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Intro to Query Optimization DB2 UDB for iSeries. Tom McKinley IBM Rochester, MN USA. Background / Foundation. IBM's DB2 UDB Family. Three code bases... Based on the system history, architecture and operating system DB2 UDB for Linux, UNIX, Windows (LUW) DB2 UDB for z/OS (S/390)

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intro to query optimization db2 udb for iseries

Intro to Query OptimizationDB2 UDB for iSeries

Tom McKinley

IBM Rochester, MN USA

ibm s db2 udb family
IBM's DB2 UDB Family
  • Three code bases...
    • Based on the system history, architecture and operating system
    • DB2 UDB for Linux, UNIX, Windows (LUW)
    • DB2 UDB for z/OS (S/390)
    • DB2 UDB for iSeries (AS/400)
db2 udb for iseries
DB2 UDB for iSeries
  • i5 + i5/OS
    • System viewed as a database server, not just an application system
    • DB2 UDB for iSeries (integrated part of OS/400 or i5/OS)
    • Universal Database support
    • Data Centric focus
    • Business logic moving into the database engine
    • SQL (DDL and DML) as primary interface to database
    • GUI to operating system and database via iSeries Navigator
iseries logical partitioning lpar
iSeries - Logical Partitioning (LPAR)

LPAR-1

LPAR-2

LPAR-3

IXS/IXA

i5/OS

Linux

AIX

Windows***

DB2

UDB

for

iSeries

DB2

UDB

for

Linux

DB2

UDB

for

AIX

DB2

UDB

for

Win

Virtual 1Gbit Ethernet LAN

*** No LPAR support

iseries i5 i5 os architecture
iSeries i5 i5/OS Architecture

M

E

M

O

R

Y

Multiple CPUs

N-way

SMP

QUERY

Single

System

Single Level Storage

64 bit

POWER

Storage Management

IOP

IOP

IOP

IOP

IOP

IOP

IOP

Table

i5 os objects
i5/OS Objects

SQL

i5/OS

library

schema/collection

physical file

table

logical file

view

keyed logical file

index

record

row

field

column

journal

log

i5 os objects8
i5/OS Objects

SELECT...

FROM Physical File

Library (Schema)

CREATE ALIAS...

Physical File (Table)

Member 1

SELECT...

FROM Alias_1

Alias_1

Member 2

SELECT...

FROM Alias_2

Alias_2

Member 3

SELECT...

FROM Alias_3

Alias_3

i5 os objects9
i5/OS Objects
  • System
    • Library
      • Object
        • Type
          • Attribute (subtype)
  • System
    • My_Schema
      • DB_Table
        • *FILE
          • PF (physical file)
  • System
    • My_Schema
      • DB_Index
        • *FILE
          • LF (logical file)
  • System
    • My_Schema
      • DB_View
        • *FILE
          • LF (logical file)

Must be unique

CREATE TABLE My_Schema.DB_Table ...

CREATE INDEX My_Schema.DB_Index ...

CREATE VIEW My_Schema.DB_View ...

i5 os objects10
i5/OS Objects

One Database Management System with multiple interfaces

  • Structured Query Language (SQL)
    • Embedded
    • ODBC
    • JDBC
    • CLI

Command Language (CL)

DB2

DB File (PF) object

CREATE TABLE

CRTPF

SELECT...

FROM...

High

Level

Language

Native I/O

sql query processing
SQL Query Processing

SQL request

Optimize

Open

Run

DB2 UDB for iSeries

v5r1 database architecture
V5R1 Database Architecture

ODBC / JDBC / ADO / DRDA / XDA

Network

Host Server

CLI / JDBC

Static

Dynamic

Extended Dynamic

The optimizer and database engine are separated at different layers of the operating system

Compiled embedded statements

Prepare once and then reference

Prepare every time

SQL

Native

(Record

I/O)

Optimizer

DB2 UDB

(Data Storage & Management)

v5r2 and v5r3 database architecture
V5R2 and V5R3 Database Architecture

ODBC / JDBC / ADO .NET / DRDA / XDA

Network

Host Server

CLI / JDBC

The optimizer and database engine merged to form the SQL Query Engine, and much of the work was moved to SLIC

Static

Dynamic

Extended Dynamic

Compiled embedded statements

Prepare once and then reference

Prepare every time

SQL

Native

(Record

I/O)

Optimizer

DB2 UDB

(Data Storage & Management)

the query dispatcher
The Query Dispatcher
  • Determines which engine will optimize and process each query request
    • Only SQL requests are considered for the SQL Query Engine
  • Initial step for all query optimization that occurs in i5/OS
  • Ability to “back up” and use the Classic Query Engine when non-standard indexes are encountered during optimization
  • Initial goal is to use SQE
the query dispatcher v5r2
The Query Dispatcher – V5R2
  • Dispatched to CQE if:
    • >1 Table (i.e. no joins)
    • OR & IN predicates
    • SMP requested
    • Non-Read (INSERT with subselect can use new path)
    • LIKE predicates
    • UNIONS
    • View or Logical File references
    • Subquery
    • Derived Tables & Common Table expressions, UDTFs
    • LOB columns
    • LOWER, TRANSLATE, or UPPER scalar function
    • CHARACTER_LENGTH, POSITION, or SUBSTRING scalar function using UTF-8/16
    • Sort Sequences & CCSID translation between columns
    • Distributed queries via DB2 Multisystem
    • Non-SQL queries (QQQQry API, Query/400, OPNQRYF)
    • ALWCPYDTA(*NO) specified
    • Sensitive Cursor

SQE support added into V5R2 - May 2003

(Latest DB Group + SI07650)

Not part of any package

the query dispatcher v5r3
The Query Dispatcher - V5R3
  • Dispatched to CQE if:
    • LIKE predicates
    • Logical File references
    • UDTFs
    • LOB columns
    • LOWER, TRANSLATE, or UPPER scalar function
    • CHARACTER_LENGTH, POSITION, or SUBSTRING scalar function using UTF-8/16
    • Sort Sequences & CCSID translation between columns
    • DB2 Multisystem
    • Non-SQL queries (QQQQry API, Query/400, OPNQRYF)
    • ALWCPYDTA(*NO) specified
    • Sensitive Cursor
  • SQE now optimizes
    • VIEWS, UNIONS, SubQueries
    • INSERT, UPDATE, DELETE
    • Star Schema Join queries
  • Only SQE optimizes
    • INTERSECT
    • EXCEPT
the query dispatcher19
The Query Dispatcher
  • Back up to CQE to complete optimization if any of the following are encountered:
    • Select/omit logical file
    • Logical file over multiple members
    • Join logical file
    • Derived key (s)
      • Native logical files that perform some intermediate mapping of the fields referenced in the key. Common ones are renaming fields, adding a translate or only selecting a subset of the columns
      • Specifying an Alternate Collating Sequence (ACS) on a field used for a key will also make a “derived key” (an implied map occurs within the index)
    • Sort Sequence (NLSS) specified for index or logical file
      • Probably the trickiest one to detect for users. The index is built while an NLSS table is specified in the query environment
    • Cost to “back up” and revert to CQE adds about 15% to the total optimization time
    • QAQQINI parameter to ignore unsupported logical files
      • Ignore_Derived_Index = *YES
optimization
Optimization
  • The Optimizer
    • Writes the best? program to fulfill your request
  • The Optimizer
    • Provides the recipe
    • Provides the methods
    • Does no cooking
optimization the intersection of various factors
Optimization... the intersection of various factors

Server attributes

Server configuration

Version/Release/Modification

Level

Server performance

The Plan

SMP

Database design

Job, Query attributes

Table sizes, number of rows

SQL Request

Static

Dynamic

Extended Dynamic

Interfaces

Views and Indexes (Radix, EVI)

Work management

query access plans
(Query) Access Plans
  • The output of query optimization (“the recipe and methods”)
  • Contents
  • A control structure that contains information on the actions necessary to satisfy each SQL request
  • These contents include:
    • Access Method
    • Info on associated tables and indexes
    • Any applicable program and/or environment information
query optimization
Query Optimization
  • Cost Based Query Optimization
  • The DB2 for iSeries Optimizer performs "cost based" optimization
  • "Cost" is defined as the estimated time it takes to run the request
  • "Costing" various plans refers to the comparison of a given set of algorithms and methods in an attempt to identify the "fastest" plan
  • Optimization is based on time, not on resource utilization
  • Usually the fastest plan is also the most resource efficient plan, but this is not necessarily true
  • The goal of the optimizer is to eliminate I/O as early as possible by identifying the best path to and through the data
  • The optimizer has the ability and freedom to "rewrite" the query
query phases
Query Phases
  • Query processing can be divided into four phases:
    • Query Validation
      • Validate the query request
      • Validate existing access plan
      • Builds internal query structures
    • Query Dispatcher
      • Determine which query engine should complete the processing
    • Query Optimization
      • Choose most efficient access method
      • Builds access plan
    • Query Execution
      • Build the structures needed for query cursor
      • Build the structures for any temporary indexes (if needed)
      • Builds and activates query cursor (ODP)
      • Generate any feedback requested
        • Debug messages in the job log
        • DB Monitor records
        • Visual Explain

We can affect this...

query optimization feedback
Query Optimization Feedback

SQE Plan Cache

Visual

Explain

DB Monitor Data

SQL request

Joblog Messages

Query Optimization

SQL Info from PGMs & PKGs

data access methods
Data Access Methods

Cost based optimization dictates that the fastest access method for a given table will vary based upon selectivity of the query

High

Response

Time

Method 3

Method 2

Method 1

Low

Few

Many

Number of rows searched / accessed

strategy for query optimization
Strategy for Query Optimization
  • Query optimization will generally follow this simplified strategy:
    • Gather meta-data and statistics for costing
      • Selectivity statistics
      • Indexes available to be costed
        • Sort the indexes based upon their usefulness
      • Environmental attributes that may affect the costs
    • Generate default cost
      • Build an access plan associated with the default plan
    • For each index:
      • Gather information needed specific to this index
      • Build an access plan based on this index
      • Cost the use of the index with this access plan
      • Compare the resulting cost against the cost from the current best plan
strategy for query optimization28
Strategy for Query Optimization
  • Optimizing indexes will generally follow this simplified strategy:
    • Gather list of indexes for statistics and costing
    • Sort the list of indexes considering how the index can be used
      • Local selection
      • Joining
      • Grouping
      • Ordering
      • Index only access
    • One index may be useful for statistics, and another useful for implementation
statistics
Statistics
  • All query optimizers rely upon statistics to make plan decisions
    • DB2 UDB for the iSeries has always relied upon indexes as its source for stats
    • Other databases rely upon manual stats collection for their source
  • SQE offers a hybrid approach where column stats will be automatically collected for cases where indexes do not already exist
sources of information
Sources of Information
  • Meta-data sources
    • Existing indexes (Radix or Encoded Vector)
      • More accurately describes multi-column key values
      • Stats available immediately as the index maintenance occurs
      • Selectivity estimates from radix by reading n keys
      • Selectivity from EVI by reading symbol table values
    • Column Statistics
      • SQE only
      • Column Cardinality, Histograms & Frequent Values List
      • Constructed over a single column in a table
      • Stored internally as a part of the table object after created
      • Collected automatically by default for the system
      • Stats not immediately maintained as the table changes
      • Stats are refreshed as they become “stale” over time
  • Default sources
    • No representation of actual values in columns

Best

Worst

sqe automatic stats collection
SQE Automatic Stats Collection
  • i5/OS Statistics collection job
    • Reactive, based on query requests
    • Automatic collection runs in this background job at very low priority
      • QDBFSTCCOL system job
    • Statistics Manager continuously analyzes entries in the Plan Cache and queues up requests for the collection job
    • Controlled by system value QDBFSTCCOL
  • iSeries Navigator graphical interface to manage stats collected by the system
    • API’s also provided to manage the stats
review
Review
  • What is the optimizer's job?
  • What is the optimizer's output?
  • What are some of the key elements used for cost based optimization?
  • What things affect the Access plan?
  • Look at resources used as well as response time.
trademarks and disclaimers
Trademarks and Disclaimers

IBM Corporation 1994-2005. All rights reserved.

References in this document to IBM products or services do not imply that IBM intends to make them available in every country.

The following terms are trademarks of International Business Machines Corporation in the United States, other countries, or both:

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Java and all Java-based trademarks are trademarks of Sun Microsystems, Inc. in the United States, other countries, or both.

Microsoft, Windows, Windows NT, and the Windows logo are trademarks of Microsoft Corporation in the United States, other countries, or both.

Intel, Intel Inside (logos), MMX and Pentium are trademarks of Intel Corporation in the United States, other countries, or both.

UNIX is a registered trademark of The Open Group in the United States and other countries.

SET and the SET Logo are trademarks owned by SET Secure Electronic Transaction LLC.

Other company, product or service names may be trademarks or service marks of others.

Information is provided "AS IS" without warranty of any kind.

All customer examples described are presented as illustrations of how those customers have used IBM products and the results they may have achieved. Actual environmental costs and performance characteristics may vary by customer.

Information concerning non-IBM products was obtained from a supplier of these products, published announcement material, or other publicly available sources and does not constitute an endorsement of such products by IBM. Sources for non-IBM list prices and performance numbers are taken from publicly available information, including vendor announcements and vendor worldwide homepages. IBM has not tested these products and cannot confirm the accuracy of performance, capability, or any other claims related to non-IBM products. Questions on the capability of non-IBM products should be addressed to the supplier of those products.

All statements regarding IBM future direction and intent are subject to change or withdrawal without notice, and represent goals and objectives only. Contact your local IBM office or IBM authorized reseller for the full text of the specific Statement of Direction.

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