A Close Look at Index Internals

7/29/2019 A Close Look at Index Internals

1/32

1

A Close Look at Oracle8/8iIndex Internals

by

Dan HotkaDirector of Database Field

Operations


2/32

2

Dan Hotka

!

Renowned author, speaker, and Oracle expert Oracle Unleashed Oracle 8 Server Unleashed

Oracle Development Unleashed Special Edition Using Oracle 8/8i

!

New book: Oracle 8i from Scratch ISBN 0-7897-2369-7

! [email protected]


3/32

3

Todays Presentation! Introduction

! B*Tree indexes What is

Structure

Tips & techniques

! Unique indexes

!

Non-unique indexes! Reverse-key indexes

! Bitmapped indexes


4/32

4

B*Tree History

! Balanced-tree index

! Originally conceived by Bayer and McCreight in

1972

! Performs well with a consistent number of reads tolocate a row/series of rows


5/32

5

B*Tree Index Structure

77 --> 110

79 --> 120

756 --> 130

778 --> 410

787 --> 410

793 --> 500

7369 --> ROWID

7499 --> ROWID

7521 --> ROWID7566 --> ROWID

7654 --> ROWID

7698 --> ROWID

7782 --> ROWID7788 --> ROWID

7839 --> ROWID

7844 --> ROWID

7876 --> ROWID

7900 --> ROWID

7902 --> ROWID

7934 --> ROWID

Branch DBA=100

Branch DBA=110 Branch DBA=120

Leaf Leaf Leaf Leaf DBA=130 DBA=410 DBA=440 DBA=500


6/32

6

B*Tree Index Split77 --> 110

79 --> 120

756 --> 130

769 --> 410778 --> 600

787 --> 410

793 --> 500

7654 --> ROWID7698 --> ROWID

7839 --> ROWID7844 --> ROWID

7876 --> ROWID

7900 --> ROWID7902 --> ROWID

7934 --> ROWID

Branch DBA=100

Branch DBA=110 Branch DBA=120

Leaf Leaf Leaf DBA=130 DBA=410 DBA=440

7700 --> ROWID

7782 --> ROWID

7788 --> ROWID

Leaf DBA=600


7/32

7

Index Block LayoutType Format Filler DBA

SCN

Base

SCN

WrapSeq Flag Chk Val Filler

Tail

Obj ID SCN of LastCleanout

No of ITLSlots

Free ListFlag

BlockType

ITL Freelist SlotDBA of next block

on Freelist

Block

Header

CacheLayer

Trans

LayerITL Index Number Trans ID Undo Address ...

No of

Rows

Affected

Committed SCN/Free

Space Credits

Index Common Header

Index Row Info

...

Index Branch or

Leaf Block Info


8/32

8

Dump Commands

!

Unix dd bs=2k if=/ora8/data/users01.dbf skip=200 count =

5 | od -x > pg

! VMS

DUMP /BLOCKS = (START:, END:,os

block #>) /OUT = blockdmp.out users01.dbf


9/32

9

Dump Commands

!

NT HEDIT

! SVRMGR

alter system dump datafile 7 block 201


10/32

10

Index Common Header

Oracle Block Header

Index Branch or Leaf

Block Info

Index Branch or Leaf

Row Info

Field

Prefix=KDXCO

Index Common Header

Block

Level

...LEV

Free

Space

...AVS

# Key

Column

...NCO

End

Freespace

...OFEO

Begin

Freespace

...OFBO

Row

Count

...NRO

Split

Cntr

...SDC

Lock

Code

...OPC

ITL

...LOK


11/32

11

Branch Block Header

Index Common

Header

Field

Prefix=KDXBR

Index Branch Block

Left mostChild Block

...LMC

Block Size

...BKSZ

Last ServiceTrans

...SNO

Oracle Block Header

Branch Block

Header

Branch BlockRows

Row # Partial KeyCol LenColDBA



...{


12/32

12

Index Leaf Block Header

Oracle Block Header

Index CommonHeader

Index Leaf Row Info

Field

Prefix=KDXLE

Split

Holes

...SPL

IOT non-key

columns

...COL

Key

Size

...DSZ

Next Leaf

Block DBA

...NXT

Deleted

Rows

...NDE

Prev Leaf

Block DBA

...PRV


13/32

13

Index Rows: Non-Unique

Oracle Block Header

Index Common

Header

Index Leaf Block Info

Row # Key DataColumnFlag Length ROWIDColumn Length

Row # Key or NULLColumnFlag Length ROWIDColumn Length

Row # Key or NULLColumnFlag Length ROWIDColumn Length

...


14/32

14

Index Rows: Unique

Oracle Block Header

Index Common

Header

Index Leaf Block Info

Row # Key DataROWIDFlag Column Length

...




15/32

15

B*Tree Tips & Techniques

!

B*Tree not best suited for sequential key inserts Deletes leave gaps to the left of the index Leaves are maintained for future possible inserts

Possible index height change (additional I/Os) REVERSE KEY Indexes solves this problem

!

Unique Indexes Better individual row performance - stores justROWID


16/32

16

B*Tree Tips & Techniques

!

Non-unique Indexes Better range-scan performance - stores ROWID

pairs

! Reverse key indexes

Setup like non-unique

Oracle reads index blocks from left to right Oracle will NOT use a reverse-key Index on a range

scan!


17/32

17

B*Tree Space Management

!

Empty blocks not removed from index Makes for longer index searches

May increase the height of the index

! High PCTFREE

Assists Oracle by doing most of the block splitting

during the initial data load


18/32

18

B*Tree Space Management

!

Low PCTFREE Helps Oracle minimize block splits due to

inserts/updates by leaving some room in each index

block behind


19/32

19

B*Tree Space Management! PCTFREE = 0

Where index key is sequentially always increasing NOT for reverse-key indexes

! Oracle 8.1.6

Recognizes types of rows being added:

If random in nature - loads blocks 70% full

If sequential in nature - loads blocks 95% full

! Index Height Change

Candidate for reorg/rebuild


20/32

20

Index-Organized Tables

SQL> CREATE TABLE emp_iot2 (emp_id number,

3 ename varchar2(20),

4 sal number(9,2),

5 deptno number,6 CONSTRAINT pk_emp_iot_index PRIMARY KEY (emp_id))

7 ORGANIZATION index

8 TABLESPACE spc_demo_ts_01

9 PCTHRESHOLD 20 INCLUDING


21/32

21

IOT Dictionary Info

SQL> select table_name, tablespace_name, iot_name, iot_type

2 from user_tables;

TABLE_NAME TABLESPACE_NAME IOT_NAME IOT_TYPE

------------------ --------------- ---------- ------------

EMP_IOT IOT

SYS_IOT_OVER_6011 SPC_DEMO_TS_02 EMP_IOT IOT_OVERFLOW

SQL> select index_name, index_type, table_name

2 from user_indexes;

INDEX_NAME INDEX_TYPE TABLE_NAME

------------------ ------------ ------------------

PK_EMP NORMAL EMPPK_EMP_IOT_ INDEX IOT - TOP EMP_IOT


22/32

22

IOT Dictionary Info

SQL> select segment_name, segment_type, tablespace_name

2 from user_segments;

SEGMENT_NAME SEGMENT_TYPE TABLESPACE_NAME

------------------ ------------ ---------------

EMP TABLE USER_DATA

SYS_IOT_OVER_6011 TABLE USER_OVERFLOW

PK_EMP INDEX USER_DATAPK_EMP_IOT_INDEX INDEX SPC_DEMO_TS_01

select object_name, status, generated

from dba_objects;

OBJECT_NAME STATUS GENERATED--------------------------- ------- -

EMP_IOT VALID N

PK_EMP_IOT_INDEX VALID N

SYS_IOT_OVER_6011 VALID Y


23/32

23

IOT Tips & Techniques

!

Keep as many rows as possible in each IOTblock

! Keep seldom accessed data in the overflow area

! Eliminates duplication of data between an index

and a table


24/32

24

Bitmap Indexes

000110000110000

100110001101000

000000010000010

001001100100110

101010001000100

100100100100100

01101101101101101000010000100001

0001000001000010

ROWIDrowidROWIDrowid


ROWIDrowidROWIDrowid...

RED

BLU

GRE

BLAAQA

VIO



ROWIDrowidROWIDrowid...

YEL

ORA

GRA

Database Block


25/32

25

Bitmap Indexes Tips &Techniques

! Resembles non-unique Index structure

! DML will add leaves if bitmap at its limit ORoutside the range of the keys

! Can be slow on DML May have to reshuffle bitmap to accommodate a

change

! Searches are efficient

! Boolean operations are efficient


26/32

26

Bitmap Indexes Tips &Techniques

!

The fewer the unique values, the faster this willperform

! No row-level locking!

DML locks entire bitmap which in turn, locks entire

underlying table

! Rule-based optimizer ignores bitmap indexes

Wh t h l d?


27/32

27

What have we learned?

! B*Tree Index

Structure

Layout Unique

Non-unique

Reverse key

Tips & techniques

!

Index-organized tables Tips & techniques

! Bitmap indexes

Tips & techniques


28/32

28

About Quest Software

A li ti


29/32

29

Application

Availability & PerformanceSQL Developers

DBAs

Application Managers

System Administrators

Webmasters

Windows Administrators

Enabling

IT professionalsto meet and exceed

end-user &

managementexpectations on

application

performance &

information

availability


30/32

30

Cover All Relevant Users

and Platforms

Relevant IT Users

Developers

DBAs

System Admins

Windows Admins

Webmasters

Relevant Platforms

Oracle

DB2

Unix

Windows

Web

Quests Free eSeminars


31/32

31

Quests Free eSeminars

Register Today!www.quest.com/events/eseminar_index.asp

! PL/SQL Best Practices with Steven Feuerstein Package Design, 5/8

Unit Testing PL/SQL Code, 6/8! A Close Look Series with Dan Hotka

Meet Your HA and Performance SLAs, 6/4! Performance Monitoring for Oracle, 5/9

!

!


32/32

32

Documents

A Close Look at Index Internals