understanding explain plans dan hot ka

8/13/2019 Understanding Explain Plans Dan Hot Ka

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Dan HotkaAuthor Instructor

Oracle ACE Director


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Dan Hotka

Author/Instructor/Oracle Expert

CEO

[email protected]

515 279 3361

Last Updated: 4/2006


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Dan is a Training

Consultant Dan Hotka

Oracle Authored Expert

31 Years in IT 26 years working with Oracle

11 books hundreds of articles

www.DanHotka.com

Training at your Desk! Web-based training

Morning lectures with hands-on lab exercises Almost like being there!

Flat Fee Training1 Course Fee Price forup to 15 Attendees!

Price includes my portable computer lab!

On-site or over the Web!

Training Courses Include: Oracle Advanced PL/SQL

Oracle SQL Tuning

TOAD and Unix/Linux

Intro to Oracle, Intro to PL/SQL Oracle Discoverer/Oracle

Analytics

Register for my quarterlyNewsletter Full of tips and techniques,

downloads, book reviews, etc


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Training at your Desk!

Web-based Training at your Desk!

Feb 22 25 Intro to PL/SQL

March 7 - 11 SQL Performance Tuning

April 4 - 8 Advanced PL/SQL April 18 21 Intro to PL/SQL

May 2 - 6 SQL Performance Tuning

June 6 - 10 Advanced PL/SQL



Additional Reading

Oracle SQL Tuning:

A Close Look at

Explain Plans

By Dan Hotka; www.Amazon.com



New Book

ISBN:978-0321649102



Introductions

Please tell me a little about your background Show of Hands:

DBAs

Developers

Working with: Oracle11, 10, 9, 8, 7 prior to 7?

Years of Oracle experience < 1, 1-3, 4-8, 8+

No tuning experience

Some tuning experience

Extensive tuning experience



Agenda

Explain Plan Tools

Xplan.display

JS Tuner

Understanding Explain Plans Reading the Explain Plan

What does it mean

Parallel Explain Plan Review



Additional Information

Ask me for:

White Papers

Free Tools

Scripts [email protected]



Explain Plan Tools



Explain Plan Tools

DBMS_XPLAN.DISPLAY

Available with Oracle 8.1.7+

Comes with Database

Used via SQL*Plus

JS Tuner

Available for Oracle8+ Download from www.DanHotka.com

Executable Jar File


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Explain Plan Tools

PLAN_TABLE

Use /rdbms/admin/utlxplan.sql for current

plan_table

Use the explain plan for syntax to populate this table Use tools to populate this table (TOAD, SQL Developer)

Use SHOW_PLAN.sql to display contents

Available on my website


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Using the Tools

All tools use the

PLAN_TABLE /rdbms/admin

/utlxplan.sql


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Explain Plan Tools


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Explain Plan Tools

JSTuner

Incorporates SQL Scripts with enhanced Trace

Index Info

Includes index statistics Enhanced Explain Plan

Works with V8+

Works with rule-based optimizer!

Available from www.DanHotka.com


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Understanding theExplain Plan


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Reading Explain Plans

Oracle:

Parses the SQL Checks SQL syntax

Checks for available indexes/stats

Reads from bottom to the top

Arrives at an Execution Plan

Decides how it will access the tables and indexes

Executes the SQL

Oracle9/10 peaks once at bind variables Oracle11 tracks explain plans with various bind vars

Called Bind Aware

Runs the Execution plan


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Explain Plan

Visualizes the execution plan

Uses the Plan_Table

Which varies slightly from release to release Utlxplan.sql

/RDBMS/ADMIN

Used heavily to tune SQL


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How does it work?

Reads from bottom up Syntax checks/tracks available indexes

Regular queries - generally does the table joins first

Utilizes Where clause predicates

Parallel queries generally does partition pruning first

Sometimes accesses tables again Oracle does rewrites

Views to joins, sub queries to joins, sub queries to inline

views, etc


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How does it work?

RBO - follows rules for index selection Arrives at an execution plan in single pass

CBO - tries a variety of combinations

Combinations called permutations Generally # perms = # tables * # where predicates

Regular queries: Uses lowest cost method

*** Queries in this course are regular queries unless otherwisenoted

Parallel queries (covered in my class)

Uses fastest access method


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CBO Permutations

9i+ Query Transformations Complex View Merging

Converts views to joins

Subquery Unesting

Converts subqueries to inline views

Join Predicate Push Down

Moves where clause predicates into subquery

- Hints for each item above- No_Query_Transformation prevents this behavior

- Discussed later in this chapter


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Single Table SQL

Single Table SQL


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Single Table SQL

Single Table SQL Answers


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Multiple Table SQL

Multiple Table SQL

Multiple Table SQL


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Multiple Table SQL

Answers

Multiple Table SQL Answers


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Access Rule Description

AND-EQUAL Index values will be used to join rows.

CONCATENATION SQL statement UNION command. FILTER FILTERs apply other criteria in the query to further qualify the

matching rows. The other criteria include correlated sub queries, and HAVINGclause.

TABLE ACCESS When not associated with a join condition, they act like

Filterprocessing additional Where Clause predicates. VIEW OF Processed SQL from a view.

FOR UPDATE SQL statement clause for update of placed row level locks onaffected rows.

INDEX (UNIQUE) SQL statement utilized a unique index to search for aspecific value.

INDEX (RANGE SCAN) SQL statement contains a non-equality or BETWEENcondition.


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INLIST ITERATOR SQL statement has an IN clause, or, values being treated as

an IN clause HASH JOIN SQL statement initiated a hash-join operation.

MERGE JOIN SQL statement references two or more tables, sorting the tworesult sets being joined over the join columns and then merging the results viathe join columns.

MERGE JOIN (CARTESIAN) SQL statement references two or more tablesbut without a joining column (generally not a good thing)

NESTED LOOPS This operation is one form of joining tables, as opposed toa merge join. One row is retrieved from the row source identified by the first

child operation, and then joined to all matching rows in the other table,identified in the second child operation.

NONUNIQUE INDEX (RANGE SCAN) The RANGE SCAN optionindicates that ORACLE expects to return multiple matches (ROWIDs) fromthe index search


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BITMAP CONVERSION Bitmap Index being merged

BITMAP MERGE Generally used with Bitmap Range Scan

BITMAP MINUS Bitmap Index handling a not = condition.

BITMAP INDEX SINGLE VALUE Bitmap index being used for

single value lookup. BITMAP INDEX (RANGE SCAN) Bitmap index being used for

multiple value lookup.


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PARTITION (CONCATTENATED) SQL statement will access a

partitioned object and merge the retrieved rows from the accessed partitions. PARTITION (SINGLE) SQL statement will access a single partition.

PARTITION (EMPTY) The SQL statement makes reference to an emptypartition.

SORT (ORDER BY) SQL statement contains an ORDER BY SORT(AGGREGATE) SQL statement initiated a sort to resolve a MIN or MAXtype function.

SORT (GROUP BY) SQL statement contains a GROUP BY

TABLE ACCESS (FULL) All rows are retrieved from the table withoutusing an index.

TABLE ACCESS (BY ROWID) A row was retrieved from a table based onthe ROWID of the row.


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Index Scans Unique Scan

via root -> branch -> leaf for a single row access Range Scan

via root -> branch -> leaf for first row, then leafto leaf for remaining rows

Full Scan (Index-Full)

Scans leaf blocks using single-block access

Fast Full Scan (Index-FFS)

scans all index leaf blocks using multi-blockread

Index Skip Scan (Index-SS)

Useful for multi-column indexes, accessing only

2nd column and first column has low cardinality

R di E l i Pl


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More Information:

Database Performance Tuning Guide & Reference

Chapter 9 Using Explain Plans

Oracle Documentation

Download from www.Oracle.com search on Documentation

R di E l i Pl


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Oracle only joins 2 tables at a time

The smaller the initial result sets, the faster thewhole query runs

Drive off the item that will eliminate the most rows first

R di E l i Pl


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Nested Loops

Inner table looped for each rowreturned in outer table

Lg table should be outer

Sm table (or unique indexed

lookup) should be inner

Rows returned to the result set

that qualify the driving WHERE

clause

Cost = outer access + (inner

table access * outer

cardinality)

Merge Scan Join

Both tables are sorted

Rows are inserted into result setbased on key value

THEN WHERE clause applied

Cost = outer access + inner

access + sort costs Hash Join

Hashes join keys and cachesobject into a hash table

Driving table should be smaller ofthe 2

Cost = inner cost + (outer cost *inner cardinality/hash partitions)



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Nested Loop Join

driving table

Default order(rule)

Merge Scan Join

sort & match

Hash Join

Full scans with no

sorts

Join column to rowaddress

R di E l i Pl


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Nested Loop Join

If join condition is ANDed, make a compound index on the inner table

Inner and Outer join column should have same data type

Outer Table: Larger of Result set

Inner Table: Smaller of Result set

Foreign key indexes helps CBO choose between nested loops and hash joins


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Join Formula


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Join Formula

RBO Join Formula

Indexes, type of indexes, Full Table Scans

CBO Join Formula High # values matching High # values

Tends to choose Merge Join

High # values matching Low # values

Tends to choose Nested Loop Low # values matching Low # values

Tends to choose Hash Join

The Oracle Optimizers


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Oracle Optimizer gives you choices:

Rule-based

Based on a set of rules (Index existence, SQL coding)

Does NOT consider object statistics

Cost-based

Uses object statistics (from DBMS_STATS command)

User has more control in tuning

2 goals:

First row: response time for interactive apps (OLTP)

All rows: throughput for batch processing (DSS)

Makes Assumptions

Hints

Cost Based directives, always used unless Oracle cant use them

Can be specified in combinations


The Oracle OptimizersC t b d O ti i


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Cost-based Optimizer

Cost based Optimizer


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Phys IO + Logical IO/1000 + NetIO*1.5

Cost Factor is an estimated number of physical I/O operations Oracle thinks it

will have to do, based on statistics. Assumes even data distribution when using

indexes (unless histograms are implemented). Costs out each join to select driving

table

Cost Factor Heavily relies upon assumed multiblock read-ahead

8i Cost reflects number of multiblock reads based on statistics

9i Cost reflects machine speed at performing multiblock reads along with 8i10g Takes into account high water mark,

10g index_fast_full_scans considers number of leaf blocks

Cost based Optimizer


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CBO Formulas

Mra Multi block read ahead Selectivity discussed with Where Clause section

Clustering Factor discussed with Index section

Full Table Scan = num rows / mra Fast Full Scan = num rows / mra

Unique Scan = #branch levels + 1

Range Scan = (#branch levels + selectivity) * (#leaf blocks +selectivity) * Clustering Factor

Index-Only Scan = (#branch levels + selectivity) * #leaf blocks

Source: Fallacies of the CBO: Wulfgang Brietling



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Complete database doc set

www.Oracle.com

Search on Documentation

380,000+ mbytes zip file

www.oracle.com/technology/documentation/database.html

Has doc sets back to Oracle9r2

View Oracle Documentation

SQL Tuning Class


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SQL Tuning Class

Hands-on Labs

Instructor-led courses

Understanding Explain Plans

Index Review

How they are used/not used

Controlling the Explain Plan

Using the Tools

Oracle Trace

StatsPack AWR

PL/SQL Tuning/Profiling

SGA Topics

Cursor management Multiple buffer caches

Result Cache

Finding the SQL problems

SQL Tuning Tips and Techniques

Training at your Desk! (sm)

Onsite

What have we learned?


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What have we learned?

Explain Plan Tools

Xplan.display

JS Tuner

Understanding Explain Plans Thank you! Keep in touch

understanding explain plans dan hot ka

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