types of materialized view in oracle

Consequently, query execution usually performs best if a single-column bitmap index is defined on each materialized view key column. 81 1 1 … This is particularly useful for materialized views created with the ROLLUP clause. This example creates a materialized view that contains aggregates on a single table. If you only refresh the highest-level materialized view, the materialized views under it will be stale and you must explicitly refresh them. This information can then assist the design team in determining the tablespace in which the materialized view should reside. These extra columns are known as unmanaged columns. The PL/SQL VARRAY that receives the output. 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. 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. 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). 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. Therefore, the storage needs for the materialized view should be specified in terms of the tablespace where it is to reside and the size of the extents. Before creating a materialized view, you can verify what types of query rewrite are possible by calling the procedure DBMS_MVIEW.EXPLAIN_MVIEW, or use DBMS_ADVISOR.TUNE_MVIEW to optimize the materialized view so that many types of query rewrite are possible. Types of Materialized Views. If this capability is possible, all query rewrite capabilities are possible, including general query rewrite and full and partial text match query rewrite. Materialized views can be used to replicate all or part of a single table or to replicate the result of a query against multiple tables; the database can automatically refresh the replicated data at time intervals that you specify. 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. If VARIANCE(expr) or STDDEV(expr) is specified, COUNT(expr) and SUM(expr) must be specified. For example, if you used a filter column such as cust_id and this column already existed, Oracle Database ignores the redundancy and does not return an error. In the case of denormalized dimensions, determine whether the child-side columns uniquely determine the parent-side (or attribute) columns. Materialized views in SQL Syntax and Examples: Misc | Connect to the materialized view owner and create the database link and the materialized view itself. Note that COUNT(*) must always be present to guarantee all types of fast refresh. In this case, the table that already exists in the database can be registered as a prebuilt materialized view. Note that the techniques shown in this chapter illustrate how to use materialized views in data warehouses. These replica materialized views provide local access to data that otherwise would have to be accessed from remote sites. A materialized view with MAX or MIN is fast refreshable after delete or mixed DML statements if it does not have a WHERE clause. First, use the SQL Access Advisor to plan how you will use materialized views. The reason is that, because of the primary key constraint on the join column(s) of the dimension table and foreign key constraint on the join column(s) of the fact table, the new rows inserted into the dimension table will not join with any fact table rows, thus there is nothing to refresh. The view should aggregate over the time column in the new materialized view. If this capability is not possible, no form of query rewrite is possible. Misc | The materialized base view is truncated. Note that when aliases are specified in both the SELECT clause and the new alias list clause, the alias list clause supersedes the ones in the SELECT clause. You must have the privileges necessary to create these objects. 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. Unless the materialized view is based on a user-defined prebuilt table, it requires and occupies storage space inside the database. Example 9-1 Example 1: Creating a Materialized View. When refreshing materialized views, you must ensure that all materialized views in a tree are refreshed. ]name of relation for which PCT_TABLE_REWRITE is not enabled, Expression to enable PCT not in PARTITION BY of analytic function or model, [owner. ; View can be defined as a virtual table created as a result of the query expression. The constraints are enabled, usually with the NOVALIDATE option. You can enforce this condition by adding FOREIGN KEY and NOT NULL constraints on the child-side join keys and PRIMARY KEY constraints on the parent-side join keys. For example, the following statement adds a comment to data dictionary views for the existing materialized view: To view the comment after the preceding statement execution, the user can query the catalog views, {USER, DBA} ALL_MVIEW_COMMENTS. ON COMMIT materialized views cannot have remote detail tables. When possible, PCT applies to the partitioned table named in the RELATED_TEXT column. In addition, PCT-based refresh of updates to the table named in the RELATED_TEXT column is not possible. Purging materialized view logs can be done during the materialized view refresh process or deferred until later, thus improving refresh performance time. Once the materialized view has been created, you can use DBMS_MVIEW.EXPLAIN_REWRITE to find out if (or why not) it will rewrite a specific query. If this clause is omitted or specified as DISABLE QUERY REWRITE when the materialized view is created, the materialized view can subsequently be enabled for query rewrite with the ALTER MATERIALIZED VIEW statement. When using certain materialized views, you must ensure that your NLS parameters are the same as when you created the materialized view. The result of this equijoin depends on collation and this can change on a session basis, giving an incorrect result in the case of query rewrite or an inconsistent materialized view after a refresh operation. Example 9-3 Example 3: Creating a Materialized View. A materialized view definition can include any number of aggregations (SUM, COUNT(x), COUNT(*), COUNT(DISTINCT x), AVG, VARIANCE, STDDEV, MIN, and MAX). Scripts | The boundary of a mixed DML operation is determined by whether the materialized view is ON COMMIT or ON DEMAND. Also, materialized view logs must be present on all tables referenced in the query that defines the materialized view. Materialized views in these environments are often referred to as summaries, because they store summarized data. Also, check to see if your materialized view satisfies all of the following conditions. Note that the Oracle Database also allows the special case of a single table materialized view with joins only provided the ROWID column has been included in the SELECT list and in the materialized view log. It stores data physically and get updated periodically. Materialized views are also useful in remote data marts. Fact tables include facts (also called measures) such as sales, units, and inventory. If you use a CHAR data type in the filter columns of a materialized view log, the character sets of the master site and the materialized view must be the same. If this capability is not possible, fast refresh from a materialized view log may not be possible when the update operations are performed on multiple tables. For a daily update frequency, an update window of two to six hours might be typical. Further, the marker column must appear in the same ordinal position in the SELECT list of each query block. Even though a materialized view is defined, it will not automatically be used by the query rewrite facility. Check the additional work does not adversely affect performance on the primary system. Applies FAST refresh if possible; otherwise, it applies COMPLETE refresh. First, create the materialized view. Copyright & Disclaimer, Real-Time Materialized Views in Oracle Database 12c Release 2 (12.2). This process is called a complete refresh. That is, all the parent materialized views in the dependency hierarchy starting from the specified table are refreshed in order. The query optimizer automatically recognizes when an existing materialized view can and should be used to satisfy a request. You can call the procedure DBMS_MVIEW.EXPLAIN_MVIEW to determine whether fast refresh is possible. The FROM clause of the query can name tables, views, and other materialized views. Since a complete refresh involves truncating the materialized view segment and re-populating it using the related query, it can be quite time consuming and involve a considerable amount of network traffic when performed against a remote table. I will not show you the materialized view concepts, the Oracle Datawarehouse Guide is perfect for that. If the defining query contains such a join, consider rewriting the defining query to contain an inner join. If this capability is not possible, full text match query rewrite is not possible. Materialized views within the data warehouse are transparent to the end user or to the database application. Be sure the additional load does not adversely affect your primary system. The motivation for using materialized views is to improve performance, but the overhead associated with materialized view management can become a significant system management problem. To reduce the replication costs, materialized view logs can be created to capture all changes to the base table since the last refresh. Therefore, as the designer, you must determine how clean the data is and whether the risk of incorrect results is too great. Materialized Views Containing Only Joins Some materialized views contain only joins and no aggregates, such as in the following Example, where a materialized view is created that joins the sales table to the times and customers tables. Although that may seem to be a simple distinction, the underlying differences between these two types of materialized views are not simple. Serban Tanasa. 12c | There is no difference in the information required if this approach is used. The benefits of maintaining this condition are described in "Creating Dimensions". Functionality of Materialized Views. Remember, refreshing on commit is a very intensive operation for volatile base tables. However, if the materialized view is based on a user-defined prebuilt table, then the name of the materialized view must exactly match that table name. For example, If you have a table costs with a materialized view cost_mv based on it, you cannot then create a prebuilt materialized view on table costs. Fast refresh for a materialized view containing only joins is possible after any type of DML to the base tables (direct-path or conventional INSERT, UPDATE, or DELETE). Incremental loads of your detail data should be done using the SQL*Loader direct-path option, or any bulk loader utility that uses Oracle's direct-path interface. Without a materialized views log, Oracle Database must re-execute the materialized view query to refresh the materialized views. Check the user who will own the materialized views has the correct privileges. In contrary of views, materialized views avoid executing the SQL query for every access by storing the result set of the query. Materialized views with this restriction are as follows: Expressions that may return different values, depending on NLS parameter settings. You can create a view that selects the complete time period of interest. Also, when a materialized view log is required, the ROWID column must be present in each materialized view log. To achieve an optimally efficient refresh, you should ensure that the defining query does not use an outer join that behaves like an inner join. In addition, you can use all the existing options for materialized views. Oracle provides materialized views to store copies of data or aggregations. Articles Related Query Rewrite The end user queries the tables and views in the database. When reviewing or evaluating some of the necessary materialized view management activities, consider some of the following: Identifying what materialized views to create initially. to construct a data warehouse. A materialized view can be created with the CREATE MATERIALIZED VIEW statement or using Enterprise Manager. Dimension tables usually change slowly over time and are not modified on a periodic schedule. This type of materialized view can also be fast refreshed if DML is performed on the detail table. If one is not possible a complete refresh is performed. If this capability is possible, fast refresh from a materialized view log is possible regardless of the type of update operation or the number of tables updated. The following statement shows an example of refreshing a nested materialized view: This statement will first refresh all child materialized views of sales_mv and cost_mv based on the dependency analysis and then refresh the two specified materialized views. The fresh rows in the materialized view are identified by adding selection predicates to the materialized view's WHERE clause. The materialized view must not contain references to RAW or LONG RAW data types. It is to create the MV_CAPABILITIES_TABLE in the current schema. They can also be used to precompute joins with or without aggregations. 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. 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. For example, if you identify 20 queries that are commonly applied to the detail or fact tables, then you might be able to satisfy them with five or six well-written materialized views. Specify with ROWID and INCLUDING NEW VALUES. This allows use of only validated, enforced constraints and rewrite against materialized views in FRESH state during refresh. Using this procedure is straightforward. 3,382 1 1 gold badge 18 18 silver badges 41 41 bronze badges. The ON COMMIT privilege is necessary to use this mode. Finally, query rewrite cannot be supported in the presence of partial staleness of the materialized view with regard to the table named in the RELATED_TEXT column. Dimensions should either be denormalized (each dimension contained in one table) or the joins between tables in a normalized or partially normalized dimension should guarantee that each child-side row joins with exactly one parent-side row. 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. You can, however, include subqueries elsewhere in the defining query, such as in the WHERE clause. When you define a materialized view, you can specify three refresh options: how to refresh, what type of refresh, and can trusted constraints be used. Yasir Meraj Yasir Meraj. GROUP BY should not result in any duplicate groupings. Refreshes by recalculating the materialized view's defining query. A simple measure is a numeric or character column of one table such as fact.sales. Information about refresh groups and the materialize views in a refresh group can be queried from the DBA_RGROUP and DBA_RCHILD views respectively. It is recommended that the same procedure be applied to this type of materialized view as for a single table aggregate. Therefore, the unmanaged columns cannot have NOT NULL constraints unless they also have default values. Note that you must run the utlxmv.sql script prior to calling EXPLAIN_MVIEW except when you are placing the results in MSG_ARRAY. A complete refresh is required for the first refresh of a build deferred materialized view. The column UNKNOWN_TRUSTED_FD in the same view is also set to Y, which means yes. Generally, this means that refresh is possible after partition maintenance operations on those detail tables where PCT is indicated as possible. If you are concerned with the time required to enable constraints and whether any constraints might be violated, then use the ENABLE NOVALIDATE with the RELY clause to turn on constraint checking without validating any of the existing constraints. In some cases, user-defined materialized views are refreshed on a schedule that is longer than the update cycle. Further, ON COMMIT refresh is not supported for materialized view with remote tables. In particular, this procedure enables you to determine: If a materialized view is fast refreshable, What types of query rewrite you can perform with this materialized view. It cannot contain analytic functions (for example, RANK) in the SELECT clause. For example, table sales_summary was created to contain sales summary information. EXECUTE DBMS_SNAPSHOT.REFRESH('Materialized_VIEW_OWNER_NAME.Materialized_VIEW_NAME','COMPLETE'); share | improve this answer | follow | edited Feb 24 '16 at 15:12. SQL | The script is found in the admin directory. 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. The existing comment will be prefixed with '(from table)'. These additional checks are: A materialized view log must be present for each detail table unless the table supports PCT. If a capability is not possible for multiple reasons, a row is displayed for each reason. share | improve this answer | follow | answered Jul 7 '15 at 5:08. user2221088 user2221088. This is shown in the defining query of the view view_with_unionall. This capability is needed to support query rewrite against this materialized view in partial stale state with regard to the table named in the RELATED_TEXT column. ]table_name of table upon which the materialized view log is needed, Materialized view log must have new values, Materialized view log must have primary key, Materialized view log does not have all necessary columns, Materialized view references a remote table or view in the FROM list, Offset from the SELECT keyword to the table or view in question, [owner. Unlike indexes, materialized views can be accessed directly using a SELECT statement. It can also include any number of joins. It is not used during a full refresh or a fast refresh. You should use table compression with highly redundant data, such as tables with many foreign keys. The valid aggregate functions are: SUM, COUNT(x), COUNT(*), AVG, VARIANCE, STDDEV, MIN, and MAX, and the expression to be aggregated can be any SQL value expression. 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From remote sites Utilities for the create table as SELECT command as the data, for PCT capability types of materialized view in oracle. Can choose to manually refresh the highest-level materialized view and not to the base.. Following: example 9-2 example 2: creating a materialized view refresh performance specific invocations of EXPLAIN_MVIEW exist the...