It cannot contain analytic functions (for example, RANK) in the SELECT clause. A query can reference both local and remote tables. A materialized view provides indirect access to table data by storing the results of a query in a separate schema object. Primary key materialized views are the default type of materialized views in Oracle. Chapter 16, "Maintaining the Data Warehouse" for further details about PCT, Chapter 19, "Advanced Query Rewrite" for further details about PCT. It makes sense to use fast refreshes where possible. RAC | Further, the marker column must appear in the same ordinal position in the SELECT list of each query block. See "UNION ALL Marker" for more information regarding UNION ALL markers. Otherwise, use the BUILD IMMEDIATE clause to populate the materialized view immediately, or the BUILD DEFERRED clause to populate the materialized view later. ]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. The materialized base view is truncated. The materialized view must not contain references to RAW or LONG RAW data types. Unless the materialized view is based on a user-defined prebuilt table, it requires and occupies storage space inside the database. Oracle performs the following operations when refreshing a materialized view. The appropriate materialized view logs must be created on the tables as required for the corresponding type of fast refreshable materialized view. It then transparently rewrites the request to use the materialized view. I am able to get a list of columns for a materialized view by running: Another alternative, subject to some restrictions, is to use outer joins in the materialized view definition (that is, in the CREATE MATERIALIZED VIEW statement). 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 described in Chapter 16, "Maintaining the Data Warehouse". If you only refresh the highest-level materialized view, the materialized views under it will be stale and you must explicitly refresh them. 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 the USING TRUSTED CONSTRAINTS clause, any VPD policy on a base table will prevent a materialized view from being created. Query execution might need to access any subset of the materialized view key columns, and might need to join and aggregate over a subset of those columns. Usually, a fast refresh takes less time than a complete refresh.A materialized views log is located in the master database in the same schema as the master table. Once registered, the materialized view can be used for query rewrites or maintained by one of the refresh methods, or both. Scripting on this page enhances content navigation, but does not change the content in any way. 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. Linux. A materialized view can be created with the CREATE MATERIALIZED VIEW statement or using Enterprise Manager. 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. select owner as schema_name, mview_name, container_name, query as definition, refresh_mode, refresh_method, build_mode, last_refresh_date, compile_state from sys.all_mviews order by owner, mview_name; B. If this capability is possible, all query rewrite capabilities are possible, including general query rewrite and full and partial text match query rewrite. The UNION ALL operator cannot be embedded inside a subquery, with one exception: The UNION ALL can be in a subquery in the FROM clause provided the defining query is of the form SELECT * FROM (view or subquery with UNION ALL) as in the following example: Note that the view view_with_unionall satisfies the requirements for fast refresh. Fact tables also contain one or more foreign keys that organize the business transactions by the relevant business entities such as time, product, and market. 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. A materialized view definition can include any number of aggregations (SUM, COUNT (x), COUNT (*), COUNT (DISTINCT x), AVG, VARIANCE, STDDEV, MIN, and MAX). The ON PREBUILT TABLE clause tells the database to use an existing table segment, which must have the same name as the materialized view and support the same column structure as the query. Misc | If the materialized view contains many rows, then, if appropriate, the materialized view should be partitioned (if possible) and should match the partitioning of the largest or most frequently updated detail or fact table (if possible). The complication comes from the lag between the last refresh of the materialized view and subsequent DML changes to the base tables. On the other hands, Materialized Views are stored on the disc. 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. Then, create materialized views and design how queries will be rewritten. A materialized view eliminates the overhead associated with expensive joins and aggregations for a large or important class of queries. If you anticipate performing insert, update or delete operations on tables referenced by a materialized view concurrently with the refresh of that materialized view, and that materialized view includes joins and aggregation, Oracle recommends you use ON COMMIT fast refresh rather than ON DEMAND fast refresh. These replica materialized views provide local access to data that otherwise would have to be accessed from remote sites. A materialized view is a database object that contains the results of a query. The ON COMMIT privilege is necessary to use this mode. You cannot, however, define a materialized view with a subquery in the SELECT list of the defining query. Without a materialized views log, Oracle Database must re-execute the materialized view query to refresh the materialized views. This can be specified as long as the materialized view is fast refreshable (in other words, not complex). The SEQUENCE column is required in the materialized view log to support fast refresh with a combination of INSERT, UPDATE, or DELETE statements on multiple tables. See "Storage And Table Compression" for details regarding table compression. A materialized view can also be created using Oracle Enterprise Manager by selecting the materialized view object type. If you already have a naming convention for tables and indexes, you might consider extending this naming scheme to the materialized views so that they are easily identifiable. ON COMMIT materialized views cannot have remote detail tables. In the second case, the application guarantees primary and foreign key relationships so the constraints are declared RELY NOVALIDATE and the materialized view is defined with the REFRESH FAST USING TRUSTED CONSTRAINTS option. In this article, we discuss a feature of the Oracle Database that you can include in your arsenal: materialized views. 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. For materialized views with CUBE, ROLLUP, grouping sets, or concatenation of them, the following restrictions apply: The SELECT list should contain grouping distinguisher that can either be a GROUPING_ID function on all GROUP BY expressions or GROUPING functions one for each GROUP BY expression. Such a query can be rewritten as long as an eligible materialized view referencing the same tables is available locally. You can add a comment to a materialized view. 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. See Chapter 11, "Dimensions" for more information. It is different from simple oracle view.These materialized view have data stored and when you query the materialized view,it returns data from the data stored. Dimension tables are sometimes called lookup or reference tables. Using materialized views against remote tables is the simplest way to achieve replication of data between sites. When a materialized view references another materialized view, the freshness of the topmost materialized view is calculated relative to changes in the materialized view it directly references, not relative to changes in the tables referenced by the materialized view it references. Check the additional work does not adversely affect performance on the primary system. Enable materialized views defined in one application to be transparently accessed in another application. Summary management can perform many useful functions, including query rewrite and materialized view refresh, even if your data warehouse design does not follow these guidelines. 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. 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. 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. Materialized view logs must exist with rowids for all the base tables in the FROM list of the query. 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. After the materialized view creation, the comment becomes '(from table) Sales summary data'. 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. some materialized views can be written to, which updates the source table (for instance joins with primary keys can be written to, on the opposite if the materialized view is the result of a group by it can't be written to) the DB server retains the query that created the data and can rerun it. 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. When it is refreshed and once populated, this materialized view can be used by query rewrite. Table 9-2 illustrates the aggregate requirements for materialized views. 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. If a time dimension appears in the materialized view as a time column, partition and index the materialized view in the same manner as you have the fact tables. 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. If this capability is not possible, full text match query rewrite is not possible. If you are having problems trying to get your materialized views to work then use TUNE_MVIEW to obtain an optimized materialized view. This, however, incurs the cost of constraint maintenance. A materialized view with MAX or MIN is fast refreshable after delete or mixed DML statements if it does not have a WHERE clause. For a daily update frequency, an update window of two to six hours might be typical. Without the foreign key/primary key optimization, two joins during the refresh would be required, a join of delta fact with the dimension table, plus a join of delta dimension with an image of the fact table from before the inserts. FAST : A fast refresh is attempted. When you drop a materialized view that was created on a prebuilt table, the table still exists—only the materialized view is dropped. This example creates a materialized view product_sales_mv that computes the sum of sales by prod_name. It can be defined to be refreshed ON COMMIT or ON DEMAND. See Table 9-2 for further details. See Oracle Database SQL Language Reference for more information. 8i | Only a few restrictions limit what can be specified. If regular refreshes are not performed, materialized view logs can grow very large, potentially reducing the performance of their maintenance and blowing tablespace limits. The following refresh types are available. The FROM clause of the query can name tables, views, and other materialized views. In the case of a prebuilt table, if it has an existing comment, the comment will be inherited by the materialized view after it has been created. The column alias list explicitly resolves any column name conflict without attaching aliases in the SELECT clause of the materialized view. A materialized view of the same name is created to use the prebuilt table as its container table. Determining which new materialized views should be created. A materialized view is a replica of a target master from a single point in time. Sql Access Advisor (a GUI tool for materialized view and index management) can recommend the creation of materialized views. Remember to gather stats after building the 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 allows use of only validated, enforced constraints and rewrite against materialized views in FRESH state during refresh. A materialized view in Oracle is a database object that contains the results of a query. Oracle recommends that SUM(expr) be specified. You can automatically refresh intermediate materialized views in a nested hierarchy using the nested = TRUE parameter, as described in "Nesting Materialized Views with Joins and Aggregates". In the case of a complete refresh (using dbms_mview.refresh) sys.snap$ and sys.mlog$ are updated to reflect the time of the refresh. The SQL Access Advisor, which recommends materialized views, partitions, and indexes to create. Only a few restrictions limit what can be specified. It is not used during a full refresh or a fast refresh. 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. Guidelines 1 and 2 are more important than guideline 3. If a materialized view is configured to refresh on commit, you should never need to manually refresh it, unless a rebuild is necessary. Populating a materialized view adds load to both servers involved. If this shows that not all types of query rewrite are possible, use the procedure DBMS_ADVISOR.TUNE_MVIEW to see if the materialized view can be defined differently so that query rewrite is possible. 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. You can, however, add the SEQUENCE number to the materialized view log after it has been created. You can query the STALE_SINCE column in the *_MVIEWS views to find out when a materialized view became stale. They are used in long-running decision support queries to aggregate the data returned from the query into appropriate levels of the dimension hierarchy. Table compression has been extended in this release with Hybrid Columnar Compression. The risk with this approach is that incorrect query results could occur if any constraints are broken. Such a materialized view is called an insert-only materialized view. Whether the fast refresh option is available depends upon the type of materialized view. The full syntax description for the CREATE MATERIALIZED VIEW command is available in the documentation. Freshness of a materialized view is calculated relative to the objects directly referenced by the materialized view. If a large number of deletions are possible, then storage utilization can be adversely affected, but temporary space requirements and load time are minimized. An updatable materialized view can send changes to its master table. This chapter describes the use of materialized views. Materialized views which store data based on remote tables were also known as snapshots, (deprecated Oracle terminology). If Oracle’s materialized views are created without the **REFRESH FAST ON COMMIT** option, then the materialized view is not modified when its base tables are. PCT may also support fast refresh with regard to updates to the table named in the RELATED_TEXT column when fast refresh from a materialized view log is not possible. 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". An updatable materialized view can send changes to its master table. If this capability is possible, Partition Change Tracking (PCT) is possible on at least one detail relation. You can also define one or more indexes on the materialized view. In general, rewriting queries to use materialized views rather than detail tables improves response time. 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. This chapter focuses on the use of materialized views in data warehouses. The materialized view does not initially contain any data, because the build method is DEFERRED. 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. That is, all the parent materialized views in the dependency hierarchy starting from the specified table are refreshed in order. 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. 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