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SAP® MaxDB Expert Sessions 

SAP MaxDB SQL Query Optimization - Part 1 

Christiane Hienger SAP AG 

Holger Becker SAP AG 

August 28, 2012 

© SAP 2012 SAP MaxDB Expert Session – Optimizer (Part 1)1

Agenda 

1. Introduction 

2 . EXPLAIN 

2.1. EXPLAIN 

2.2. EXPLAIN QUERYREWRITE 

3. Single Table Optimizer Strategies 

© SAP 20102/MaxDB 7.8 Expert Session– Optimizer Part 1 /Page 2

After this presentation, you will be able to: 

Use several EXPLAIN commands to analyze SQL queries 

Understand and interpret the EXPLAIN output 

Understand the usage of SQL strategies of the single table optimizer 

© SAP 20102/MaxDB 7.8 Expert Session – Optimizer Part 1/Page 3

Goals of Optimization 

Goal: Minimizing resource-consumption like 

CPU-time 

I/O-load 

Memory 

Disk space 

SQL Commands affected by optimization 

SELECT 

UPDATE 

DELETE 

INSERT/SELECT 

© SAP 2012 /MaxDB 7.8 Expert Session – Optimizer Part 1/Page 4

Cost-based Optimizer 

Cost-based optimizer 

Search strategy is determined via 

– current column content (values) 

– available indexes 

– estimated count of (page) accesses 

‚Lowest cost‘ strategy will be used. 


Which condition will be evaluated ? 

Single table SELECT 

Column = value 

Column value 

Column BETWEEN value AND value 

Column IN ( value, value, ... ) 

Column LIKE string value (including %,?,...) 

Column = (ANY) 

Column IN 


Introduction into SQL Optimization 

SELECT * FROM zztele WHERE STR = ‚Wexstr‘ 


Primary and Secondary keys (indexes) 

Primary key 

The primary key is stored in the data tree (clustered) 

No separate tree for primary key ! 

Parts of the primary keys are used as separator in B*trees 

The records are stored in primary key order 

Secondary key (index) 

Construction of a separate B*tree for the secondary key values 

A secondary key does not contain physical addresses pointing to the base data but 

logical addresses in terms of primary keys 


Table Examples: ZZTELE 

# of records: around 115,000 

© SAP 2012 /MaxDB 7.8 Expert Session – Optimizer Part 1/Page 9 

: 

Create Table ZZTELE 

( NAME CHAR(40), 

VORNAME CHAR(20), 

STR CHAR(40), 

NR INT, 

PLZ CHAR(5), 

ORT CHAR(25), 

CODE CHAR(31), 

ADDINFO CHAR(31), 

PRIMARY KEY 

(NAME,VORNAME,STR) )

Table Examples: ZZSTADTTEIL , ZZMASTER 

around 20,000 

records 


Create Table ZZSTADTTEIL 

( PLZ CHAR(5), 

ORT CHAR(25), 

STADTTEIL CHAR(40), 

PRIMARY KEY 

(PLZ) ) 

Create Table ZZMASTER 

( YEAR INT, 

NAME CHAR(40), 

VORNAME CHAR(20), 

UNI CHAR (40), 

ORT CHAR (25), 

PRIMARY KEY 

(YEAR,NAME,VORNAME) ) 

10 records

Indexes created on table ZZTELE 


Storing data in a B*tree 

Root Level 

Page 

Root node (Root) of table ZZTELE 

8 KByte D S U .... 

Level 1 

Pages 

Leaf Level 

Pages 

Base data as well as index data (secondary key) is stored in B*tree structures. 


Brau Caes Dart Pete... Ste... 

Aaron Anton 

Steig 5 

... 

Brauner E. 

Ilseweg 11 ... 

Caesar 

S.Gartenweg 5

Information used by Optimization 

SELECT ... FROM tab1 WHERE tab1.col1 = 'Walldorf' 

Parse Statement 

Best Access Path 


Final Strategy Execution 

DDL Information 

Value Evaluation

Explain (1) 

Input : EXPLAIN 

Output : Description of search strategy 

EXPLAIN is used with SELECT commands that access tables and views 

EXPLAIN does not execute the specified SELECT command. 

The explain command cannot be used with 

UPDATE, DELETE or INSERT commands 


Explain (2) 

SCHEMANAME 

Schema Table 1 

Schema Table 2 

TABLENAME COLUMN_OR_INDEX STRATEGY PAGECOUNT 

Result 

name 


Names of key or 

index columns 

Names of key or 

index columns 

Name of chosen strategy 

for this table 

Name of chosen strategy 

for this table 

RESULT IS (NOT) 

COPIED, COSTVALUE IS 

Number of pages 

In system table 

Optimizerstatistics 

Number of pages 

in system table 

Optimizerstatistics 

Estimated 

costs 

Applied 

Query Rewrite rules 1

Query Rewrite 

Query Rewrite rebuilds SQL statements by the use of rules to enable the optimizer 

to find the best strategy. 

Example: ConvertOrToIn 

select * from zztele 

where PLZ = '10967' or PLZ = '15099' or PLZ = '12047' 

SELECT "NAME", "VORNAME", "STR", "NR", 

"PLZ", "ORT", "CODE", "ADDINFO" 

FROM "SAPR3"."ZZTELE" AS "_T1“ 

WHERE PLZ in ('10967' , '15099' , '12047' ) 

Parameter: EnableQueryRewrite=YES, QueryAnalysisMode=EXTENDED 


Query Rewrite Rules 

Changing the column ACTIVE in the view QUERYREWRITERULES effects if a rule 

is switched on or off. 


Monitoring Query Rewrite 

EXPLAIN QUERYREWRITE shows the result of Query Rewrite as SQL statement. 

The view MONITOR indicates how often rules were applied. 


Explain (3) 


Explain Queryrewrite 


Search Condition and Search Strategies (1) 

SELECT VORNAME,NAME,PLZ,ORT,STR,ADDINFO 

FROM ZZTELE 

WHERE ADDINFO LIKE 'Training%' 


Search Strategy

Index and Table 

Index 

ZZTELE~3 

SELECT VORNAME,NAME,PLZ,ORT,STR,ADDINFO 

FROM ZZTELE 

WHERE PLZ = '10967' 

10967 


Secondary Key Primary Key List 

Aimonsri Kano...Dummy Diel... 

Index List 

Aimonsri Kanopkon Dummy1 

: 

Dielschn Korina Dummy4 

10967 Berlin Fiance 

Primary Key Fieldvalues 

A 1 

Base Record 

10967 Berlin IBU Pub 

Base Table 

ZZTELE

EQUAL CONDITION FOR KEY 

SELECT * FROM zztele 

WHERE Name = 'Aaron' 

AND Vorname = 'Anton' 

AND Str = 'Alt Moabit' 

Key Value = 


Base Table

EQUAL CONDITION FOR KEY - Example 


WHERE Name = 'Aaron' 

AND Vorname = 'Anton' 

AND Str = 'Alt Moabit' 


RANGE CONDITION FOR KEY 

SELECT * FROM zztele WHERE Name = 'Schmidt' 

AND Vorname like 'A%' 


STARTKEY 

CONDITION(S) TABLE SCAN RANGE 

STOPKEY 


Base Table

RANGE CONDITION FOR KEY - Example 

SELECT * FROM zztele WHERE Name = 'Schmidt' 

AND Vorname like 'A%‘ 



IN CONDITION FOR KEY 


WHERE Name IN ('Mueller', 'Schmidt') 

Key Value 1 

Key Value 2 

'Mueller' 

'Schmidt' 


Base Table 

RANGE Cond for In

IN CONDITION FOR KEY - Example 


WHERE Name IN ('Mueller', 'Schmidt') 


WHERE Name IN ('Scheu', 'Schmidt') 


EQUAL CONDITION FOR INDEX 

CREATE INDEX "ZZTELE~3" ON ZZTELE(PLZ) 


WHERE plz = ‘12345' 

Single 

Index 


Index List 

12345 

primary key list 

Base Table

EQUAL CONDITION FOR INDEX - Example 



WHERE plz = '12345' 


RANGE CONDITION FOR INDEX 



WHERE plz BETWEEN '10100' AND '10967' 

Index 

SCAN 

RANGE 

Index 


Index List 

Index List 



Base Table

RANGE CONDITION FOR INDEX - Example 



WHERE plz BETWEEN '10100' AND '10967' 


IN CONDITION FOR INDEX 

CREATE INDEX "ZZTELE~2" ON ZZTELE ( STR, NR ) 


WHERE str = 'Wexstr' AND nr IN (3, 255) 

Multiple 

Index 


Index List 

3 

Index List 

255 



Base Table

IN CONDITION FOR INDEX - Example 



WHERE str = 'Wexstr' AND nr IN (3, 255) 


Properties which influence the Optimizer 

Only Index 

Create intermediate result Yes / NO ( e.g. order by) 

Min / Max Optimization 

Distinct 


ONLY INDEX ACCESSED 

Index 


Secondary Key Primary Key List 

plz name 

12345 Antoa Dry Huebel Marek ... 

SELECT name, plz 

FROM zztele 


Base Table

ONLY INDEX ACCESSED - Example 

SELECT name, plz 

FROM zztele 



ORDER BY Index column(s) 

CREATE INDEX "ZZTELE~2" ON zztele ( str, nr ) 

SELECT * FROM zztele WHERE name BETWEEN 'A' and 'T' 

ORDER BY str, nr 

Index 


Index List 

Index List 



Base Table

ORDER BY Index column(s) - Example 

SELECT * FROM zztele WHERE name 

BETWEEN 'A' and 'T' 

ORDER BY str, nr 


ORDER BY key column(s) 



WHERE str = 'Wexstr' AND nr BETWEEN 20 AND 23 

ORDER BY name, vorname, str 

Single 

Index 

20 

Index List 

22 

Kernel parameter: IndexlistsMergeThreshold 




Index List 

23 


Merged 

List 

Base Table

ORDER BY key column(s) - Example 


WHERE str = 'Wexstr' AND nr 

BETWEEN 20 AND 23 

ORDER BY name, vorname, str 


MIN/MAX Optimization - Example 

CREATE INDEX "ZZTELE~2" ON zztele ( str, nr ) 

SELECT MAX (nr) 

FROM zztele 

WHERE str = 'Wexstr' 


DISTINCT Optimization - Example 

CREATE INDEX "ZZTELE~1" ON zztele ( ort, str ) 

select distinct (ort) 

from zztele 


NO STRATEGY NOW (ONLY AT EXECUTION 

TIME) 

Strategy will be determined first during execution of the command 

This is valid for queries if the access path will be determined first when they are 

executed. 

Usually this output is used for queries containing sub-queries or correlated subqueries: 

strategy will first be determined when interim results become available. 


SUBQUERY allows CONDITION FOR KEY 


WHERE name IN (SELECT name FROM zzmaster WHERE year = 2000) 

Internal 

Subquery File 

Key Value 1 

Key Value 2 

Key Value 3 

Key Value 4 

Key Value 5 


‘Aimonsri' 

Base Table

SUBQUERY – Example (1) 

EXPLAIN SELECT * FROM zztele 

WHERE name IN 

(SELECT name FROM zzmaster 

WHERE year = 2000) 


SUBQUERY allows CONDITION FOR INDEX 


SELECT * FROM zztele WHERE plz IN 

(SELECT plz FROM zzstadtteil WHERE stadtteil = 'Kreuzberg' ) 

Internal 

Subquery 

File 

Value 1 

Value 1 

Value 1 

Value 2 

Value 3 

Value 3 

Value 4 


Single 

Index 

Index List 

val 1 

Index List 

val 3 



Base Table

SUBQUERY – Example (2) 




WHERE plz IN 

(SELECT plz FROM zzstadtteil 

WHERE stadtteil = 'Kreuzberg' )

Nice to know 

SELECT plz FROM zzstadtteil WHERE stadtteil like ‚%berg' 

SELECT * FROM zzmaster WHERE vorname = 'Lars' 


WHERE name IN ('Aimonsri', 'Hofmann', 'Lueck', 'MORONI', 'Reijer') 

AND vorname IN ( 'Jan', 'Walter') 

SELECT * FROM zzmaster 

WHERE year + 1 = 2001 


SELECT * FROM zzmaster 

WHERE year = 2001 - 1

© SAP AG 2009. All rights reserved. / Page 50 

Questions and Answers

Thank You! 

Bye, Bye – And Remember Next Session 


Feedback and further information: 

http://www.sdn.sap.com/irj/sdn/maxdb 

Next Session: 29.08.2012 

SAP® MaxDB SQL Query Optimization - Part 2

Disclaimer 

This presentation reflects current planning. 


Contents may be changed 

without prior notice, and are in no way 

binding upon SAP.

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© SAP 2012 SAP MaxDB Expert Session – Optimizer (Part 1)53

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