Example 9-4 Materialized View Containing Only Joins. For example, table sales_summary was created to contain sales summary information. When a master table is modified, the related materialized view becomes stale and a refresh is necessary to have the materialized view up to date. Refresh can use validated constraints and QUERY_REWRITE_INTEGRITY = ENFORCED during refresh. A nested materialized view is a materialized view whose definition is based on another materialized view. However, if the prebuilt table, sales_summary, does not have any comment, the following comment is added: 'Sales summary data'. Expressions that generate internal conversion to character data in the SELECT list of a materialized view, or inside an aggregate of a materialized aggregate view. 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. Another example of this refresh optimization is when both the fact and dimension tables have inserts since the last refresh. 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. Using the tables and their columns from the sh sample schema, the following materialized views illustrate how nested materialized views can be created. With DBMS_MVIEW.REFRESH_DEPENDENT, the entire chain is refreshed from the bottom up. ]name of the table involved with the join or filter condition (or NULL when not available), Expression not supported for fast refresh, Offset from the SELECT keyword to the expression in question, The alias name in the SELECT list of the expression in question, SELECT lists must be identical across the UNION operator, Offset from the SELECT keyword to the first different select item in the SELECT list, The alias name of the first different select item in the SELECT list, [owner. Table 9-4 describes the refresh modes. To improve the performance of queries against large materialized views, store the rows in the materialized view in the order specified in the ORDER BY clause. Materialized View Concepts Oracle uses materialized views (also known as snapshots in prior releases) to replicate data to nonmaster sites in a replication environment and to cache expensive queries in a data warehouse environment. The compatibility initialization parameter must be set to 9.2.0 or higher to create a fast refreshable materialized view with UNION ALL. Alternatively, you can use EXPLAIN_MVIEW on a potential materialized view using its SELECT statement or the complete CREATE MATERIALIZED VIEW statement. If you do not know how much space the materialized view requires, then the DBMS_MVIEW.ESTIMATE_MVIEW_SIZE package can estimate the number of bytes required to store this uncompressed materialized view. You can create a nested materialized view on materialized views, but all parent and base materialized views must contain joins or aggregates. 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. This is because the refresh operation is performed as part of the commit process. 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. An analysis of the dimensions, combined with an understanding of the typical work load, can be used to create materialized views. If you define a materialized view as BUILD DEFERRED, it is not eligible for query rewrite until it is populated with data through a complete refresh. This example creates a materialized view product_sales_mv that computes total number and value of sales for a product. If you have privilege on dba_mviews. That is, all the parent materialized views in the dependency hierarchy starting from the specified table are refreshed in order. This is because the refresh operation is performed as part of the commit process. If the defining query contains such a join, consider rewriting the defining query to contain an inner join. 19c |
You simply call DBMS_MVIEW.EXPLAIN_MVIEW, passing in as a single parameter the schema and materialized view name for an existing materialized view. Oracle performs the following operations when refreshing a materialized view. This technique is described in "Registering Existing Materialized Views". Materialized Views in Oracle A materialized view, or snapshot as they were previously known, is a table segment whose contents are periodically refreshed based on a query, either against a local or remote table. To create a nested materialized view on the table join_sales_cust_time, you would have to create a materialized view log on the table. That is, perform one type of change (direct-path INSERT or DML) and then refresh the materialized view. If dimensions are denormalized or partially denormalized, hierarchical integrity must be maintained between the key columns of the dimension table. Materialized view logs are defined using a CREATE MATERIALIZED VIEW LOG statement on the base table that is to be changed. In addition, PCT-based refresh of updates to the table named in the RELATED_TEXT column is not possible. Nested materialized views must have a join or aggregate. As a general rule, though, you should create materialized view logs if you want to use fast refresh. The syntax of the materialized view column alias list is illustrated in the following example: In this example, the defining query of sales_mv now matches exactly with the user query Q1, so full text match rewrite takes place. Table 9-7 lists explanations for values in the CAPABILITY_NAME column. If a capability is not possible, N will appear in the P column and an explanation in the MSGTXT column. Once you have selected the materialized views you want to create, follow these steps for each materialized view. 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. If query rewrite is enabled during refresh, these can improve the performance of refresh by enabling more performant query rewrites. These operations are expensive in terms of time and processing power. 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. Hierarchical integrity in a denormalized dimension can be verified by calling the VALIDATE_DIMENSION procedure of the DBMS_DIMENSION package. See Oracle Database Advanced Replication for further information. 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. It makes sense to use fast refreshes where possible. You need to use the SEQ column in an ORDER BY clause so the rows will display in a logical order. This example creates a materialized view that contains aggregates on a single table. Materialized views within the data warehouse are transparent to the end user or to the database application. Otherwise, you may be limited to fast refresh after inserts only. This allows the work to be done once and used repeatedly by multiple sessions, reducing the total load on the server. If this capability is not possible, at least partial text match query rewrite and general query rewrite are not possible. The benefits of maintaining this condition are described in "Creating Dimensions". 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. FORCE : A fast refresh is attempted. 81 1 1 … Table compression has been extended in this release with Hybrid Columnar Compression. If the materialized view has remote tables in the FROM clause, all tables in the FROM clause must be located on that same site. At minimum they will require the CREATE MATERIALIZED VIEW privilege. Key Differences Between View and Materialized View. See Oracle Database Utilities for the restrictions and considerations when using SQL*Loader with the DIRECT or PARALLEL keywords. If you only refresh the highest-level materialized view, the materialized views under it will be stale and you must explicitly refresh them. For example, consider the following example: Note: If the compatibility is set to 10.0.1 or higher, COMMENT ON TABLE will not be allowed for the materialized view container table. The EXPLAIN_MVIEW procedure has the following parameters: An optional parameter. They are updatable if the materialized view was created as part of a materialized view group and FOR UPDATE was specified when defining the materialized view. Materialized views improve query performance by precalculating expensive join and aggregation operations on the database prior to execution and storing the results in the database. The introduction of summary management eased the workload of the database administrator and meant the user no longer needed to be aware of the summaries that had been defined. Query rewrite is not possible with all materialized views. A materialized view, or snapshot as they were previously known, is a table segment whose contents are periodically refreshed based on a query, either against a local or remote table. Bulk-loading the data into the warehouse. If you think the materialized view did not refresh, check the alert log or trace file. Refresh performance benefits from partitioning, because it can take advantage of parallel DML capabilities and possible PCT-based refresh. ; View can be defined as a virtual table created as a result of the query expression. Although that may seem to be a simple distinction, the underlying differences between these two types of materialized views are not simple. Like a regular view, the data in a materialized view … Copyright & Disclaimer, Real-Time Materialized Views in Oracle Database 12c Release 2 (12.2). These materialized view have data stored and when you query the materialized view,it returns data from the data stored. RAC |
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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… Materialized view logs must exist with rowids for all the base tables in the FROM list of the query. The name of an existing materialized view or the query definition or the entire CREATE MATERIALIZED VIEW statement of a potential materialized view you want to analyze. Each query block in the UNION ALL query must satisfy the requirements of a fast refreshable materialized view with aggregates or a fast refreshable materialized view with joins. ]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. 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. Oracle recommends that you include the optional aggregates in column Z in the materialized view in order to obtain the most efficient and accurate fast refresh of the aggregates. When using certain materialized views, you must ensure that your NLS parameters are the same as when you created the materialized view. They can also be used to precompute joins with or without aggregations. Also, materialized view logs must be present on all tables referenced in the query that defines the materialized view. If the defining queries for a materialized view do not contain joins or aggregates, it cannot be nested. 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. Whether the fast refresh option is available depends upon the type of materialized view. A BUILD DEFERRED materialized view is disabled for use by query rewrite until the first COMPLETE REFRESH, after which it is automatically enabled, provided the ENABLE QUERY REWRITE clause has been specified. 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. TUNE_MVIEW, which shows you how to make your materialized view fast refreshable and use general query rewrite. Equijoins where one side of the join is character data. If there are no outer joins, you may have arbitrary selections and joins in the WHERE clause. In contrary of views, materialized views avoid executing the SQL query for every access by storing the result set of the query. Checking which materialized views have been used. The temporary table is then dropped. It can also include any number of joins. Materialized views which store data based on remote tables were also known as snapshots, (deprecated Oracle terminology). 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). 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. It is to create the MV_CAPABILITIES_TABLE in the current schema. This is shown in the defining query of the view view_with_unionall. Types of Materialized Views. The FROM clause of the query can name tables, views, and other materialized views. 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. A materialized view is a precomputed table comprising aggregated and joined data from fact and possibly from dimension tables. This foreign key/primary key optimization can significantly improve refresh performance. select owner as view_schema, name as view_name, referenced_owner as referenced_schema, referenced_name as referenced_table from sys.all_dependencies where type = 'MATERIALIZED VIEW' and referenced_type = 'TABLE' order by view_schema, view_name; B. 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. Materialized views in these environments are often referred to as summaries, because they store summarized data. 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. The SELECT clause in the materialized view creation statement defines the data that the materialized view is to contain. When using materialized views to improve performance of transformations and aggregations, the. You can, however, add the SEQUENCE number to the materialized view log after it has been created. In addition, you can use all the existing options for materialized views. To reduce the replication costs, materialized view logs can be created to capture all changes to the base table since the last refresh. Therefore this method may not be suitable if many users are concurrently changing the tables upon which the materialized view is based. All the underlying objects are treated as if they were tables. In the past, organizations using summaries spent a significant amount of time and effort creating summaries manually, identifying which summaries to create, indexing the summaries, updating them, and advising their users on which ones to use. Before using materialized views and materialized view logs, consider the following: Home |
The SQL Access Advisor, which recommends materialized views, partitions, and indexes to create. A read-only materialized view is implemented as a create table as select command. In some cases, user-defined materialized views are refreshed on a schedule that is longer than the update cycle. You must have the privileges necessary to create these objects. share | improve this answer | follow | answered Jul 7 '15 at 5:08. user2221088 user2221088. Create a materialized view to perform the aggregation in advance, making sure you specify the ENABLE QUERY REWRITE clause. This is particularly a problem for queries from Discoverer, which makes extensive use of column aliases. A simple measure is a numeric or character column of one table such as fact.sales. Some data warehouses have implemented materialized views in ordinary user tables. 8i |
Alternatively, we could have used a prebuilt table, as shown below. Determining which existing materialized views should be dropped. Two build methods are available for creating the materialized view, as shown in Table 9-3. Note that support for materialized views with selection predicates is a prerequisite for this type of rewrite. Materialized views, which store data based on remote tables are also, know as snapshots. In a data warehouse, you typically create many aggregate views on a single join (for example, rollups along different dimensions). 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. Collectively these source objects are called master tables (a replication term) or detail tables (a data warehousing term). In this case, ensure that the materialized view behaves correctly. It cannot contain multiple detail tables at different sites. This restriction does not apply to expressions that involve only numeric data, for example, a+b where a and b are numeric fields. Ania 31st December, 2018 Article for ... All materialized views accessible to the current user. When refreshing materialized views, you must ensure that all materialized views in a tree are refreshed. An existing comment 'Sales summary data' was associated with the table. That is, aggregates such as AVG(AVG(x)) or AVG(x)+ AVG(x) are not allowed. When it is refreshed and once populated, this materialized view can be used by query rewrite. Rather than using a refresh group, you can schedule DBMS_MVIEW.REFRESH called using the Oracle Scheduler. The materialized view does not initially contain any data, because the build method is DEFERRED. SQL> create index mv_testtabobj_idx1 on mv_testtabobj (OWNER,TABLE_NAME); Index created. Materialized views stored in the same database as their base tables can improve query performance through query rewrites. Example 9-7 Example of Refreshing a Nested Materialized View. What is materialized views in oracle. If this capability is possible, at least full and partial text match query rewrite are possible. You can also specify if it is acceptable to use trusted constraints and QUERY_REWRITE_INTEGRITY = TRUSTED during refresh. If indexes are built on the columns by which the materialized view is ordered, accessing the rows of the materialized view using the index often reduces the time for disk I/O due to the physical clustering. SQL> create materialized view mv_testtabobj refresh on demand as select a.table_name, a.owner, b.object_id, b.object_type from test_tab a, test_obj b where a.table_name=b.object_name; Materialized view created. Although materialized views are usually accessed through the query rewrite mechanism, an end user or database application can construct queries that directly access the materialized views. They are local copies of data located remotely, or are used to create summary tables based on aggregations of a table's data. The SELECT list must contain all GROUP BY columns. The database administrator creates one or more materialized views, which are the equivalent of a summary. The SELECT clause in the materialized view creation statement defines the data that the materialized view is to contain. In this article, we discuss a feature of the Oracle Database that you can include in your arsenal: materialized views. 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. DEFERRED : The materialized view is populated on the first requested refresh. Materialized views are also useful in remote data marts. Archiving old detail and materialized view data that is no longer useful. Only a few restrictions limit what can be specified. This reference uses the term master tables for consistency. 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. If aggregate X is present, aggregate Y is required and aggregate Z is optional. For example, instead of naming the materialized view sum_of_sales, it could be called sum_of_sales_mv to denote that this is a materialized view and not a table or view. It cannot contain a SELECT list subquery. Hybrid Columnar Compression, a feature of certain Oracle storage systems, utilizes a combination of both row and columnar methods for storing data. One technique employed in data warehouses to improve performance is the creation of summaries. Materialized views with named views or subqueries in the FROM clause can be fast refreshed provided the views can be completely merged. Storing column data together, with the same data type and similar characteristics, drastically increases the storage savings achieved from compression. Materialized views, which store data based on remote tables are also, know as snapshots. Materialized Views in Oracle; Introduction. 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. The following code creates a refresh group defined to refresh every minute and assigns a materialized view to it. Purging materialized view logs can be done during the materialized view refresh process or deferred until later, thus improving refresh performance time. An updatable materialized view can send changes to its master table. Further, the marker column must appear in the same ordinal position in the SELECT list of each query block. Populating a materialized view adds load to both servers involved. 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. 13c |
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. For ON COMMIT, the mixed DML statements occur within the same transaction because the refresh of the materialized view will occur upon commit of this transaction. A materialized view is a database object that contains the results of a query. 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. 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. ON DEMAND : The refresh is initiated by a manual request or a scheduled task. Note that the techniques shown in this chapter illustrate how to use materialized views in data warehouses. The existence of a materialized view is transparent to SQL applications, so that a database administrator can create or drop materialized views at any time without affecting the validity of SQL applications. 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. Remember, refreshing on commit is a very intensive operation for volatile base tables. 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. Any nonvalidated RELY constraint is a trusted constraint. Ensuring that all materialized views and materialized view indexes are refreshed properly each time the database is updated. Queries go directly to the materialized view and not to the underlying detail tables. 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. Another alternative, subject to some restrictions, is to use outer joins in the materialized view definition (that is, in the CREATE MATERIALIZED VIEW statement). The FROM clause of the query can name tables, views, and other materialized views. This chapter focuses on the use of materialized views in data warehouses. You can choose different options for when the purge will occur, using a PURGE clause, as in the following: You can also query USER_MVIEW_LOGS for purge information, as in the following: In addition to setting the purge when creating a materialized view log, you can also modify an existing materialized view log by issuing a statement resembling the following: A materialized view can also be created using Enterprise Manager by selecting the materialized view object type. Generally support fast parallel or fast materialized view refresh. Table compression can also speed up query execution at the expense of update cost. If this capability is possible, all query rewrite capabilities are possible, including general query rewrite and full and partial text match query rewrite. Note whether the joins between the dimension tables can guarantee that each child-side row joins with one and only one parent-side row. Certification |
If the materialized view has one of the following, then fast refresh is supported only on conventional DML inserts and direct loads. An explanation of the various capabilities is in Table 9-7, and all the possible messages are listed in Table 9-8. Create the materialized view definition but do not populate it with data. If a materialized view is configured to refresh on commit, you should never need to manually refresh it, unless a rebuild is necessary. Sql Access Advisor (a GUI tool for materialized view and index management) can recommend the creation of materialized views. Usually, the vast majority of the data is stored in a few very large fact tables. The boundary of a mixed DML operation is determined by whether the materialized view is ON COMMIT or ON DEMAND. See "Restrictions on Fast Refresh on Materialized Views with Aggregates". E.g. If regular refreshes are not performed, materialized view logs can grow very large, potentially reducing the performance of their maintenance and blowing tablespace limits. 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. For fast refresh of materialized views, the definition of the materialized view logs must normally specify the ROWID clause. 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. The full text match rewrite fails. Fact tables describe the business transactions of an enterprise. When possible, PCT applies to the partitioned table named in the RELATED_TEXT column. If this capability is not possible, full text match query rewrite is not possible. You can achieve better fast refresh performance for local materialized views if you use a materialized view log that contains a WITH COMMIT SCN clause. You cannot, however, define a materialized view with a subquery in the SELECT list of the defining query.
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