It contains the following topics: Overview of Data Warehousing with Materialized Views. You can refer either to the table name with scope_table_name or to a column alias. In contrary of views, materialized views avoid executing the SQL query for every access by storing the result set of the query. You can make an MV which queries a view and another MV fast refresh on commit. Table 5-4 describes the refresh options. If a materialized view using aggregates is fast refreshable, then an index appropriate for the fast refresh procedure is created unless USING NO INDEX is specified in the CREATE MATERIALIZED VIEW statement. Otherwise, use the BUILD IMMEDIATE clause to populate the materialized view immediately, or the BUILD DEFERRED clause to populate the materialized view later. The advantage of creating this type of materialized view is that expensive joins are precalculated. Restriction on Primary Key Materialized Views. Before starting to define and use the various components of summary management, you should review your schema design to abide by the following guidelines wherever possible. In some cases, user-defined materialized views are refreshed on a schedule that is longer than the update cycle. The following statement creates a rowid materialized view on the sample table oe.orders: Periodic Refresh of Materialized Views: Example. Table 5-7 lists the semantics for RELATED_TEXT and RELATED_NUM columns. The column alias list explicitly resolves any column name conflict, eliminating the need to specify aliases in the SELECT clause of the materialized view. The cost of maintaining a Materialized View Log is not unlike the cost of maintaining an index structure, the benefits of which can potentially far outweigh the overheads. The following statement creates and populates a materialized aggregate view on the sample sh.sales table and specifies the default refresh method, mode, and time. If the materialized view was prebuilt on a table, then the table is not dropped, but it can no longer be maintained with the refresh mechanism or used by query rewrite. If the materialized view has remote tables in the FROM clause, all tables in the FROM clause must be located on that same site. Determining which new materialized views should be created. You cannot create remote materialized views on master tables with commit SCN-based materialized view logs. The logging characteristic affects the creation of the materialized view and any nonatomic refresh that is initiated by way of the DBMS_REFRESH package. Specify FAST to indicate the fast refresh method, which performs the refresh according to the changes that have occurred to the master tables. It can also include any number of joins. Specify ON DEMAND to indicate that database will not refresh the materialized view unless the user manually launches a refresh through one of the three DBMS_MVIEW refresh procedures. Hierarchies describe the business relationships and common access patterns in the database. The index_org_table_clause of CREATE TABLE, Restrictions on Index-Organized Materialized Views. Oracle Database implicitly refreshes objects materialized WITH OBJECT ID. If there are outer joins, all the joins must be connected by ANDs and must use the equality (=) operator. You cannot alter an existing materialized view and enable ON STATEMENT refresh for it. Any number of tables can be joined together. When a materialized view is maintained using the ON COMMIT method, the time required to complete the commit may be slightly longer than usual. Alternatively, if the previous example did not include the columns times_rid and customers_rid, and if the refresh method was REFRESH FORCE, then this materialized view would be fast refreshable only if the sales table was updated but not if the tables times or customers were updated. This clause is not supported for materialized views with remote tables. The owner of the materialized view must have the CREATE TABLE system privilege. About Nesting Materialized Views with Joins and Aggregates, Nested Materialized View Usage Guidelines, Restrictions When Using Nested Materialized Views. This is also the case for indexes created on the materialized view. You cannot subsequently alter the materialized view to use ON STATEMENT refresh. A materialized view is a database object that contains the results of a query. Oracle Database PL/SQL Packages and Types Reference for further information about the DBMS_MVIEW package, MV_CAPABILITIES_TABLE.CAPABILITY_NAME Details. You need to know your update window for the following activities: Updating or rebuilding the indexes on the detail data, Performing quality assurance tests on the data, Updating the indexes on the materialized views. Oracle Database Data Warehousing Guide for restrictions relating to using fast refresh and synchronous refresh, Oracle Database Administrator’s Guidefor more information on restrictions relating to replication, "Creating Materialized Join Views: Example", "Creating Subquery Materialized Views: Example", and "Creating a Nested Materialized View: Example". logging_clause for a full description of this clause and Oracle Database PL/SQL Packages and Types Reference for more information on atomic and nonatomic refresh. Specify the tablespace in which the materialized view is to be created. This, however, incurs the cost of constraint maintenance. In this case, you must ensure that the materialized view behaves correctly. Also, the refresh method you choose will not be optimally efficient if: The defining query uses an outer join that behaves like an inner join. Using materialized views against remote tables is … You can add a column alias list to a CREATE MATERIALIZED VIEW statement. You can, however, add the SEQUENCE number to the materialized view log after it has been created. A query rewrite capability to transparently rewrite a query to use a materialized view. Specify EDITION edition to search edition. Referential integrity constraints on the target table are disabled, and the local index in the target partition is marked unusable. Materialized views are not eligible for fast refresh if the defining query contains an analytic function or the XMLTable function. Thus, if the user specifies select aliases in the materialized view's defining query while there is no alias in the query, the full text match comparison fails. Incrementally maintaining these distinct materialized aggregate views can take a long time, because the underlying join has to be performed many times. Performing CPU-intensive calculations 4. Specifying NULL EDITION is equivalent to omitting the evaluation_edition_clause. You cannot fast refresh a materialized view if any of its columns is encrypted. If you specify REFRESH FAST, Oracle Database performs further verification of the query definition to ensure that fast refresh can be performed if any of the detail tables change. Materialized view logs are required if you want to use fast refresh, with the exception of partition change tracking refresh. When you drop a materialized view that was created on a prebuilt table, the table still exists—only the materialized view is dropped. You cannot specify the ON PREBUILT TABLE clause when creating a rowid materialized view. When using certain materialized views, you must ensure that your NLS parameters are the same as when you created the materialized view. This example creates a materialized view product_sales_mv that computes the sum of sales by prod_name. Refresh can use trusted constraints and QUERY_REWRITE_INTEGRITY = TRUSTED during refresh.This allows use of non-validated RELY constraints and rewrite against materialized views in UNKNOWN or FRESH state during refresh. For example, "GROUP BY a, ROLLUP(a, b)" is not fast refreshable because it results in duplicate groupings "(a), (a, b), AND (a)". Applies to: Oracle Database Cloud Service - Version N/A and later Oracle Database - Enterprise Edition - Version and later Oracle Database Cloud Schema Service - … Refer to the table_compression clause in the documentation on CREATE TABLE for the full semantics of this clause. In Oracle, if you specify REFRESH FAST for a single-table aggregate Oracle materialized view, you must have created a materialized view log for the underlying table, or the refresh command will fail. Oracle Database 12c Enterprise Edition Release - 64bit Production desc Par_trials; Column Name Pk Null? You then decide to recreate the materialized view and you know that the master tables of the view have not changed. If no such log exists, then only full refresh of mv1 is possible. The column alias list explicitly resolves any column name conflict without attaching aliases in the SELECT clause of the materialized view. Any materialized view that can use TRUSTED constraints for refresh is left in a state of trusted freshness (the UNKNOWN state) after refresh. If this capability is possible, at least full and partial text match query rewrite are possible. "Creating Prebuilt Materialized Views: Example". When creating an Oracle materialized view, you have the option of specifying whether the refresh occurs manually (ON DEMAND) or automatically (ON COMMIT, DBMS_JOB). Even though a materialized view is defined, it will not automatically be used by the query rewrite facility. The preceding statement also establishes storage characteristics that the database uses to maintain the materialized view. If aggregate X is present, aggregate Y is required and aggregate Z is optional. If query rewrite is enabled during refresh, these can improve the performance of refresh by enabling more performant query rewrites. It cannot reference a table on which an XMLIndex index is defined. You must specify this clause if subquery references an editioned object. The XML data types include XMLType and URI data type columns. The default refresh method is FORCE. Only a few restrictions limit what can be specified. The TUNE_MVIEW package, which shows you how to make your materialized view fast refreshable and use general query rewrite. In addition, optimizations can be performed for this class of single-table aggregate materialized view and thus refresh is very efficient. Unlike indexes, materialized views can be accessed directly using a SELECT statement. If you specify ON COMMIT, then you cannot also specify START WITH or NEXT. The ALTER MATERIALIZED VIEW is used to modify an existing materialized view in one or more of the following ways: To change its storage characteristics; To change its refresh method, mode, or time; To alter its structure so that it is a different type of materialized view They cannot have GROUP BY clauses or aggregates. The script is found in the admin directory. Starting with Oracle 12cR2, Materialized views can be refreshed ON STATEMENT. You can typically achieve better fast refresh performance of local materialized views containing aggregates or joins by using a WITH COMMIT SCN clause. After the initial effort of creating and populating the data warehouse or data mart, the major administration overhead is the update process, which involves: Periodic extraction of incremental changes from the operational systems. Once you create one based on your query, Oracle can get the results direct from the MV instead of executing the statement itself. Refresh performance benefits from partitioning, because it can take advantage of parallel DML capabilities and possible PCT-based refresh. Creating Subquery Materialized Views: Example. A materialized view definition includes any number of aggregates, as well as any number of joins. Neither the materialized view nor the master tables of the materialized view can be remote. However, you can specify WITH REDUCED PRECISION to allow the precision of columns in the defining query to be different from that of the table columns. If you use the REFRESH procedure with the nested parameter value set to TRUE, only specified materialized views and their child materialized views in the tree are refreshed, and not their top-level materialized views. This clause may increase the time taken to complete the commit, because the database performs the refresh operation as part of the commit process. Materialized views are not eligible for fast refresh if the defining query references a table on which an XMLIndex index is defined. See Oracle Database Utilities for the restrictions and considerations when using SQL*Loader with the DIRECT or PARALLEL keywords. The column UNKNOWN_TRUSTED_FD in the same view is also set to Y, which means yes. If query rewrite is not occurring when expected, DBMS_MVIEW.EXPLAIN_REWRITE can help provide reasons why a specific query is not eligible for rewrite. Without a materialized view log, Oracle Database must reexecute the materialized view query to refresh the materialized view. This clause lets you specify that the materialized view is not eligible for query rewrite in an edition and its descendants. You can select data from a materialized view as you would from a table or view. Materialized views based on prebuilt tables are eligible for selection by query rewrite provided the parameter QUERY_REWRITE_INTEGRITY is set to STALE_TOLERATED or TRUSTED. Guidelines 1, 2, and 3 affect both query rewrite performance and materialized view refresh performance. "Requirements for Using Materialized Views with Aggregates". Refresh occurs when a user manually executes one of the available refresh procedures contained in the DBMS_MVIEW package (REFRESH, REFRESH_ALL_MVIEWS, REFRESH_DEPENDENT). For example, you can create a summary table to contain the sums of sales by region and by product. This example also requires a materialized view log on oe.inventories. If this capability is not possible, full text match query rewrite is not possible. A materialized view consumes storage space. Besides tables, other elements such as views, inline views (subqueries in the FROM clause of a SELECT statement), subqueries, and materialized views can all be joined or referenced in the SELECT clause. Materialized Views in Oracle. The constraints can be in RELY mode. Because this will be a single-table aggregate materialized view on join_sales_cust_time, you must log all the necessary columns and use the INCLUDING NEW VALUES clause. The rowids of all the detail tables must appear in the SELECT list of the materialized view query definition. If this capability is possible, full text match query rewrite is possible. About Refresh Modes for Materialized Views, About Using Trusted Constraints and Materialized View Refresh. About Materialized Views Containing Only Joins. Creating a Nested Materialized View: Example. Performing data summarization (for example, sums and averages) 2. Automatic Refresh Times for Materialized Views: Example. Refreshing Materialized Views for further details about partition change tracking, Advanced Query Rewrite for Materialized Views for further details about partition change tracking. "About Partition Change Tracking (PCT) Refresh for Materialized Views". Use the REFRESH_DEPENDENT procedure with the nested parameter value set to TRUE if you want to ensure that all materialized views in a tree are refreshed. For example, table sales_summary was created to contain sales summary information. This is shown in the defining query of the view view_with_unionall. The materialized view warranty_orders requires that materialized view logs be defined on order_items (with product_id as a join column) and on inventories (with quantity_on_hand as a filter column). materialized view is built from many tables, and changes to the base tables require an update to the materialized view (via a snapshot refresh or full refresh).
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