CS4432: Database Systems II

CS 4432 lecture #9 1

CS4432: Database Systems IILecture #8

(Basic indexing)

Professor Elke A. Rundensteiner


Indexing : helps to retrieve data quicker for certain queries

value= 1,000,000

Select * FROM Emp WHERE salary = 1,000,000;Select * FROM Emp WHERE salary = 1,000,000;

Indexing (Chapter 14 )

value

record


Topics

• Sequential Index Files • Secondary Indexes


Sequential File

2010

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6050

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10090


Sequential File

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Dense Index

10203040

50607080

90100110120

Every record

is in index.


Sequential File

2010

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6050

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10090

Sparse Index

10305070

90110130150

170190210230

Only first record

per block in index.


Sequential File

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Sparse 2nd level

10305070

90110130150

170190210230

1090

170250

330410490570


Note : DATA FILE or INDEX can be both “ordered files”.

Question:How would we lay them out on disk ?

- contiguous layout on disk ? - block-chained layout on disk ?


Questions:

• Do we want to build a dense 2nd-level index for a dense index?

• Can we even do this ?

Sequential File2010

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2nd level?1030507090

110130150170190210230

1090

170250330410490570

1st level?


Notes on pointers:

(1)Block pointer (used in sparse index) can be smaller than record pointer (used in dense index)

BP

RP


K1

K3

K4

K2

R1

R2

R3

R4

say:1024 Bper block

• if we want K3 block:• get it at offset (3-1)*1024 = 2048 bytes

Note : If file is contiguous, then we can omit pointers


Sparse vs. Dense Tradeoff

• Sparse: Less index space per record can keep more of index in

memory (Later: sparse better for insertions)

• Dense: Can tell if any record exists without accessing file

(Later: dense needed for secondary indexes)


Terms

• Index sequential file• Search key ( primary key)• Primary index (on sequencing field)• Secondary index• Dense index (contains all search

key values)• Sparse index• Multi-level index


Next:

• Duplicate keys

• Deletion/Insertion

• Secondary indexes


Duplicate keys

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1010

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Dense index ! Point to each value !

Duplicate keys


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Dense index. Point to each distinct value!

10203040

Duplicate keys


1010

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10102030

Sparse index: point to start of block !

Duplicate keys

care

ful if lookin

gfo

r 2

0 o

r 3

0!


1010

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10203030

Sparse index, another way ?

Duplicate keys

– place first new key from block

shouldthis be40?


Duplicate values, primary index

• Index may point to first instance ofeach value only

File Index

Summary

aaa

b


Next:

• Duplicate keys




Deletion from sparse index

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90110130150



2010

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– delete record 40



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90110130150


4040



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10305070

90110130150

– delete records 30 & 40

5070


Deletion from dense index

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50607080


Deletion from dense index

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50607080


4040


Insertion, sparse index case

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10304060



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– insert record 34

34

• our lucky day! we have free space where we need it!



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15

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• Immediate reorganization• Other variations?


• Just Illustrated: -Immediate reorganization

• Now Variation:– insert new block (chained file)– otherwise leave data file– update index only



2010

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10304060


25

overflow blocks(reorganize later...)


Insertion, dense index case

• Similar

• Often more expensive . . .


Next:

• Duplicate keys




Secondary indexesSequencefield

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Can I make a

secondary

index sparse ?



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• Sparse index

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100

90...



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• Sparse index

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100

90...

?



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• Sparse index

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90...

does not make sense!



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• Must be dense index !10203040

506070...

105090...

sparsehighlevel

allowed?


Reminder : With secondary indexes:• Lowest level is dense• Other levels are sparse

Also: Pointers are record pointers

(not block pointers; nor off-sets)


Duplicate values & secondary indexes

1020

4020

4010

4010

4030



1020

4020

4010

4010

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10101020

20304040

4040...

one option...

Problem:excess overhead!

• disk space• search time



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4010

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10

another option...

4030

20Problem:variable sizerecords inindex!



1020

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Another idea :Chain records with same key !

Problems:• Need to add fields to data records for each index• Need to follow chain to know records


Summary : Indexing Basics

– Basic Ideas: sparse, dense, multi-level…

– Duplicate Keys– Deletion/Insertion– Secondary Indexes

CS4432: Database Systems II

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