I will not show you the materialized view concepts, the Oracle Datawarehouse Guide is perfect for that. A materialized view can be stored in the same database as its base table(s) or in a different database. 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. The query rewrite mechanism in the Oracle server automatically rewrites the SQL query to use the summary tables. Storing column data together, with the same data type and similar characteristics, drastically increases the storage savings achieved from compression. You simply call DBMS_MVIEW.EXPLAIN_MVIEW, passing in as a single parameter the schema and materialized view name for an existing materialized view. See Chapter 18, "Basic Query Rewrite" for details about integrity levels. These operations are expensive in terms of time and processing power. If the materialized view contains only joins, the ROWID columns for each table (and each instance of a table that occurs multiple times in the FROM list) must be present in the SELECT list of the materialized view. Determining which new materialized views should be created. Determining how effective each materialized view has been on workload performance. You need to use the SEQ column in an ORDER BY clause so the rows will display in a logical order. In any database management system following the relational model , a view is a virtual table representing the result of a database query . A materialized view in Oracle is a database object that contains the results of a query. A complete refresh is required for the first refresh of a build deferred materialized view. This example creates a materialized view product_sales_mv that computes the sum of sales by prod_name. 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 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. The QUERY REWRITE clause tells the optimizer if the materialized view should be consider for query rewrite operations. 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. Neither the detail tables nor the materialized view can be owned by SYS. Materialized views in Oracle; A materialized view is a stored summary containing precomputes results (originating from an SQL select statement). Generally, this means that refresh is possible after partition maintenance operations on those detail tables where PCT is indicated as possible. You can use the DBMS_MVIEW.EXPLAIN_MVIEW procedure to learn what is possible with a materialized view or potential materialized view. First, create the materialized view. Also, unique constraints must exist on the join columns of the inner join table. A refresh can be triggered in one of two ways. A materialized view definition includes any number of aggregates, as well as any number of joins. ]name of the table or view in question, Name of the first different node, or NULL if the first different node is local, [owner. You should keep in mind the following restrictions: The defining query of the materialized view cannot contain any non-repeatable expressions (ROWNUM, SYSDATE, non-repeatable PL/SQL functions, and so on). This initial ordering provides physical clustering of the data. SQL - Materialized View in Oracle. If you specify REFRESH FAST, Oracle performs further verification of the query definition to ensure that fast refresh can be performed if any of the detail tables change. Connect to the materialized view owner and create the database link and the materialized view itself. Functionality of Materialized Views. If the materialized view has one of the following, then fast refresh is supported only on conventional DML inserts and direct loads. The SELECT column in the defining query cannot be a complex expression with columns from multiple base tables. Materialized join views and materialized aggregate views with a GROUP BY clause cannot select from an index-organized table. 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. Figure 9-1 illustrates how query rewrite works. 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. answered Jan 20 '15 at 7:39. 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. However, you realize significantly greater query execution performance and materialized view refresh performance benefits and you require fewer materialized views if your schema design complies with these guidelines. 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. It cannot contain a [START WITH …] CONNECT BY clause. It is used only during the initial creation of the materialized view. The following query does an aggregation of the data in the EMP table. This is particularly useful for materialized views created with the ROLLUP clause. On the other hands, Materialized Views are stored on the disc. The same query is now rewritten to take advantage of the pre-aggregated data in the materialized view, instead of the session doing the work for itself. 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. 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. When data is loaded, groups of rows are stored in columnar format, with the values for a given column stored and compressed together. It can also include any number of joins. Archiving old detail and materialized view data that is no longer useful. 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. 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). If VARIANCE(expr) or STDDEV(expr) is specified, COUNT(expr) and SUM(expr) must be specified. 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. Therefore this method may not be suitable if many users are concurrently changing the tables upon which the materialized view is based. Use the CREATE MATERIALIZED VIEW statement to create and, optionally, populate the materialized view. Alternatively, you can use EXPLAIN_MVIEW on a potential materialized view using its SELECT statement or the complete CREATE MATERIALIZED VIEW statement. A simple measure is a numeric or character column of one table such as fact.sales. The SQL Access Advisor, which recommends materialized views, partitions, and indexes to create. Immediately after loading the detail data and updating the indexes on the detail data, the database can be opened for operation, if desired. You can refresh a tree of nested materialized views in the appropriate dependency order by specifying the nested = TRUE parameter with the DBMS_MVIEW.REFRESH parameter. The column alias list explicitly resolves any column name conflict without attaching aliases in the SELECT clause of the materialized view. If QUERY_REWRITE_INTEGRITY is set to STALE_TOLERATED, access to the materialized view can be allowed at the session level to any users who do not require the materialized views to reflect the data from the latest load by issuing an ALTER SESSION SET QUERY_REWRITE_ENABLED = TRUE statement. 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. Consequently, query execution usually performs best if a single-column bitmap index is defined on each materialized view key column. These additional checks are: A materialized view log must be present for each detail table unless the table supports PCT. The materialized view does not initially contain any data, because the build method is DEFERRED. 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. Such a materialized view is called an insert-only materialized view. In this case, the table that already exists in the database can be registered as a prebuilt materialized view. The benefits of maintaining this condition are described in "Creating Dimensions". 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. In the case of denormalized dimensions, determine whether the child-side columns uniquely determine the parent-side (or attribute) columns. The SELECT clause in the materialized view creation statement defines the data that the materialized view is to contain. 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. Materialized views which store data based on remote tables were also known as snapshots, (deprecated Oracle terminology). 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. Hierarchies describe the business relationships and common access patterns in the database. You cannot create both a materialized view and a prebuilt materialized view on the same table. We can define a materialized view on a base/master table (at a master site), partitioned table, view, synonym or a master materialized view (at a materialized view site). It then transparently rewrites the request to use the materialized view. Usually, the vast majority of the data is stored in a few very large fact tables. If the materialized view has remote tables in the FROM clause, all tables in the FROM clause must be located on that same site. If it is ON COMMIT, the refresh is performed at commit time of the transaction that does DML on the materialized view's detail table. This is because the refresh operation is performed as part of the commit process. PCT is needed to support fast fresh after partition maintenance operations on the table named in the RELATED_TEXT column. A read-only materialized view is implemented as a create table as select command. Purging materialized view logs can be done during the materialized view refresh process or deferred until later, thus improving refresh performance time. 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. to construct a data warehouse. This is also the case for indexes created on the materialized view. Query rewrite is not possible with all materialized views. 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. A nested materialized view is considered to be fresh as long as its data is synchronized with the data in its detail tables, even if some of its detail tables could be stale materialized views. For example, "GROUP BY a, ROLLUP(a, b)" is not fast refreshable because it results in duplicate groupings "(a), (a, b), AND (a)". 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. 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. Also, check to see if your materialized view satisfies all of the following conditions. This example creates a materialized view product_sales_mv that computes total number and value of sales for a product. The materialized base view is truncated. Table 9-4 describes the refresh modes. Although materialized view logs improve the performance of materialized view refreshes, they do increase the work needed to perform DML on the base table. A refresh mechanism to ensure that all materialized views contain the latest data. 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. 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