Oracle OptimizerOracle Optimizer

Types of OptimizersTypes of Optimizers

• There are different modes for the optimizer

• RULE: Rule-based optimizer (RBO)– Deprecated; not updated since 1994

• CHOOSE: Default option. Chooses cost-based optimizer (CBO) if statistics available, rule-based optimizer otherwise.

ALTER SESSION SET optimizer_mode = {choose|rule|first_rows(_n)|all_rows}

Types of OptimizersTypes of Optimizers

• ALL_ROWS (CBO): Execute the query so that all of the rows are returned as quickly as possible– Merge Join has priority over Block Nested Loop Join

• FIRST_ROWS(n) (CBO): execute the query so that all of the first n rows are returned as quickly as possible– Block Nested Loop Join has priority over Merge

Join

analyze table | index

<table_name> | <index_name>

compute statistics |

estimate statistics [sample <integer>

rows | percent] |

delete statistics;

analyze table Sailors estimate statistics sample 25 percent;

Analyzing the DataAnalyzing the Data

Viewing the Execution PlanViewing the Execution Plan

• You need a PLAN_TABLE table. So, the first

time that you want to see execution plans,

run the command:

• Set autotrace on to see all plans

– Display the execution path for each query, after

being executed

@$ORACLE_HOME/rdbms/admin/utlxplan.sql

Operations that Access Operations that Access TablesTables

• TABLE ACCESS FULL: sequential table scan

– Oracle optimizes by reading multiple blocks

– Used whenever there is no where clause on a

query

select * from Sailors

• TABLE ACCESS BY ROWID: access rows by

their RowID values.

– How do you get the rowid? From an index!

select * from Sailors where sid > 10

Types of IndexesTypes of Indexes

• Unique: each row of the indexed table

contains a unique value for the

indexed column

• Nonunique: the row’s indexed values

can repeat

Operations that Use Operations that Use IndexesIndexes

• INDEX UNIQUE SCAN: Access of an

index that is defined to be unique

• INDEX RANGE SCAN: Access of an

index that is not unique or access of a

unique index for a range of values

When are Indexes Used/Not When are Indexes Used/Not Used?Used?

• If you set an indexed column equal to a value, e.g., sname = 'Jim'

• If you specify a range of values for an indexed column, e.g., sname like 'J%'– sname like '%m': will not use an index

– UPPER(sname) like 'J%' : will not use an index

– sname is null: will not use an index, since null values are not stored in the index

– sname is not null: will not use an index, since every value in the index would have to be accessed

When are Indexes Used? When are Indexes Used? (cont)(cont)

• 2*age = 20: Index on age will not be used. Index on 2*age will be used.

• sname != 'Jim': Index will not be used.

• MIN and MAX functions: Index will be used

• Equality of a column in a leading column of a multicolumn index. For example, suppose we have a multicolumn index on (sid, bid, day)– sid = 12: Can use the index

– bid = 101: Cannot use the index

Optimizer HintsOptimizer Hints

• You can give the optimizer hints about

how to perform query evaluation

• Hints are written in /*+ */ right after

the select

• Note: These are only hints. The Oracle

optimizer can choose to ignore your

hints

HintsHints

• FULL hint: tell the optimizer to perform a

TABLE ACCESS FULL operation on the

specified table

• ROWID hint: tell the optimizer to perform a

TABLE ACCESS BY ROWID operation on the

specified table

• INDEX hint: tells the optimizer to use an

index-based scan on the specified table

ExamplesExamples

Select /*+ FULL (sailors) */ sidFrom sailorsWhere sname=‘Joe’;

Select /*+ INDEX (sailors) */ sidFrom sailorsWhere sname=‘Joe’;

Select /*+ INDEX (sailors s_ind) */ sidFrom sailors S, reserves RWhere S.sid=R.sid AND sname=‘Joe’;

Combining Output From Combining Output From Multiple Index ScansMultiple Index Scans

• Suppose we have 2 indexes: sname, rating

• select * from sailorswhere sname = 'Jim' and rating = 10----------------------------------

0 SELECT STATEMENT Optimizer=CHOOSE

1 0 TABLE ACCESS (BY INDEX ROWID) OF 'SAILORS‘

2 1 AND-EQUAL

3 2 INDEX (RANGE SCAN) OF 'SNAME_IND' (NON-UNIQUE)

4 2 INDEX (RANGE SCAN) OF 'RAT' (NON-UNIQUE)

Operations that Manipulate Data Operations that Manipulate Data SetsSets

• Up until now, all operations returned the rows as they were found

• There are operations that must find all rows before returning a single row

• Try to avoid these operations for online users!– SORT ORDER BY: query with order by

select sname, age

from Sailors

order by age;

Operations that Manipulate Data Operations that Manipulate Data SetsSets

– SORT UNIQUE: sorting records while

eliminating duplicates (e.g., query

with distinct; query with minus,

intersect or union)

– SORT AGGREGATE, SORT GROUP BY:

queries with aggregate or grouping

functions (like MIN, MAX)

Is the table always Is the table always accessed?accessed?

What if there is no index?

Operations that Manipulate Data Operations that Manipulate Data SetsSets

• Consider the query:

– select sname from sailors

union

select bname from boats;

Operations that Manipulate Data Operations that Manipulate Data SetsSets

• Consider the query:

– select sname from sailors

minus

select bname from boats;

How do you think that

Oracle implements intersect?union all?

• Select MIN(age), COUNT(*)

from Sailors

GROUP BY rating

------------------------------------------0 SELECT

STATEMENT Optimizer=CHOOSE

1 0 SORT (GROUP BY)

2 1 TABLE ACCESS (FULL) OF 'SAILORS'

Operations that Manipulate Data Operations that Manipulate Data SetsSets

DistinctDistinct

• What should Oracle do when

processing the query (assuming that

sid is the primary key):

– select distinct sid

from Sailors

• Try it!!

Join MethodsJoin Methods

• Select * from Sailors, Reserves

where Sailors.sid = Reserves.sid

• Oracle can use an index on Sailors.sid

or on Reserves.sid (note that both will

not be used)

• Join Methods: MERGE JOIN, NESTED

LOOPS, HASH JOIN

Nested Loops JoinsNested Loops Joins

• Block nested loop join

NESTED LOOPS

TABLE ACCESS FULL OF our_outer_table

TABLE ACCESS FULL OF our_inner_table

• Index nested loop joinNESTED LOOPS

TABLE ACCESS FULL OF our_outer_table

TABLE ACCESS BY ROWID OF our_inner_table

INDEX RANGE SCAN OF inner_table_index

Nested Loops JoinsNested Loops Joins

• Select *from reserves r, sailors swhere r.sid=s.sid;

-------------------------------------------------------0 SELECT STATEMENT Optimizer=CHOOSE1 0 NESTED LOOPS2 1 TABLE ACCESS (FULL) OF 'RESERVES‘3 1 TABLE ACCESS (BY INDEX ROWID) OF 'SAILORS‘4 3 INDEX (UNIQUE SCAN) OF 'SYS_C00628777' (UNIQUE)

When Are Nested Loops When Are Nested Loops Joins Used?Joins Used?

• If tables are of unequal size

• If results should be returned

online

Hash Join PlanHash Join Plan

HASH JOINTABLE ACCESS FULL OF table_ATABLE ACCESS FULL OF table_B

When Are Hash Joins When Are Hash Joins Used?Used?

• If tables are small

• If results should be returned online

Sort-Merge Join PlanSort-Merge Join Plan

MERGE JOINSORT JOINTABLE ACCESS FULL OF table_ASORT JOINTABLE ACCESS FULL OF table_B

When Are Sort/Merge Joins When Are Sort/Merge Joins Used?Used?

• Performs badly when tables are

of unequal size. Why?

Hint Examples for JoinsHint Examples for Joins

Select /*+ USE_NL (sailors) */ sidFrom sailors S, reserves RWhere S.sid=R.sid AND sname=‘Joe’;

Select /*+ USE_MERGE (sailors, reserves) */ sidFrom sailors S, reserves RWhere S.sid=R.sid AND sname=‘Joe’;

Select /*+ USE_HASH */ sidFrom sailors S, reserves RWhere S.sid=R.sid AND sname=‘Joe’;

inner table

XML (Extensible Markup XML (Extensible Markup Language) Language)

andand

the Semi-Structured Data Modelthe Semi-Structured Data Model

MotivationMotivation

• We have seen that relational databases

are very convenient to query. However:

– There is a LOT of data not in relational

databases!!

• Perhaps the most widely accessed

database is the web, and it certainly

isn’t a relational database.

Querying the WebQuerying the Web

• The web can be queried using a search engine, however, we can’t ask questions like:– What is the lowest price for which a Jaguar

is sold on the web?

• Problems:– There are no facilities for asking complex

questions, such as aggregation of data

Understanding the WebUnderstanding the Web

• In order to query the web, we must be

able to understand it.

• 2 Computer Science Approaches:

– Artificial Intelligence Approach

– Database Approach

Database ApproachDatabase Approach

“The web is unstructured and we will structure it”

• Sometimes problems that are very difficult can be solved easily by enforcing a standard

• Encourage the use of XML as a standard for data exchange on the web

<addresses >

<person friend="yes">

<name> Jeff Cohen</name>

<tel> 04-828-1345 </tel>

<tel> 054-470-778 </tel>

<email> jeffc@cs.technion.ac.il </email>

</person>

<person friend="no">

<name> Irma Levy</name>

<tel> 03-426-1142 </tel>

<email>irmal@yourmail.com</email>

</person>

</addresses>

Example XML DocumentExample XML Document

Opening Tag

AttributeElement

Closing Tag

Very Unstructured XMLVery Unstructured XML

<?xml version=“1.0”?>

<DamageReport>

The insured’s <Vehicle Make = “Toyota”>

Corolla </Vehicle> broke through the guard rail and plummeted into the ravine. The cause was determined to be <Cause>faulty brakes </Cause>. Amazingly there were no casualties.

</DamageReport>

XML Vs. HTMLXML Vs. HTML

• XML and HTML are brothers. They are both

special cases of SGML.

• HTML has specific tag and attribute names.

These are associated with a specific meaning

• XML can have any tag and attribute name.

These are not associated with any meaning

• HTML is used to specify visual style

• XML is used to specify meaning

Characteristics of Characteristics of Semistructured DataSemistructured Data

(From http://www2.cs.uh.edu/~ceick/6340/intro-xml.ppt)

• Missing or additional attributes

• Multi-valued attributes

• Different types in different objects

• Heterogeneous collections

Self-describing, irregular data, no a priori structure

Relational vs Semi-Relational vs Semi-Structured Structured

RelationalSemi-Structured

Abstract

Model

Sets of

tuples

Labeled Directed

Graph

Concrete

Model

TablesXML Documents

Standard

for

Storing

Data

Data Exchange

Separating Content

from Style

Data ExchangeData Exchange

• Problem: Many data sources, each of a different type (different vendor), with a different schema. – How can the data be combined and used

together?

– How can different companies collaborate on their data?

– What format should be used to exchange the data?

Separating Content from Separating Content from StyleStyle

• Web sites develop over time

• Important to separate style from data in order to allow changes to the site structure and appearance

• Using XML, we can store data alone

• CSS separates style from data only in a limited way

• Using XSL, this data can be translated into HTML

• The data can be translated differently as the site develops

Write Once Use Write Once Use EverywhereEverywhere

XML Data

XSL

WML(hand-held

devices)

XSL

HTML(web browser

XSL

TEXT(Excel)

Using XMLUsing XML

• Quering and Searching XML: There are query languages and search engines that query XML and return XML. Examples: Xpath, Xquery /SQL4X, Equix, XSEarch

• Displaying XML: An XML document can have an associated style-sheet which specifies how the document should be translated to HTML. Examples: CSS, XSL