Oracle Optimizer
Types of Optimizers
• There are different modes for the optimizer
ALTER SESSION SET optimizer_mode =
{choose|rule|first_rows(_n)|all_rows}
• RULE: Rule-based optimizer (RBO)
– Deprecated; not updated since 1994
• CHOOSE: Default option. Chooses cost-based
optimizer (CBO) if statistics available, rulebased optimizer otherwise.
Types 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
Analyzing the Data
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;
Viewing the Execution Plan
• You need a PLAN_TABLE table. So, the first
time that you want to see execution plans, run
the command:
@$ORACLE_HOME/rdbms/admin/utlxplan.sql
• Set autotrace on to see all plans
– Display the execution path for each query, after
being executed
Operations that Access Tables
• 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 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 Indexes
• 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 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? (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 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
Hints
• 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 indexbased scan on the specified table
Examples
Select /*+ FULL (sailors) */ sid
From sailors
Where sname=‘Joe’;
Select /*+ INDEX (sailors) */ sid
From sailors
Where sname=‘Joe’;
Select /*+ INDEX (sailors s_ind) */ sid
From sailors S, reserves R
Where S.sid=R.sid AND sname=‘Joe’;
Combining Output From
Multiple Index Scans
• Suppose we have 2 indexes: sname, rating
• select * from sailors
where sname = 'Jim' and rating = 10
----------------------------------
0
SELECT STATEMENT Optimizer=CHOOSE
1
0
TABLE ACCESS (BY INDEX ROWID) OF 'SAILORS‘
2
1
3
2
INDEX (RANGE SCAN) OF 'SNAME_IND' (NON-UNIQUE)
4
2
INDEX (RANGE SCAN) OF 'RAT' (NON-UNIQUE)
AND-EQUAL
Operations that Manipulate Data Sets
• 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 Sets
– 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 accessed?
What if there is no index?
Operations that Manipulate Data Sets
• Consider the query:
– select sname from sailors
union
select bname from boats;
Operations that Manipulate Data Sets
• Consider the query:
– select sname from sailors
minus
select bname from boats;
How do you think
that Oracle
implements
intersect?
union all?
Operations that Manipulate Data Sets
• Select MIN(age), COUNT(*)
from Sailors
GROUP BY rating
-----------------------------------------0
SELECT STATEMENT Optimizer=CHOOSE
1
0
2
1
SORT (GROUP BY)
TABLE ACCESS (FULL) OF 'SAILORS'
Distinct
• What should Oracle do when processing
the query (assuming that sid is the
primary key):
– select distinct sid
from Sailors
• Try it!!
Join 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 Joins
• Block nested loop join
NESTED LOOPS
TABLE ACCESS FULL OF our_outer_table
TABLE ACCESS FULL OF our_inner_table
• Index nested loop join
NESTED 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 Joins
• Select *
from reserves r, sailors s
where r.sid=s.sid;
------------------------------------------------------0
SELECT STATEMENT Optimizer=CHOOSE
1 0 NESTED LOOPS
2 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 Joins
Used?
• If tables are of unequal size
• If results should be returned online
Hash Join Plan
HASH JOIN
TABLE ACCESS FULL OF table_A
TABLE ACCESS FULL OF table_B
When Are Hash Joins Used?
• If tables are small
• If results should be returned online
Sort-Merge Join Plan
MERGE JOIN
SORT JOIN
TABLE ACCESS FULL OF table_A
SORT JOIN
TABLE ACCESS FULL OF table_B
When Are Sort/Merge Joins
Used?
• Performs badly when tables are of
unequal size. Why?
Hint Examples for Joins
Select /*+ USE_NL (sailors) */ sid
From sailors S, reserves R
Where S.sid=R.sid AND sname=‘Joe’;
inner
table
Select /*+ USE_MERGE (sailors, reserves) */
sid
From sailors S, reserves R
Where S.sid=R.sid AND sname=‘Joe’;
Select /*+ USE_HASH */ sid
From sailors S, reserves R
Where S.sid=R.sid AND sname=‘Joe’;
XML (Extensible Markup Language)
and
the Semi-Structured Data Model
Motivation
• 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 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 Web
• In order to query the web, we must be
able to understand it.
• 2 Computer Science Approaches:
– Artificial Intelligence Approach
– Database Approach
Database 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
Example XML Document
<addresses >
<person friend="yes">
Opening
Tag
<name> Jeff Cohen</name>
<tel> 04-828-1345 </tel>
<tel> 054-470-778 </tel>
<email> [email protected] </email>
</person>
Attribute
<person friend="no">
<name> Irma Levy</name>
<tel> 03-426-1142 </tel>
<email>[email protected]</email>
</person>
</addresses>
Closing
Tag
Element
Very 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. 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
Semistructured 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-Structured
Relational
Semi-Structured
Abstract
Sets of
Labeled Directed
Model
tuples
Graph
Concrete
Tables
XML Documents
Standard
Storing
Data Exchange
for
Data
Separating Content
Model
from Style
Data 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 Style
• 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 Everywhere
XML Data
XSL
WML
(hand-held
devices)
HTML
(web browser
TEXT
(Excel)
Using 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