materialized view in oracle 12c

If the materialized view is partitioned, then, after doing a partition maintenance operation on the materialized view, the indexes become unusable, and they need to be rebuilt for fast refresh to work. The preceding statement also establishes storage characteristics for both the materialized view and the index that the database uses to maintain it: The first STORAGE clause establishes the sizes of the first and second extents of the materialized view as 50 kilobytes each. To create a materialized view in another user's schema: You must have the CREATE ANY MATERIALIZED VIEW system privilege. The contents of the materialized view must be updated when the underlying detail tables are modified. Creating Primary Key Materialized Views: Example. The master table must contain an enabled primary key constraint, and the defining query of the materialized view must specify all of the primary key columns directly. Dropping the evaluation edition invalidates the materialized view. If the materialized view is complex, then the database ignores any master rollback segment you specify. This condition must be declared, and optionally enforced, by adding FOREIGN KEY and NOT NULL constraints on the fact key column(s) and PRIMARY KEY constraints on the dimension key column(s), or by using outer joins. It is used only during the initial creation of the materialized view. It cannot reference a table on which an XMLIndex index is defined. You need to be aware of its performance impact. If this capability is not possible, at least partial text match query rewrite and general query rewrite are not possible. Query rewrite is not possible with all materialized views. Then, create materialized views and design how queries will be rewritten. Materialized view logs must exist with rowids for all the base tables in the FROM list of the query. Note that the techniques shown in this chapter illustrate how to use materialized views in data warehouses. For example, you cannot insert into the master by selecting from a remote table. Performing CPU-intensive calculations 4. They are used in long-running decision support queries to aggregate the data returned from the query into appropriate levels of the dimension hierarchy. If this capability is possible, fast refresh is possible at least under certain circumstances. Oracle strongly recommends that you ensure that this assumption is true in order to ensure that the materialized view correctly reflects the data in its master tables. If this capability is possible, at least full text match query rewrite is possible. You can specify how you want your materialized views to be refreshed from the detail tables by selecting one of four options: COMPLETE, FAST, FORCE, and NEVER. The EXPLAIN_MVIEW procedure has the following parameters: An optional parameter. 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. Oracle Database Administrator’s Guide for restrictions on fast refresh in replication environments, Oracle Database Data Warehousing Guide for restrictions on fast refresh in data warehousing environments, The EXPLAIN_MVIEW procedure of the DBMS_MVIEW package for help diagnosing problems with fast refresh and the TUNE_MVIEW procedure of the DBMS_MVIEW package for correction of query rewrite problems, "Creating a Fast Refreshable Materialized View: Example". Use the table_compression clause to instruct the database whether to compress data segments to reduce disk and memory use. This process is called a complete refresh. This is particularly a problem for queries from Discoverer, which makes extensive use of column aliases. Materialized views cannot contain columns of data type LONG or LONG RAW. Dimension tables usually change slowly over time and are not modified on a periodic schedule. This clause lets you specify that the materialized view is not eligible for query rewrite in an edition and its descendants. The table and the materialized view must have the same name, but the table retains its identity as a table and can contain columns that are not referenced in the defining query of the materialized view. Oracle Database Tips by Donald BurlesonNovember 5, 2015. When refreshing materialized views, you must ensure that all materialized views in a tree are refreshed. The FROM clause of the query can name tables, views, and other materialized views. It cannot contain a HAVING clause with a subquery. As a general rule, though, you should create materialized view logs if you want to use fast refresh. 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. If you specify EDITION edition, then the materialized view is not eligible for query rewrite in the specified edition and its descendants. If you are using the synchronous refresh method, then you must specify ON DEMAND and USING TRUSTED CONSTRAINTS. Oracle Database Administrator’s Guide for information on the types of materialized views used to support replication. This is the default and should be used in all cases except those described for WITH ROWID. A computed measure is an expression involving measures of one table, for example, fact.revenues - fact.expenses. Oracle Database PL/SQL Packages and Types Reference for information on specifying the local materialized view rollback segment using the DBMS_REFRESH package. Materialized views can perform a number of roles, such as improving query performance or providing replicated data. "Creating Materialized Aggregate Views: Example" and "Creating Rowid Materialized Views: Example". Some basic data warehousing terms are defined as follows: Dimension tables describe the business entities of an enterprise, represented as hierarchical, categorical information such as time, departments, locations, and products. You can specify ON STATEMENT only when creating a materialized view. Oracle Database uses these objects to maintain the materialized view data. You can also specify if it is acceptable to use trusted constraints and QUERY_REWRITE_INTEGRITY = TRUSTED during refresh. It is not uncommon in a data warehouse to have already created summary or aggregation tables, and you might not wish to repeat this work by building a new materialized view. This is because the refresh operation is performed as part of the commit process. Basic Query Rewrite for Materialized Views for details about integrity levels. Specify CURRENT EDITION to search the edition in which this DDL statement is executed. Currently, when a materialized view is created, if its defining query contains same-name columns in the SELECT list, the name conflicts need to be resolved by specifying unique aliases for those columns. However, it is recommended that you try to avoid writing SQL statements that directly reference the materialized view, because then it is difficult to change them without affecting the application. Oracle Database Administrator’s Guide for detailed information about primary key materialized views and "Creating Primary Key Materialized Views: Example". The cursors of queries that directly access real-time materialized views are not shared. If VARIANCE(expr) or STDDEV(expr) is specified, COUNT(expr) and SUM(expr) must be specified. 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. Until then, the materialized view has a staleness value of UNUSABLE, so it cannot be used for query rewrite. Example 5-3 Creating a Materialized View (Aggregates on a Single Table). Example 5-4 Materialized View Containing Only Joins. Specify a datetime expression for calculating the interval between automatic refreshes. Also, unique constraints must exist on the join columns of the inner join table. Materialized views in these environments are often referred to as summaries, because they store summarized data. Specify ENABLE to enable the materialized view for query rewrite. This is reflected in the column STALENESS in the view USER_MVIEWS. You must also have access to any master tables of the materialized view that you do not own, either through a READ or SELECT object privilege on each of the tables or through the READ ANY TABLE or SELECT ANY TABLE system privilege. A popular and efficient way to load data into a data warehouse or data mart is to use SQL*Loader with the DIRECT or PARALLEL option, Data Pump, or to use another loader tool that uses the Oracle direct-path API. 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. This allows use of only validated, enforced constraints and rewrite against materialized views in FRESH state during refresh. Purpose. The column_properties clause lets you specify the storage characteristics of a LOB, nested table, varray, or XMLType column. The FROM clause of the query can name tables, views, and other materialized views. This is the default. In the case of normalized or partially normalized dimension tables (a dimension that is stored in multiple tables), identify how these tables are joined. If the materialized view is dropped, then the preexisting table reverts to its identity as a table. Data Type PARTY_TRIAL_ID 1 N NUMBER (11) PARTY_ID N NUMBER (11) PARTY_ROLE_TYPE_ID N NUMBER (11) CLINICAL_TRIAL_ID N NUMBER (11) START_DT Y DATE END_DT Y DATE CORE_CREATE_DT N DATE CORE_LAST_UPDT Y DATE EDC_FLAG Y VARCHAR2 (1 … The table_partitioning_clauses let you specify that the materialized view is partitioned on specified ranges of values or on a hash function. The following error message will be thrown if it is issued. To create the materialized view with query rewrite enabled, in addition to the preceding privileges: If the schema owner does not own the master tables, then the schema owner must have the GLOBAL QUERY REWRITE privilege or the QUERY REWRITE object privilege on each table outside the schema. This clause has the same semantics in CREATE MATERIALIZED VIEW and CREATE TABLE. The ORDER BY clause is not considered part of the materialized view definition. If you think the materialized view did not refresh, check the alert log or trace file. The constraints can be in RELY mode. A materialized view can be partitioned, and you can define a materialized view on a partitioned table. If there are no outer joins, you may have arbitrary selections and joins in the WHERE clause. For example, nonvalidated foreign key/primary key relationships, functional dependencies defined in dimensions or a materialized view in the UNKNOWN state. Another alternative, subject to some restrictions, is to use outer joins in the materialized view definition (that is, in the CREATE MATERIALIZED VIEW statement). Two build methods are available for creating the materialized view, as shown in Table 5-2. Key Differences Between View and Materialized View. Defining queries for materialized views with joins only and no aggregates have the following restrictions on fast refresh: All restrictions from "General Restrictions on Fast Refresh". You can enable query rewrite only if expressions in the statement are repeatable. Query rewrite is disabled by default, so you must specify this clause to make materialized views eligible for query rewrite. I just want to introduce a really cool new feature introduced in Oracle Database 12c Release 2 called Real-Time Materialized Views. Rowids of all the tables in the FROM list must appear in the SELECT list of the query. In distributed environments, you can use materialized views to replicate data at distributed sites and to synchronize updates done at those sites with conflict resolution methods. This restriction does not apply to expressions that involve only numeric data, for example, a+b where a and b are numeric fields. Materialized Views in Oracle. Such a query can be rewritten as long as an eligible materialized view referencing the same tables is available locally. For ON DEMAND, the mixed DML statements occur between refreshes. It contains the following topics: Overview of Data Warehousing with Materialized Views. You can create the following types of local materialized views (including both ON COMMIT and ON DEMAND) on master tables with commit SCN-based materialized view logs: Materialized aggregate views, including materialized aggregate views on a single table, Primary-key-based and rowid-based single table materialized views, UNION ALL materialized views, where each UNION ALL branch is one of the above materialized view types. In general, rewriting queries to use materialized views rather than detail tables improves response time. This feature is similar to the CREATE TABLE ... ORDER BY capability. Create the materialized view and then populate it with data. If this capability is possible, at least full and partial text match query rewrite are possible. The SELECT list must contain all GROUP BY columns. Creating a Fast Refreshable Materialized View: Example. Table 5-1 Requirements for Materialized Views with Aggregates. You must not create dimensions in any schema that does not satisfy these relationships. This is also the case for indexes created on the materialized view. The SELECT list of each query must include a UNION ALL marker, and the UNION ALL column must have a distinct constant numeric or string value in each UNION ALL branch. If your schema design does not follow guidelines 1 and 2, it does not then matter whether it follows guideline 3. Using nested materialized views, you can create multiple single-table materialized views based on a joins-only materialized view and the join is performed just once. Verifying that the incremental changes are correct, consistent, and complete. Note, however, that materialized views used in replication, which do not contain joins or aggregates, can be fast refreshed when UNION ALL or remote tables are used. If this capability is not possible, full text match query rewrite is not possible. In most cases, these foreign keys are non-null, form a unique compound key of the fact table, and each foreign key joins with exactly one row of a dimension table. The data is normally processed in a staging file before being added to the data warehouse. Some features such as outer joins, insert-only aggregate materialized view queries and remote tables are not supported for materialized views with UNION ALL. Materialized aggregate views with outer joins are fast refreshable after conventional DML and direct loads, provided only the outer table has been modified. Even though query rewrite is enabled by default, you also must specify the ENABLE QUERY REWRITE clause if the materialized view is to be considered available for rewriting queries. In addition to the EXPLAIN_MVIEW procedure, which is discussed throughout this chapter, you can use the DBMS_ADVISOR.TUNE_MVIEW procedure to optimize a CREATE MATERIALIZED VIEW statement to achieve REFRESH FAST and ENABLE QUERY REWRITE goals. Oracle stores some statistical information on them by default but they have no indexes and when you tried to drop an MV oracle try to delete those tables for that MV but because lack of indexes (probably) it gets an internal timeout. You cannot alter the definition of an existing materialized view that uses the ON STATEMENT refresh mode. Whether the fast refresh option is available depends upon the type of materialized view. DEFAULT specifies that Oracle Database will choose automatically which rollback segment to use. Also, in this case, the view should aggregate over the time column in the new materialized view. The vast majority of data in a data warehouse is stored in a few very large fact tables that are updated periodically with data from one or more operational OLTP databases. This scenario does not apply when QUERY_REWRITE_INTEGRITY is either ENFORCED or TRUSTED because the system ensures in these modes that only materialized views with updated data participate in a query rewrite. This allows you to directly query up-to-date data from the materialized view by specifying the FRESH_MV hint in the SELECT statement. FAST refresh is subject to the following restrictions: When you specify FAST refresh at create time, Oracle Database verifies that the materialized view you are creating is eligible for fast refresh. If you specify DEFAULT, then you cannot specify rollback_segment. Incrementally maintaining these distinct materialized aggregate views can take a long time, because the underlying join has to be performed many times. A possible workaround to this is to use a nested materialized view. Measuring the space being used by materialized views. The WITH REDUCED PRECISION clause allows for differences between the precision of the materialized view columns and the precision of the values returned by the subquery. The index_org_table_clause of CREATE TABLE, Restrictions on Index-Organized Materialized Views. The materialized view log resides in … This can be specified as long as the materialized view is fast refreshable (in other words, not complex). Specify WITH PRIMARY KEY to create a primary key materialized view. Oracle Database Development Guide for more information on specifying the evaluation edition for a materialized view, { ENABLE | DISABLE } ON QUERY COMPUTATION. Scripting on this page enhances content navigation, but does not change the content in any way. That is, all the parent materialized views in the dependency hierarchy starting from the specified table are refreshed in order. An alternative approach is to use the BUILD DEFERRED clause, which creates the materialized view without data, thereby enabling it to be populated at a later date using the DBMS_MVIEW.REFRESH package. The TUNE_MVIEW package, which shows you how to make your materialized view fast refreshable and use general query rewrite. Otherwise, you may be limited to fast refresh after inserts only. In this section, you learn about the following uses of these views, as they are applicable to the topic of large databases. This is the default. Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production desc Par_trials; Column Name Pk Null? For rollback_segment, specify the remote rollback segment to be used during materialized view refresh. The following is an example of the problem. They are not created on the materialized view unless there is another materialized view on top of that materialized view, which is the case with nested materialized views. The constraints are enabled, usually with the NOVALIDATE option. Refresh can use validated constraints and QUERY_REWRITE_INTEGRITY = ENFORCED during refresh. For example, a monthly materialized view might be updated only at the end of each month, and the materialized view values always refer to complete time periods. Range/composite partition your tables by a monotonically increasing time column if possible (preferably of type DATE). CONSTRAINTS clause lets Oracle Database choose more rewrite options during the refresh operation, resulting in more efficient refresh execution. The following statement creates the complex materialized view all_customers that queries the employee tables on the remote and local databases: Oracle Database automatically refreshes this materialized view tomorrow at 11:00 a.m. and subsequently every Monday at 3:00 p.m. The second STORAGE clause, appearing with the USING INDEX clause, establishes the sizes of the first and second extents of the index as 25 kilobytes each. The ON PREBUILT TABLE clause could be useful in the following scenarios: You have a table representing the result of a query. If you specify any column alias in this clause, then you must specify an alias for each data source referenced in the SELECT clause. Oracle Database will ignore any REFRESH statement on the materialized view issued from such a procedure. "Requirements for Using Materialized Views with Aggregates". The defining query of a materialized view cannot select from a V$ view or a GV$ view. Some materialized views contain only joins and no aggregates, such as in Example 5-4, where a materialized view is created that joins the sales table to the times and customers tables. Further, ON COMMIT refresh is not supported for materialized view with remote tables. Queries go directly to the materialized view and not to the underlying detail tables. However, if the prebuilt table, sales_summary, does not have any comment, the following comment is added: 'Sales summary data'. Use this clause to specify the edition that is searched during name resolution of the editioned object—the evaluation edition. Use this clause to determine when the segment for this materialized view should be created. The use of summary management features imposes no schema restrictions, and can enable some existing DSS database applications to improve performance without the need to redesign the database or the application. To specify an edition in the evaluation_edition_clause or the unusable_editions_clause, you must have the USE privilege on the edition. Oracle Database SQL Language Reference for syntax of the CREATE MATERIALIZED VIEW and CREATE MATERIALIZED VIEW LOG statements. This clause lets you create a real-time materialized view or a regular view. MASTER specifies the remote rollback segment to be used at the remote master site for the individual materialized view. The reports should be modified to refer to the view instead of referring directly to the user-defined materialized view. That is, aggregates such as AVG(AVG(x)) or AVG(x)+ AVG(x) are not allowed. Specify ENABLE ON QUERY COMPUTATION to create a real-time materialized view by enabling on-query computation. The default collation is used as the derived collation for all the character literals included in the defining query of the materialized view. NOCACHE has no effect on materialized views for which you specify KEEP in the storage_clause. Equijoins where one side of the join is character data. If the materialized view has one of the following, then fast refresh is supported only on conventional DML inserts and direct loads. You cannot, however, define a materialized view with a subquery in the SELECT list of the defining query. Specify DEFERRED to indicate that the materialized view is to be populated by the next REFRESH operation. It can also include any number of joins. Refresh performance benefits from partitioning, because it can take advantage of parallel DML capabilities and possible PCT-based refresh. This includes INSERT ... AS SELECT with the APPEND or PARALLEL hints, where the hints cause the direct loader log to be used during the insert. Explain an existing or potential materialized view with output to MV_CAPABILITIES_TABLE: Explain an existing or potential materialized view with output to a VARRAY: One of the simplest ways to use DBMS_MVIEW.EXPLAIN_MVIEW is with the MV_CAPABILITIES_TABLE, which has the following structure: You can use the utlxmv.sql script found in the admin directory to create MV_CAPABILITIES_TABLE. Notes on the Defining Query of a Materialized View. ]name of relation for which PCT is not enabled, Expression to enable PCT cannot be rolled up, No partition key or PMARKER in the SELECT list. Drop Materialized View Hangs with 'Enq: JI - Contention' (Doc ID 1126123.1) Last updated on NOVEMBER 07, 2019. You cannot specify the mapping_table_clause. 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 ORA-32414. "Restrictions on Fast Refresh on Materialized Views with Joins Only" for more information regarding the conditions that cause refresh performance to degrade. You cannot create a materialized view log on a temporary table. The materialized view will be populated with data as soon as the statement executes successfully. Use the segment_attributes_clause to establish values for the PCTFREE, PCTUSED, and INITRANS parameters, the storage characteristics for the materialized view, to assign a tablespace, and to specify whether logging is to occur. For complete information on this clause, refer to parallel_clause in the documentation on CREATE TABLE. A materialized view eliminates the overhead associated with expensive joins and aggregations for a large or important class of queries. The materialized view or potential materialized view is then analyzed and the results are written into either a table called MV_CAPABILITIES_TABLE, which is the default, or to an array called MSG_ARRAY. Use this clause to specify the default collation for the materialized view. A materialized view of the same name is created to use the prebuilt table as its container table. The following restrictions apply to the ON STATEMENT clause: This clause can be used only with materialized views that are fast refreshable. The following examples require the materialized logs that are created in the "Examples" section of CREATE MATERIALIZED VIEW LOG. Materialized join views and materialized aggregate views with a GROUP BY clause cannot select from an index-organized table. Design the materialized view. Immediately after loading the detail data and updating the indexes on the detail data, the database can be opened for operation, if desired. Materialized views are also useful in remote data marts. The risk with this approach is that incorrect query results could occur if any constraints are broken. 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. The first (deferred) refresh must always be a complete refresh. 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. Because of these limitations, and because existing materialized views can be extremely large and expensive to rebuild, you should register your existing materialized view tables whenever possible. See Requirements for Using Materialized Views with Aggregates for further details. Here are some examples of materialized views with aggregates. Instead of modifying the materialized view, the optimizer writes a query that combines the existing rows in the materialized view with changes recorded in log files (either materialized view logs or the direct loader logs). All the underlying objects (materialized views or tables) on which the materialized view is defined must have a materialized view log. This example creates a materialized view product_sales_mv that computes the sum of sales by prod_name. You should keep the following in mind when deciding whether to use nested materialized views: If you want to use fast refresh, you should fast refresh all the materialized views along any chain. A materialized view is a database object that contains the results of a query. If a column or expression is present in the GROUP BY clause of the materialized view, it must also be present in the SELECT list. If complete refresh is the only refresh option supported for a particular nested materialized view, then a complete refresh is performed even when a fast refresh is specified. Some data warehouses have implemented materialized views in ordinary user tables. This process is called a complete refresh. In order to create a materialized view whose defining query selects from a master table that has a Virtual Private Database (VPD) policy, you must specify the REFRESH USING TRUSTED CONSTRAINTS clause. Restrictions on the Default Collation for Materialized Views. If you want the benefits of this index, then you can create it manually. Oracle recommends that you qualify each table and view in the FROM clause of the defining query of the materialized view with the schema containing it. If this capability is not possible, fast refresh from a materialized view log may not be possible when the update operations (other than INSERT) affect multiple tables. The following statement creates a fast-refreshable materialized view that selects columns from the order_items table in the sample oe schema, using the UNION set operator to restrict the rows returned from the product_information and inventories tables using WHERE conditions. A materialized viewis a database object that contains the results of a query. Freshness of a materialized view is calculated relative to the objects directly referenced by the materialized view. PCT is also needed to support query rewrite in the presence of partial staleness of the materialized view with regard to the table named in the RELATED_TEXT column.