1 The Optimizer How ORACLE optimizes SQL statements David Konopnicky 1997, Revised by Mordo Shalom 2004

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The OptimizerThe Optimizer

How ORACLE optimizes SQL How ORACLE optimizes SQL statementsstatements

David Konopnicky 1997, Revised by Mordo Shalom 2004David Konopnicky 1997, Revised by Mordo Shalom 2004

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David Konopnicki –1997, MS 2004David Konopnicki –1997, MS 2004

What is optimization?What is optimization?

Whenever a DML statement is issued, Whenever a DML statement is issued, ORACLE must determine how to execute ORACLE must determine how to execute it.it.

There may be different ways to execute the There may be different ways to execute the statement.statement.

The The optimizeroptimizer’s job is to choose one of ’s job is to choose one of these ways.these ways.

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Execution PlansExecution Plans

To execute a DML statement, ORACLE To execute a DML statement, ORACLE may have to perform many steps.may have to perform many steps.

Each step may:Each step may: retrieve rows from the DB.retrieve rows from the DB. prepare rows for the user in some way.prepare rows for the user in some way.

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ExampleExample

SELECT ename,job,sal,dnameSELECT ename,job,sal,dname

FROM emp,deptFROM emp,dept

WHERE emp.deptno=dept.deptnoWHERE emp.deptno=dept.deptno

AND NOT EXISTS AND NOT EXISTS

(SELECT * (SELECT *

FROM salgradeFROM salgrade

WHERE emp.sal BETWEEN WHERE emp.sal BETWEEN

lowsal AND hisal)lowsal AND hisal)

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An Execution PlanAn Execution Plan

3T A B L E A C C E S S (F U L L)

e m p

5IN D E X (U N IQ U E S C A N )

p k_ de p tno

4T A B L E A C C E S S (B Y R O W ID )

d ep t

2N E S T E D L O O P S


sa lg ra de

1F IL T E R

Physical accessto the DB(Access Paths)

Read allthe rows

For each emp, usethe deptno value to

search the index.It returns a ROWID

Use the ROWIDto find the row

Join rowsfrom empand dept

Filter the results

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Order of ExecutionOrder of Execution


e m p


p k_ de p tno


d ep t



sa lg ra de

1F IL T E R

Get all the rows andreturn them, one at atime to step 2

For each emp...

Find the ROWID of the dept

Find the rowFind the row

Join the rows

Select the rows

Return the row(or not)

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The explain plan commandThe explain plan command

ID Operation Options Object_name1 Filter

2 Nested loops

3 Table access Full EMP4 Table access By Rowid DEPT5 Index Unique scan pk_dep tno

6 Filter Full S A L G R A D E

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Two approaches to optimizationTwo approaches to optimization

Rule-based:Rule-based:

Choose an execution Choose an execution plan based on the plan based on the access path available access path available and choose the access and choose the access path using a heuristic path using a heuristic ranking.ranking.

Cost-basedCost-based Generate a set of Generate a set of

possible access paths.possible access paths. Evaluate the cost of Evaluate the cost of

each access path based each access path based on the data distribution on the data distribution and statistics.and statistics.

Choose the plan with Choose the plan with the smallest cost.the smallest cost.

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How the optimization is doneHow the optimization is done

Evaluation of expression and conditionsEvaluation of expression and conditions Statement transformationStatement transformation View mergingView merging Choice: rule-based or cost-basedChoice: rule-based or cost-based Choice of access pathsChoice of access paths Choice of join ordersChoice of join orders Choice of join operationChoice of join operation

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SQL Processing ArchitectureSQL Processing Architecture

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Optimizer ArchitectureOptimizer Architecture

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Choosing an Optimization Approach and GoalChoosing an Optimization Approach and Goal

OPTIMIZER_MODEOPTIMIZER_MODE:: CHOOSE: If there are some statistics then cost-CHOOSE: If there are some statistics then cost-

based else rule-based.based else rule-based. ALL_ROWS (best throughput)ALL_ROWS (best throughput) RULE: rule-based approach (for backward RULE: rule-based approach (for backward

compatibility)compatibility) FIRST_ROWS_n (best response time)FIRST_ROWS_n (best response time)

Hints in the query.Hints in the query.

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Evaluating Expressions and conditionsEvaluating Expressions and conditions

Constants: Constants: sal > 24000/12 sal > 24000/12 sal > 2000 sal > 2000 sal*12 > 2000 sal*12 > 2000 No evaluation No evaluation

LIKE:LIKE: ename LIKE ‘SMITH’ ename LIKE ‘SMITH’ ename = ‘SMITH’ (if variable length)ename = ‘SMITH’ (if variable length)

BETWEEN:BETWEEN: sal BETWEEN 2000 AND 3000sal BETWEEN 2000 AND 3000 sal >= 2000 AND sal <= 3000sal >= 2000 AND sal <= 3000

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Evaluating...(cont)Evaluating...(cont)

Transitivity:Transitivity:

Select *Select *

From emp, deptFrom emp, dept

Where emp.deptno = 20 andWhere emp.deptno = 20 and

emp.deptno = dept.deptnoemp.deptno = dept.deptno

dept.deptno=20

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Estimator’s ToolsEstimator’s Tools

SelectivitySelectivity CardinalityCardinality

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Transforming statementsTransforming statements

OR’s into Compound Queries:OR’s into Compound Queries:

SELECT * SELECT *

FROM EMPFROM EMP

WHERE job = ‘Clerk’WHERE job = ‘Clerk’

OR deptno = 10OR deptno = 10

UNION ALL SELECT * FROM EMP WHERE deptno = 10

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Optimizing Complex Statement Optimizing Complex Statement

Transform the complex statement in a join Transform the complex statement in a join statement and optimize the join statementstatement and optimize the join statement

Optimize the complex statement as it isOptimize the complex statement as it is

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ExampleExample SELECT *SELECT *

FROM accounts FROM accounts

WHERE custno IN (SELECT custno FROM WHERE custno IN (SELECT custno FROM

customers)customers)

SELECT accounts.*SELECT accounts.*

FROM accounts,customersFROM accounts,customers

WHERE account.custno = WHERE account.custno =

customers.custnocustomers.custno

customer.custnomust be a primary key

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The corresponding execution planThe corresponding execution plan


a ccou n ts


p k_ cu s tom e rs

1N e ste d L oo ps

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If it cannot be transformedIf it cannot be transformed

Optimize and execute the query and the Optimize and execute the query and the subqueries independently.subqueries independently.

For example:For example:

SELECT *SELECT *

FROM accountsFROM accounts

WHERE accounts.balance > (WHERE accounts.balance > (

SELECT AVG(balance) FROM SELECT AVG(balance) FROM accounts);accounts);

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Transforming statements that access viewsTransforming statements that access views

Merge the view’s query into the statementMerge the view’s query into the statement Predicate PushingPredicate Pushing Then optimize the resulting statement.Then optimize the resulting statement.

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Merging the view’s queryMerging the view’s query

View:View:

CREATE VIEW CREATE VIEW emp_10emp_10

AS SELECT * AS SELECT *

FROM empFROM emp

WHERE deptno = 10;WHERE deptno = 10;

Statement:Statement:

SELECT empnoSELECT empno

FROM emp_10FROM emp_10

WHERE empno > 7800WHERE empno > 7800

SELECT empnoFROM empWHERE empno>7800and deptno = 10;

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Complex View MergingComplex View Merging

if the view’s query contains(and enabled by if the view’s query contains(and enabled by parameter):parameter):

Set operatorSet operator Group byGroup by DISTINCTDISTINCT Group FunctionGroup Function

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ExampleExample

View:View:

CREATE VIEW groupCREATE VIEW group

AS SELECTAS SELECT

AVG(sal) avg_sal,AVG(sal) avg_sal,

MIN(sal) min_sal,MIN(sal) min_sal,

MAX(sal) max_salMAX(sal) max_sal

FROM empFROM emp

GROUP BY deptno;GROUP BY deptno;


SELECT * FROMSELECT * FROM

groupgroup

WHERE deptno=10WHERE deptno=10Select AVG(sal) avg_sal,MIN(sal) min_sal,MAX(sal) max_salFROM empWHERE deptno = 10 GROUP BYdeptno

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The execution planThe execution plan

4 IN D E X (R A N G E S C A N )e m p _ de p tn o ind ex

3 T A B L E A C C E S S (B Y R O W ID )e m p

2 S O R T (G R O U P B Y)

1 V IE Wg roup

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Predicate PushingPredicate Pushing

View:View:

CREATE VIEW empCREATE VIEW emp

AS AS


emp1emp1

UNIONUNION


emp2;emp2;


SELECT empno,enameSELECT empno,ename

FROM empFROM emp

WHERE deptno=20;WHERE deptno=20;

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We will execute...We will execute...

SELECT * FROM emp1 WHERE deptno=20SELECT * FROM emp1 WHERE deptno=20

UNIONUNION

SELECT * FROM emp2 WHERE deptno=20SELECT * FROM emp2 WHERE deptno=20

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The execution planThe execution plan

4 T A B L E A C C E S S (F U L L)e m p1

5 T A B L E A C C E S S (F U L L)e m p2

3U N IO N A LL

2S O R T (U N IQ U E )

1V IE We m p

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Optimizing other statements that access viewsOptimizing other statements that access views

ORACLE cannot always merge views’ ORACLE cannot always merge views’ queries and statements.queries and statements.

In these cases, ORACLE issues the view’s In these cases, ORACLE issues the view’s query, collects the rows and then access this query, collects the rows and then access this set of rows with the original statement as set of rows with the original statement as thought it was a table.thought it was a table.

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ExampleExample

View:View:CREATE VIEW groupCREATE VIEW group

AS SELECT deptno,AS SELECT deptno,

AVG(sal) avg_sal,AVG(sal) avg_sal,

MIN(sal) min_sal,MIN(sal) min_sal,

MAX(sal) max_salMAX(sal) max_sal

FROM empFROM emp

GROUP BY deptnoGROUP BY deptno

Statement:Statement:SELECT group.deptno,SELECT group.deptno,

avg_sal, min_sal, max_sal,avg_sal, min_sal, max_sal,

dname,locdname,loc

FROM group,deptFROM group,dept

WHERE WHERE group.deptno=dept.deptnogroup.deptno=dept.deptno

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Execution PlanExecution Plan


e m p

3S O R T (G R O U P B Y )

2V IE Wg roup


p k_d ep t


d ep t


Documents

1 The Optimizer How ORACLE optimizes SQL statements David Konopnicky 1997, Revised by Mordo Shalom 2004