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12 Copyright © 2005, Oracle. All rights reserved. Query Rewrite.

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Presentation on theme: "12 Copyright © 2005, Oracle. All rights reserved. Query Rewrite."— Presentation transcript:

1 12 Copyright © 2005, Oracle. All rights reserved. Query Rewrite

2 12-2 Copyright © 2005, Oracle. All rights reserved. Objectives After completing this lesson, you should be able to do the following: Define query rewrite Enable query rewrite Determine whether query rewrite occurred Explain the various methods used to rewrite queries List query rewrite restrictions

3 12-3 Copyright © 2005, Oracle. All rights reserved. Tries to use materialized views instead of base tables to return query results Can save orders of magnitude of CPU and elapsed time to return results as queries are precomputed Can significantly reduce I/O Not necessary for query to be in the exact form of the materialized view query to rewrite Various requirements for query rewrite to take place Query Rewrite: Overview

4 12-4 Copyright © 2005, Oracle. All rights reserved. Cost-Based Query Rewrite Process Pick the best Generate plan Compare plan costs Query rewrite Generate plan

5 12-5 Copyright © 2005, Oracle. All rights reserved. What Can Be Rewritten? Queries and subqueries in the following types of SQL statements: – SELECT – CREATE TABLE … AS SELECT – INSERT INTO … SELECT Subqueries in DML statements: – INSERT – UPDATE – DELETE Subqueries in the set operators: – UNION – UNION ALL – INTERSECT – MINUS

6 12-6 Copyright © 2005, Oracle. All rights reserved. Setting Initialization Parameters for Query Rewrite QUERY_REWRITE_ENABLED : Set at instance and session level – TRUE : Default – FORCE QUERY_REWRITE_INTEGRITY : Set at instance and session level – ENFORCED : Default – TRUSTED – STALE_TOLERATED

7 12-7 Copyright © 2005, Oracle. All rights reserved. Full Notes Page

8 12-8 Copyright © 2005, Oracle. All rights reserved. Using Trusted Constraints TRUSTED CONSTRAINTS : Allows use of nonvalidated RELY constraints and rewrite against materialized views in UNKNOWN or FRESH state during refresh –Refresh can use trusted constraints. – QUERY_REWRITE_INTEGRITY = TRUSTED during refresh ENFORCED CONSTRAINTS : Allows use of only validated, enforced constraints and rewrite against materialized views in FRESH state during refresh –Refresh can use validated constraints. – QUERY_REWRITE_INTEGRITY = ENFORCED during refresh

9 12-9 Copyright © 2005, Oracle. All rights reserved. Enabling Query Rewrite Specify ENABLE QUERY REWRITE when the materialized view is created. Enable query rewrite after creation of the materialized view with ALTER MATERIALIZED VIEW.

10 12-10 Copyright © 2005, Oracle. All rights reserved. Query Rewrite Requirements Either all or part of the result requested by the query must be obtainable from the materialized view. Constraints or dimensions are defined.

11 12-11 Copyright © 2005, Oracle. All rights reserved. Query Rewrite Hints REWRITE and NOREWRITE – REWRITE[(mv1, mv2, …)] : Forces the optimizer to use a materialized view (if any exist) to rewrite the query regardless of the cost – NOREWRITE : Prevents the optimizer from rewriting REWRITE_OR_ERROR : Generates an ORA error if the query failed to rewrite

12 12-12 Copyright © 2005, Oracle. All rights reserved. Did Query Rewrite Occur? 1.Execute the query: 2.Examine the execution plan: SELECT s.zip, p.product_type, sum(s.amount) FROM sales s, product p WHERE s.prod_id = p.prod_id GROUP BY s.zip, p.prod_type; OPERATION NAME MAT_VIEW REWRITE ACCESS FULL SALES_SUMMARY

13 12-13 Copyright © 2005, Oracle. All rights reserved. Verifying That Query Rewrite Occurred Goal is to understand: –If rewrite is possible, which materialized view is used? –Why was the query not rewritten? Use the DBMS_MVIEW.EXPLAIN_REWRITE procedure with the following parameters: –SQL statement (never executed) –Optional materialized view name Results are stored in the REWRITE_TABLE table or in a VARRAY. utlxrw.sql must be executed in the current schema to create REWRITE_TABLE.

14 12-14 Copyright © 2005, Oracle. All rights reserved. Using DBMS_MVIEW.EXPLAIN_REWRITE DECLARE qrytext VARCHAR2(2000) := 'SELECT cust_last_name, SUM(amount_sold) FROM sales s, customers c WHERE s.cust_id = c.cust_id GROUP BY cust_last_name'; BEGIN dbms_mview.explain_rewrite(qrytext,'smv','124'); END; SELECT message FROM rewrite_table WHERE statement_id = '124' ORDER BY sequence; MESSAGE QSM-01001: query rewrite not enabled

15 12-15 Copyright © 2005, Oracle. All rights reserved. Full Notes Page

16 12-16 Copyright © 2005, Oracle. All rights reserved. Query Rewrite Methods Two categories of query rewrite methods: Text match General rewrite

17 12-17 Copyright © 2005, Oracle. All rights reserved. Text Match Rewrite Methods The optimizer uses two methods: –Exact text match: Entire text of query is compared with entire text of materialized view detail query. –Partial text match: Only text starting from the FROM clause of the query and materialized view are compared. White spaces are ignored. A complex materialized view is one for which: –Rewrite capability is limited to text match rewrite (exact or partial) – REWRITE_CAPABILITY=TEXTMATCH in DBA_MVIEWS

18 12-18 Copyright © 2005, Oracle. All rights reserved. Expression Matching Query Rewrite Rewrite based on equivalent matching of: –Expressions –Subexpressions Expressions should be stored in the materialized views SELECT list. If a querys expression does not match materialized views expression, subexpressions matching is tried. Canonical expressions taken into account: –Commutativity –Associativity –Distributivity If multiple queries share a common subexpression, store it in the materialized view to avoid future computations.

19 12-19 Copyright © 2005, Oracle. All rights reserved. Expression Matching: Example CREATE MATERIALIZED VIEW sales_by_age_mv (age_bracket, sum_amount_sold) ENABLE QUERY REWRITE AS SELECT TO_CHAR( (2000-c.cust_year_of_birth)/10-0.5,999), SUM(s.amount_sold) FROM sales s, customers c WHERE s.cust_id = c.cust_id GROUP BY TO_CHAR( (2000-c.cust_year_of_birth)/10-0.5,999);

20 12-20 Copyright © 2005, Oracle. All rights reserved. Expression Matching: Example SELECT TO_CHAR( ((2000-c.cust_year_of_birth)/10)-0.5,999), SUM(s.amount_sold) FROM sales s, customers c WHERE s.cust_id = c.cust_id GROUP BY TO_CHAR( (2000-c.cust_year_of_birth)/10-0.5,999); SELECT age_bracket, sum_amount_sold FROM sales_by_age_mv; Query rewritten

21 12-21 Copyright © 2005, Oracle. All rights reserved. General Query Rewrite Methods Selection compatibility (filtered) Join compatibility (join back) Data sufficiency (rollup) Grouping compatibility Aggregate computability

22 12-22 Copyright © 2005, Oracle. All rights reserved. Partition Change Tracking (PCT) and Query Rewrite Uses metadata to maintain staleness at a finer granularity: Only specific sections of the materialized view are considered stale. Query rewrite can use a materialized view in ENFORCED (or TRUSTED ) mode for rows considered fresh only. A materialized views fresh rows are identified by adding selection predicates to the materialized views definition. A materialized views fresh rows are identified by using partitions markers in the materialized view definition.

23 12-23 Copyright © 2005, Oracle. All rights reserved. PCT and Query Rewrite: Example CREATE TABLE sales (time_id DATE NOT NULL, prod_id NUMBER NOT NULL, cust_id NUMBER NOT NULL, channel_id CHAR(1) NOT NULL, promo_id NUMBER NOT NULL, quanty_sold NUMBER NOT NULL, amount_sold NUMBER NOT NULL) PARTITION BY RANGE (time_id) ( PARTITION q1 VALUES LESS THAN ('1-APR-2001'), PARTITION q2 VALUES LESS THAN ('1-JUL-2001'), PARTITION q3 VALUES LESS THAN ('1-OCT-2001'), PARTITION q4 VALUES LESS THAN ('1-JAN-2002') );

24 12-24 Copyright © 2005, Oracle. All rights reserved. PCT and Query Rewrite: Example 1.Create the SSP_MV materialized view: 2.Insert data into the second partition of the SALES table: 3.Materialized view query reflecting fresh rows: CREATE MATERIALIZED VIEW ssp_mv ENABLE QUERY REWRITE AS SELECT time_id, prod_name, SUM(amount_sold) AS samt FROM products p, sales s WHERE s.prod_id = p.prod_id GROUP BY time_id, prod_name; WHERE s.prod_id = s.store_id AND time_id = '1-OCT-2001' INSERT …('1-JUL-2001' <= time_id < '1-OCT-2001')…

25 12-25 Copyright © 2005, Oracle. All rights reserved. PCT and Query Rewrite: Example The following query: Can be rewritten: SELECT time_id, SUM(amount_sold) AS samt FROM products p, sales s WHERE s.prod_id = p.prod_id AND time_id < '1-JUL-2001' GROUP BY time_id; SELECT time_id, SUM(samt) FROM ssp_mv WHERE time_id < '1-JUL-2001' GROUP BY time_id;

26 12-26 Copyright © 2005, Oracle. All rights reserved. Query Rewrite and Bind Variables Consider two situations: Materialized view detail query has literals. –If the application query uses bind variables for those literals, there is no possible rewrite. –If CURSOR_SHARING is set to FORCE or SIMILAR, general rewrite can occur if the application query does not use bind variables for those literals. Materialized view detail query does not have literals. –If the application query uses bind variables, general rewrite is possible. –If CURSOR_SHARING is set to FORCE or SIMILAR, general rewrite is possible.

27 12-27 Copyright © 2005, Oracle. All rights reserved. Creating Indexes for Materialized Views Recommendations: Create a single-column bitmap index on each materialized view key column for query execution performance. Create indexes on columns containing row IDs for materialized views containing only joins.

28 12-28 Copyright © 2005, Oracle. All rights reserved. Query Rewrite Restrictions The materialized views detail query should not contain: –Any nonrepeatable expression such as ROWNUM, SYSDATE, bind variables, and so on –Reference to RAW, LONG RAW, or object REFs data types Only local tables in the query are considered for potential rewrite. Neither detail tables nor the materialized view can be owned by SYS.

29 12-29 Copyright © 2005, Oracle. All rights reserved. Summary In this lesson, you should have learned how to: Enable query rewrite Set the various parameters to enable query rewrite Use the EXPLAIN_REWRITE procedure to check query rewrite operations Create valid materialized views for query rewrite to be possible

30 12-30 Copyright © 2005, Oracle. All rights reserved. Practice 12: Overview This practice covers the following topics: Setting query rewrite integrity in your session Using aggregate computability check Using PCT rewrite in ENFORCED mode Using V$SQL_PLAN to verify that a query is rewritten Using dimensions to rewrite queries


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