greater network traffic, and might require a dedicated network link. 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