Module 8 Improving Performance through Nonclustered Indexes.
SQL/Lesson 7/Slide 1 of 32 Implementing Indexes Objectives In this lesson, you will learn to: *...
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Transcript of SQL/Lesson 7/Slide 1 of 32 Implementing Indexes Objectives In this lesson, you will learn to: *...
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
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 i.e. Indexes can enforce entity integrity.
Indexes can improve the performance of queries that use joins to retrieve tables.
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
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
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
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
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.
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]…)
SQL/Lesson 7/Slide 9 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 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
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
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: 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 13 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 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 Employee
sp_helpindex Department
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
SQL/Lesson 7/Slide 16 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 17 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 18 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 19 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 20 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 21 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 22 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 23 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 24 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 25 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 26 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 27 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 28 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.