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lnrlcxing

rtrrcl

I lashing

lndcx

A datat-rasc

incicx

is

n

clata

s1r'uclurc that inrprovcs

tltc

spccd

ol'data

rctrieval

opclations on

lt

databersc

tablc

at

thc cost

ol'slowcr

writes

ancl

thc

use

of'more

storage space.

llasic Concepts

N4auv clucrics rclcrcncc

only

a

srlall

proportion

of

thc rccorcls

in

a ljlc.

Ior cxanrplc, a

clLicrv

lil

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Inscrtion

timc:

'l'lrc

tirnc

il

lalics

to iuscrl n ncrv

dala

itcrl.'l'his

vrlrrc

inclrr'.le:

iir,'

tiurc

it

tulics

to

ljrtl

thc

corlcct

place

trl iuscrt tlrc

ncw

clata itcn-r, as

wcll

us tlre tirirc

il

tal

7/24/2019 Advance DBMS 1

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lSri.girtori

Jtighton

I-)or,r'niLrlvn

liantrs

L)o\v'nlo\,\'n

edt".rrood

lVlianrLs

Pcrryrirlge

Pcrrvri.ige

Penyridge

A-222

|

liercln'oor{

Iioum.{ I{ill

looking.

We start

at tlic

t'ccord

pointcd

to

by

that indcx entry,

in

llrc

fllc tnttil

n,c

lnttl

thc

tlcsil'ctl

rccol'tl.

and lbllow

thc

poinlcrs

Irig.

2: Sparsc

lncicx

Comparison:

It

is

generaily i'astcr

1o

locate

a rccord

if

we

l-ravc

a dense index rather

than

a sparsc

incicx.

I

lowcvcr,

sparsc

indiccs

havc

advantagcs

over

dcnse

indiccs in

that

they

rcqr-rire

lcss

spacc

and

they impose

less maintenance

ovcrhcad

lbr

insertions

ancl delctions.

'l-hclc

is

a

lraclc-olf

that

tl-ie

systcr-r-r

dcsigner

uust n-iakc bctwcen acccss

timc

and

spacc

.',.:::,'*.:"

.i.:i;.ri;:.;;.;,-'i.-ci-'io;r

icgardin-u,

this

tradc-o11 depcnds ott

thc

sltccil-ic

apllliclrttorl.

a

good compromise

is

to have a

sparse

index

with

one index entry

per

block.

The

rcason this

dcsign

is

a

good

trade-off

is

tl"rat

the

dominant

cost

in

processing

a database

reqllcst

is

thc

tinrc that

it

tahes

to

bring

a

block

liom

clisi