sql xp 07
TRANSCRIPT
©NIIT SQL/Lesson 7/Slide 1 of 32
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 2 of 32
Implementing Indexes
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
©NIIT SQL/Lesson 7/Slide 3 of 32
Implementing Indexes
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
©NIIT SQL/Lesson 7/Slide 4 of 32
Implementing Indexes
Getting Started (Contd.)
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:
The physical order of the rows is not the same as the index order
©NIIT SQL/Lesson 7/Slide 5 of 32
Implementing Indexes
Getting Started (Contd.)
Nonclustered indexes are typically created on columns used in joins and WHERE clauses, and whose values may be modified frequently
SQL Server creates nonclustered indexes by default when the CREATE INDEX command is given
There can be as many as 249 nonclustered indexes per table
©NIIT SQL/Lesson 7/Slide 6 of 32
Implementing Indexes
Getting Started (Contd.)
Indexes and Heap Structures
SQL Server supports indexes defined on any column in a table, including computed columns
If a table does not have any clustered index, data is not stored in a particular order. This structure is called a heap
©NIIT SQL/Lesson 7/Slide 7 of 32
Implementing Indexes
Getting Started (Contd.)
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.
©NIIT SQL/Lesson 7/Slide 8 of 32
Implementing Indexes
Getting Started (Contd.)
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]…)
©NIIT SQL/Lesson 7/Slide 9 of 32
Implementing Indexes
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.
©NIIT SQL/Lesson 7/Slide 10 of 32
Implementing Indexes
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
©NIIT SQL/Lesson 7/Slide 11 of 32
Implementing Indexes
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
©NIIT SQL/Lesson 7/Slide 12 of 32
Implementing 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
©NIIT SQL/Lesson 7/Slide 13 of 32
Implementing Indexes
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
©NIIT SQL/Lesson 7/Slide 14 of 32
Implementing Indexes
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:
In the Query Analyzer window, type:
sp_helpindex ExternalCandidate
Press F5 to execute the command
In the Query Analyzer window, type:
sp_helpindex RecruitmentAgencies
Press F5 to execute the command
©NIIT SQL/Lesson 7/Slide 15 of 32
Implementing Indexes
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
©NIIT SQL/Lesson 7/Slide 16 of 32
Implementing Indexes
Just a Minute…
How many clustered indexes can be created per table?
Which index organizes data logically but does not store data physically?
©NIIT SQL/Lesson 7/Slide 17 of 32
Implementing Indexes
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
©NIIT SQL/Lesson 7/Slide 18 of 32
Implementing Indexes
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)
©NIIT SQL/Lesson 7/Slide 19 of 32
Implementing Indexes
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
©NIIT SQL/Lesson 7/Slide 20 of 32
Implementing Indexes
Task List
Identify a method to analyze the use of indexes
Perform the analysis
Verify that the analysis has been performed
©NIIT SQL/Lesson 7/Slide 21 of 32
Implementing Indexes
Identify a method to analyze the use of indexes
Result:
Use the Index Tuning Wizard to analyze the use of indexes
©NIIT SQL/Lesson 7/Slide 22 of 32
Implementing Indexes
Perform the analysis
Action:
Perform the analysis using the Index Tuning Wizard
©NIIT SQL/Lesson 7/Slide 23 of 32
Implementing Indexes
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
©NIIT SQL/Lesson 7/Slide 24 of 32
Implementing Indexes
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 the intermediate levels of the index
©NIIT SQL/Lesson 7/Slide 25 of 32
Implementing Indexes
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])]
©NIIT SQL/Lesson 7/Slide 26 of 32
Implementing Indexes
Index Enhancements (Contd.)
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 } )
©NIIT SQL/Lesson 7/Slide 27 of 32
Implementing Indexes
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?
©NIIT SQL/Lesson 7/Slide 28 of 32
Implementing Indexes
Index Enhancements (Contd.)
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
©NIIT SQL/Lesson 7/Slide 29 of 32
Implementing Indexes
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
©NIIT SQL/Lesson 7/Slide 30 of 32
Implementing Indexes
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.
©NIIT SQL/Lesson 7/Slide 31 of 32
Implementing Indexes
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.
©NIIT SQL/Lesson 7/Slide 32 of 32
Implementing Indexes
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.