Download Implementing

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
SQL/Lesson 7/Slide 1 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 i.e. Indexes can enforce
entity integrity.
 Indexes can improve the performance of queries that use
joins to retrieve tables.
SQL/Lesson 7/Slide 2 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
SQL/Lesson 7/Slide 3 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
SQL/Lesson 7/Slide 4 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
SQL/Lesson 7/Slide 5 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
SQL/Lesson 7/Slide 6 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.
SQL/Lesson 7/Slide 7 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]…)
SQL/Lesson 7/Slide 8 of 32
Implementing Indexes
Optimizing Query Execution
The Employee table contains a large amount of data. The
first name of each employee and the name of the Department
are required to create a report. However, it takes a long time
to execute the following query.
SELECT
vFirstName, cDeptName
FROM Employee JOIN Department
ON
Employee.cDeptno = Department.cDeptno
Suggest and implement a solution for faster data retrieval.
SQL/Lesson 7/Slide 9 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
SQL/Lesson 7/Slide 10 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
SQL/Lesson 7/Slide 11 of 32
Implementing Indexes
Draft the statement to create an index
Action:
 The tables on which the index would be created
are: Department and Employee
 The attributes on which the index would be created
are: cDeptNo of Employee and cDeptNo of
Department
 The types of indexes to be created are:
Employee - Nonclustered index;
Department - Clustered index
SQL/Lesson 7/Slide 12 of 32
Implementing Indexes
Create the index in the database
Action:
 In the Query Analyzer window, type:
CREATE NONCLUSTERED INDEX idxdno
ON Employee(cDeptNo)
CREATE CLUSTERED INDEX
idxDeptDno
ON Department(cDeptno)
 Press F5 to execute the code
SQL/Lesson 7/Slide 13 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 Employee
sp_helpindex Department
SQL/Lesson 7/Slide 14 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
SQL/Lesson 7/Slide 15 of 32
Implementing Indexes
Wait a while…
How many clustered indexes can be created per
table?
Which index organizes data logically but does not
store data physically?
SQL/Lesson 7/Slide 16 of 32
Implementing Indexes
Composite Indexes
It is simply an index that has more than one column in its
key.
Helpful wit queries that search for rows based on all of the
values in multiple columns, such as LastName & FirstName.
Up to 16 columns can be used in an index. The combined
size of the columns cannot exceed 900 bytes.
SQL Server doesn’t use the composite index unless the first
column in the key is specified.
When building a composite index, use the column with the
most unique values as the first column.
SQL/Lesson 7/Slide 17 of 32
Implementing Indexes
Unique Indexes
A unique index will ensure that the values must be unique
within the table.
Unique indexes are how SQL Server enforces PRIMARY
KEY & UNIQUE constraints for entity integrity.
SQL/Lesson 7/Slide 18 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
SQL/Lesson 7/Slide 19 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
SQL/Lesson 7/Slide 20 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])
]
SQL/Lesson 7/Slide 21 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
SQL/Lesson 7/Slide 22 of 32
Implementing Indexes
Wait a while…
 Neha wants to minimize the amount of page
splitting that occurs each time an index page is
full. What should she use?
SQL/Lesson 7/Slide 23 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
SQL/Lesson 7/Slide 24 of 32
Implementing Indexes
Choosing columns for Index
Primary keys and foreign keys should always be indexed.
Create indexes on columns that you often search on.
Create indexes on columns that are often used to sort the
results of a query.
Create unique indexes on a primary key or alternate key.
If you often retrieve data in sorted order by a particular
column, consider putting a clustered index on that column.
Consider using a non-clustered index on the primary key of a
table if the table uses an IDENTITY column as its primary
key.
SQL/Lesson 7/Slide 25 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.
SQL/Lesson 7/Slide 26 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.
SQL/Lesson 7/Slide 27 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.
SQL/Lesson 7/Slide 28 of 32