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Index Fragmentation
Paul Randal
Dev Lead, Microsoft SQL Server

Introduction

Structural details of an index

How these structures are used

Index fragmentation

SQL Server 2000 DBCC SHOWCONTIG

SQL Server 2000 DBCC INDEXDEFRAG

"Yukon" dm_db_index_physical_stats

"Yukon" ALTER INDEX

Records (1)

How is data stored in SQL Server?

Example record types

Data

Index


Text

Ghost data/index

Records (2)

‘Inside SQL Server 2000’ has the details

Things to note:

Non-leaf index records contain a child page
Id in every record

All non-clustered index leaf records contain
a base table RID

“Yukon” allows INCLUDE’d columns in an
index record

Pages (1)

How are records stored?

Example page types

Data page

Index page

Allocation maps (GAM, SGAM, IAM) pages


PFS page

Pages (2)

Layout is the same for all page types
8k
96 byte header
slot array
records
free space

Extents

How are pages grouped?

Extents are:

A group of 8 contiguous pages

Start on an 8 page boundary

Tracked in IAM, GAM, SGAM pages
0 1 2 3 54 6 7

Heap (1)

Simplest storage arrangement

Comprised entirely of data pages


Unordered, pages unlinked

Records are located using physical RID

create table foo (
first char(100), last char (100), city char (100))

Clustered index (1)

Alternative table structure to heap

Data stored in defined order

Fast lookup through B-tree

Records located through logical RID

create clustered index foo_c on foo (city)

Clustered index (2)
index tree pages
P
L L
P
L L
P
L L
R
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)
P
L L
P
L L
P
L L
R
index leaf pages
index tree 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

Reduces amount of data to apply predicates to

Singleton lookup
Matching record

Range scan

Allocation-order scan
1 23 45 6

Side note: merry-go-rounds

Why don’t I get the index order when I do ‘select *’?
D A T A
scan 1
scan 2
scan 3
scan 1
starts
scan 2
starts
scan 3
starts

Readahead (1)

Why use readahead?

Keep the CPUs busy, maximize throughput

Feedback mechanism to determine how far ahead of
the scan point to read

Driven from parent level

Issues 1, 8, or 32 page IOs

Better contiguity = bigger IOs

Readahead (2)
L

What causes fragmentation (1)
Index leaf level of newly built index
Red arrow is the allocation order
Black arrows are following the logical order

What causes fragmentation (2)
Newly built index leaf after a single page split
Red arrow is the allocation order
Black arrows are following the logical order

What causes fragmentation (3)

Index leaf level after random inserts/deletes
Red arrow is the allocation order
Black arrows are following the logical order

Logical scan fragmentation

Occurs when the next logical page is not the next
physical page

Prevents optimal readahead

Reduces range scan performance

Does not affect cached pages

Smaller indexes affected less

Extent scan fragmentation

Occurs when the extents in an index are not
contiguous

Also affects readahead but not as much
Index A Index B Index A Index B Index A Index A
1 2 3 4 5 6

DBCC SHOWCONTIG

Your tool for determining fragmentation


Keys to success are:

Knowing which indexes to look at

Knowing which options to use

Knowing how to interpret the results