Sql xp 07

Implementing Indexes

Objectives
In this lesson, you will learn to:
Create a clustered index
Create a nonclustered index
Use the Index Tuning Wizard
Understand Index Enhancements

©NIIT SQL/Lesson 7/Slide 1 of 32


Getting Started
An index is an internal table structure that SQL Server uses
to provide quick access to rows of a table based on the
values of one or more columns
Advantages of Using Indexes
Improve the speed of the execution of queries
Enforce uniqueness of data
Speed up joins between tables



Getting Started (Contd.)
Disadvantages of Using Indexes
Takes time to create an index
Takes large amount of disk space to store data along with
the original data source—the table
Gets updated each time the data is modified
Types of Indexes
Clustered index
Nonclustered index



Clustered Index
In a clustered index:
® The data is physically sorted
® Only one clustered index can be created per table
Nonclustered Index
In a nonclustered index:
® Thephysical order of the rows is not the same as the
index order



® Nonclustered indexes are typically created on columns
used in joins and WHERE clauses, and whose values
may be modified frequently
® SQLServer creates nonclustered indexes by default
when the CREATE INDEX command is given
® There can be as many as 249 nonclustered indexes per
table



Indexes and Heap Structures
SQL Server supports indexes defined on any column in a
table, including computed columns
® Ifa table does not have any clustered index, data is not
stored in a particular order. This structure is called a
heap



Features of Indexes
Indexes accelerate queries that join tables, and perform
sorting and grouping.
Indexes can be used to enforce uniqueness of rows.
Indexes are useful on columns in which the majority of
data is unique.
When you modify the data of an indexed column, the
associated indexes are updated automatically.
You require time and resources to maintain indexes. You
should not create an index that is not used frequently.



A clustered index should be created before a
nonclustered index. A clustered index changes the order
of rows. A nonclustered index would need to be rebuilt if it
is built before a clustered index
Typically, nonclustered indexes are created on foreign
keys.
Syntax
CREATE [UNIQUE] [CLUSTERED|NONCLUSTERED]
INDEX index_name
ON table_name(column_name[,column_name]…)



7.D.1 Optimizing Query Execution
The ExternalCandidate table contains a large amount of data.
The first name of each candidate and the name of the
recruitment agency are required to create a report. However,
it takes a long time to execute the following query.
SELECT vFirstName, cName
FROM ExternalCandidate JOIN
RecruitmentAgencies
ON
ExternalCandidate.cAgencyCode =
RecruitmentAgencies.cAgencyCode
Suggest and implement a solution for faster data retrieval.


Task List
Identify how to speed up data retrieval
Draft the statement to create an index
Create the index in the database
Verify that the index has been created
Verify that the query execution is faster



Identify how to speed up data retrieval
Indexes are used to:
Speed up data retrieval
Enforce the uniqueness of rows
Result:
To speed up data retrieval, use indexes



Draft the statement to create an index
Action:
The tables on which the index would be created are:
ExternalCandidate and RecruitmentAgencies
The attributes on which the index would be created are:
cAgencyCode of ExternalCandidate and cAgencyCode of
RecruitmentAgencies
The types of indexes to be created are:
ExternalCandidate - Nonclustered index;
RecruitmentAgencies - Clustered index
The names of the indexes to be created are idxRecruitment
and idxExternalCandidate


Create the index in the database
Action:
In the Query Analyzer window, type:
CREATE NONCLUSTERED INDEX idxExternalCandidate
ON ExternalCandidate(cAgencyCode)
CREATE CLUSTERED INDEX idxRecruitment
ON RecruitmentAgencies(cAgencyCode)
Press F5 to execute the code



Verify that the index has been created
To verify that the index has been created, use the sp_helpindex
command
Syntax
sp_helpindex table_name
Action:
sp_helpindex ExternalCandidate
Press F5 to execute the command
sp_helpindex RecruitmentAgencies
Press F5 to execute the command


Verify that the query execution is faster
Action:
Execute the query after creating the index. If there is a lot of
data, you can note the difference in speed



Just a Minute…
How many clustered indexes can be created per table?
Which index organizes data logically but does not store data
physically?



Index Tuning Wizard
Index Tuning Wizard available in SQL Server is used to select
and create the best possible set of indexes and information
regarding a database
Uses of the Index Tuning Wizard
For a given workload, the best possible combination of
indexes for a database is recommended
The effects of the proposed recommendation about the
indexes, distribution of queries among tables, and the query
performance in the workload will be analyzed
For a small set of problem queries, the way to tune the
database will be recommended
It will specify the advanced options such as disk space
constraints that can be customized


7.D.2 Using the Index Tuning Wizard
You have created the following indexes on the Department
table:
CREATE NONCLUSTERED INDEX idxdepcode
ON DEPARTMENT(cDepartmentCode)
CREATE NONCLUSTERED INDEX idxdepname
ON DEPARTMENT(vDepartmentName)
CREATE NONCLUSTERED INDEX idxdephead
ON DEPARTMENT(vDepartmentHead)
CREATE NONCLUSTERED INDEX idxdeplocation
ON DEPARTMENT(vlocation)


7.D.2 Using the Index Tuning Wizard (Contd.)
Analyze the use of these indexes on the Department table
when the following query is executed:
SELECT Requisition.cRequisitionCode,
Position.vDescription,vDepartmentName
FROM Position
JOIN Requisition
ON Requisition.cPositionCode=
Position.cPositionCode
JOIN Department
ON Requisition.cDepartmentCode=
Department.cDepartmentCode



Task List
Identify a method to analyze the use of indexes
Perform the analysis
Verify that the analysis has been performed



Identify a method to analyze the use of indexes
Result:
Use the Index Tuning Wizard to analyze the use of
indexes



Perform the analysis
Action:
Perform the analysis using the Index Tuning Wizard



Verify that the analysis has been performed
Action:
You can use the following command to view the indexes
present on the Department table:
sp_helpindex Department
Observe that, as recommended by the Index Tuning Wizard,
the extra indexes have been removed



Index Enhancements
Fill Factor
FILLFACTOR clause improves performance of the system
by minimizing the amount of page splitting that occurs each
time an index page becomes full
Syntax
CREATE CLUSTERED INDEX index_name
ON table_name (column_name)
WITH FILLFACTOR =
percentage_fillfactor
Pad_Index
Specifies the space to leave open on each page (node) in
©NIIT the intermediate levels of the index SQL/Lesson 7/Slide 24 of 32


Index Enhancements (Contd.)
Implications of NULL in Unique Indexes:
In a table, a unique index cannot be created on a single
column if that column contains NULL in more than one row
DBCC SHOWCONTIG:
The DBCC SHOWCONTIG command is primarily used to
find out why the table or the index is heavily fragmented
Syntax
DBCC SHOWCONTIG
[
(table_id [, index_id])
]


The DBCC INDEXDEFRAG:
The DBCC INDEXDEFRAG command is used to
defragment clustered and secondary indexes of the
specified table or view
Syntax
DBCC INDEXDEFRAG
( { database_name | database_id | 0 }
, { table_name | table_id | 'view_name' | view_id }
, { index_name | index_id }
)



Just a Minute…
Susan wants to minimize the amount of page splitting that
occurs each time an index page is full. What should she use?



Index Selection:
A detailed query analysis is required to determine which
index to involve in a query process. This involves:
® Examining the search clauses to identify the columns
referenced
® Knowing the importance of the data to determine the
usefulness of the index
® Ranking the queries in the order of importance



Performance Considerations (Contd.)
Index Usage Criteria:
SQL Server cannot use an index until and unless the query
contains a column in a valid search argument or join clause
that matches at least the first column of the index



Summary
In this lesson, you learned that:
Indexes are created to enhance the performance of queries.
There are two types of indexes – clustered and nonclustered.
Indexes are created using the CREATE INDEX statement.
Data is physically sorted in a clustered index.
Clustered indexes should be built on an attribute whose values
are unique and do not change often.
In a nonclustered index, the physical order of rows is not the
same as that of the index order.



Summary (Contd.)
A nonclustered index should be built on an attribute which is
normally used in joins and the WHERE clause. The values of
this attribute may often change.
A nonclustered index is the default index that is created with
the CREATE INDEX command.
The Index Tuning Wizard can be used to analyze the optimal
use of indexes in the query entered in the Query Analyzer
window.
SQL Server provides the FILLFACTOR clause to improve
performance of the system by minimizing the amount of page
splitting that occurs each time an index page becomes full.



Summary (Contd.)
The DBCC SHOWCONTIG command is mainly used to find
out whether the table or index is heavily fragmented. Table
fragmentation normally occurs when a large number of insert
and update operations are performed on the table.
The DBCC INDEXDEFRAG command is used to defragment
clustered and secondary indexes of the specified table or view.


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