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Query Rewrite → Query rewrite is an optimization technique that transforms a user request written in terms of … . Because materialized views contain precomputed aggregates and joins, query rewrite can quickly answer … in the SQL statement. Because query rewrite is transparent, materialized views can
Query Rewrite Restrictions → Query rewrite is not possible with all materialized views. If query rewrite is not occurring when … for rewrite. 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
Controlling Query Rewrite → A materialized view is only eligible for query rewrite if the ENABLE QUERY REWRITE clause has been … , ALTER SESSION can be used to disable or enable query rewrite for that session only. An example is … the following: ALTER SESSION SET QUERY_REWRITE_ENABLED = TRUE; You can set the level
Query Rewrite Using Dimensions → … Figure 19-2 Query Rewrite Subgraphs Description of \"Figure 19-2 Query Rewrite Subgraphs\" Common … rewrite, the materialized view is joined to the appropriate tables in the query delta. For example … rewrite the query if this join is lossless
Other Query Rewrite Considerations → The following discusses some of the other cases when query rewrite is possible: Query Rewrite Using … Nested Materialized Views Query Rewrite in the Presence of Inline Views Query Rewrite Using Remote … Tables Query Rewrite
General Query Rewrite Methods → The optimizer has a number of different types of query rewrite methods that it can choose from to … answer a query. When text match rewrite is not possible, this group of rewrite methods is known as … general query rewrite. The advantage of using these more advanced
Query Rewrite Considerations: Statistics → the materialized view and the tables in the query to make a cost-based choice. Materialized views
Accuracy of Query Rewrite → Query rewrite offers three levels of rewrite integrity that are controlled by the session parameter … relationships specified using dimensions. This mode offers greater query rewrite capabilities but also creates … contain stale data as well as those that contain fresh data. This mode offers the maximum
Query Rewrite Considerations: Dimensions → QUERY_REWRITE_INTEGRITY to TRUSTED or STALE_TOLERATED for query rewrite to take advantage of the relationships declared in dimensions.
Overview of Query Rewrite → Database employs an extremely powerful process called query rewrite to quickly answer the query using … ability to take advantage of query rewrite, which transforms a SQL statement expressed in terms of … intervention and no reference to the materialized view in the SQL statement. Because query
Query Rewrite Considerations: Hints → You can include hints in the SELECT blocks of your SQL statements to control whether query rewrite … occurs. Using the NOREWRITE hint in a query prevents the optimizer from rewriting it. The REWRITE hint … with no argument in a query forces the optimizer to use a materialized view (if any) to rewrite
Query Rewrite Considerations: Constraints → Make sure all inner joins referred to in a materialized view have referential integrity (foreign key/primary key constraints) with additional NOT NULL constraints on the foreign key columns. Since constraints tend to impose a large overhead, you could make them NO VALIDATE and RELY and set the parameter QUERY_REWRITE_INTEGRITY to STALE_TOLERATED or TRUSTED. However, if you set QUERY_REWRITE_INTEGRITY
Query Rewrite Considerations: Aggregates → To get the maximum benefit from query rewrite, make sure that all aggregates which are needed to … aggregates are quite similar to those for incremental refresh. For instance, if AVG(x) is in the query
19 Advanced Query Rewrite → This chapter discusses advanced query rewrite topics in Oracle, and contains: How Oracle Rewrites … Queries Types of Query Rewrite Other Query Rewrite Considerations Advanced Query Rewrite Using … Equivalences Creating Result Cache Materialized Views
Enabling Query Rewrite → Before creating a materialized view, you can verify what types of query rewrite are possible by … materialized view so that many types of query rewrite are possible. Once the materialized view has been … rewrite facility. Even though query rewrite is enabled by default,
18 Basic Query Rewrite → This chapter discusses query rewrite in Oracle, and contains: Overview of Query Rewrite Ensuring … that Query Rewrite Takes Effect Example of Query Rewrite
Types of Query Rewrite → original tables. Query rewrite transforms an incoming query to fetch the results from materialized … answered almost instantaneously. For considerations regarding query rewrite of cube organized … used to rewrite a query: Text Match Rewrite Join Back
Benefits of Using Dimensions A dimension defines a hierarchical parent/child relationships between columns,… → If the dimensions are normalized, that is, stored in multiple tables, then check that a join between the dimension tables guarantees that each child-side row joins with one and only one parent-side row. In the case of denormalized dimensions, check that the child-side columns uniquely determine the parent-side (or attribute) columns. Failure to abide by these rules may result in incorrect results
Query Rewrite in the Presence of Grouping Sets → row in the materialized view originates. Query rewrite can use the marker to distinguish what rows … materialized view: CREATE MATERIALIZED VIEW cust_postal_mv ENABLE QUERY REWRITE AS SELECT 1 AS marker … represent female customers in the first subselect of the UNION ALL. Note that query rewrite
Using the EXPLAIN_REWRITE Procedure with Query Rewrite → It can be difficult to understand why a query did not rewrite. The rules governing query rewrite … addition, you may want to know why query rewrite chose a particular materialized view instead of another … , you can take the appropriate action needed to make a query