12 copyright © 2005, oracle. all rights reserved. query rewrite

12Copyright © 2005, Oracle. All rights reserved.

Query Rewrite

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


• 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


Cost-Based Query Rewrite Process

Pick the best

Generate plan

Compare plan costs

Query rewrite

Generate plan


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


Setting Initialization Parameters forQuery 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


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


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.


Query Rewrite Requirements

• Either all or part of the result requested bythe query must be obtainable from the materialized view.

• Constraints or dimensions are defined.


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-30393 error if the query failed to rewrite


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 pWHERE s.prod_id = p.prod_idGROUP BY s.zip, p.prod_type;

OPERATION NAME---------------------- ---------------MAT_VIEW REWRITE ACCESS FULL SALES_SUMMARY


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.


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


Query Rewrite Methods

Two categories of query rewrite methods:

• Text match

• General rewrite


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


Expression Matching Query Rewrite

• Rewrite based on equivalent matching of:– Expressions– Subexpressions

• Expressions should be stored in the materialized view’s SELECT list.

• If a query’s expression does not match materialized view’s 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.


Expression Matching: Example

CREATE MATERIALIZED VIEW sales_by_age_mv(age_bracket, sum_amount_sold)ENABLE QUERY REWRITE ASSELECT 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_idGROUP BY TO_CHAR((2000-c.cust_year_of_birth)/10-0.5,999);


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_idGROUP 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


General Query Rewrite Methods

• Selection compatibility (filtered)

• Join compatibility (join back)

• Data sufficiency (rollup)

• Grouping compatibility

• Aggregate computability


Partition Change Tracking (PCT) andQuery 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 view’s fresh rows are identified by adding selection predicates to the materialized view’s definition.

• A materialized view’s fresh rows are identified by using partitions markers in the materialized view definition.


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'));



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_mvENABLE QUERY REWRITE ASSELECT time_id, prod_name, SUM(amount_sold) AS samtFROM products p, sales sWHERE s.prod_id = p.prod_idGROUP BY time_id, prod_name;

WHERE s.prod_id = s.store_id ANDtime_id < '1-JUL-2001' OR time_id >= '1-OCT-2001'

INSERT …('1-JUL-2001' <= time_id < '1-OCT-2001')…



• The following query:

• Can be rewritten:

SELECT time_id, SUM(amount_sold) AS samtFROM products p, sales sWHERE s.prod_id = p.prod_id AND time_id < '1-JUL-2001'GROUP BY time_id;

SELECT time_id, SUM(samt)FROM ssp_mvWHERE time_id < '1-JUL-2001'GROUP BY time_id;


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.


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.


Query Rewrite Restrictions

• The materialized view’s 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.


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


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

12 copyright © 2005, oracle. all rights reserved. query rewrite

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