Bài giảng Client/Server - Chương 11: Flag (Phần 1) pdf

Clustered index

(1)



Alternative table structure to heap



Data stored in defined order



Fast lookup through B-tree



Records located through logical RID
create table foo (
first char(100), last char (100), city char (100))
create clustered index foo_c on foo (city)






Clustered index

(2)

R
index tree pages
P

L

P

L

L

P

L

L

L

data pages


Non-clustered index

(1)



A way to provide a different ordering



Define on heaps or clustered indexes



Leaf records contain RID of matching record in base
table



create index foo_nc on foo (last)
create index foo_nc on foo (last) include (first)




Non-clustered index

(2)

R
index tree pages
P

L

P

L

L

P

L

L

L

index leaf pages


Why use an index?


Allows a variety of access modes:






Singleton lookup
Range scan
Allocation order scan

Allows skipping of sort step in query


Singleton lookup

Matching record


Range scan


Allocation-order scan

3

1

5

2

4

6


What causes fragmentation
Index leaf level of newly built index

Red arrow is the allocation order

Black arrows are following the logical order

(1)


What causes fragmentation
Newly built index leaf after a single page split

Red arrow is the allocation order
Black arrows are following the logical order

(2)


What causes fragmentation
Index leaf level after random inserts/deletes

Red arrow is the allocation order
Black arrows are following the logical order

(3)


Logical scan fragmentation


Occurs when the next logical page is not the next
physical page

Extent scan fragmentation


Occurs when the extents in an index are not
contiguous

Index A Index B Index A Index B Index A Index A
1
2
3
4
5
6


Inside INDEXDEFRAG


(1)

Stage 1: Page compaction







Make pages have ‘fullness’ near to original
FILLFACTOR

Operates on leaf level only
Compacts pages by shuffling rows towards
left side of the B-tree
Delete pages made empty


Inside INDEXDEFRAG
Page compaction example

(2)


Inside INDEXDEFRAG


(3)

Stage 2: Page defragment



Make logical order same as allocation order
Operates on leaf level only


Inside INDEXDEFRAG

(4)

Page reordering example

AB
VW
BC
WZ

DE
JK
JK
KL
EF
KL

AB
JK
AB
BC
KL
BC

VW
DE
DE
WZ
EF
EF

1
1

2
2

3
3

4
4

VW
WZ

2104
2104

Physical Page ID
Physical Page ID

17