February 6, 2011

SQL 2005 - Clustered and NonClustered index in sql server

1. Introduction

We all know that data entered in the tables persisted in the physical drive in the form of database files. Think about a table say Customer (For any leading bank India) that has around 16 Million records. When we try to retrieve records for two or three customers based on his customer id, all 16 million records or taken and a comparison is made to get a match on the supplied customer ids. Think about how much time will that take, if it is a web application and there are 25 to 30 customers wants to access their data through the internet. Does the database server 16 Million x 30 searches? The answer is no because all modern database uses the concept of "Index".

2. What is a Database Index?

An index is a database object, which can be created on one or more columns (16 Max column combination). While creating the index, SQL Server will read the column(s) and forms a relevant data structure to minimise the number of data comparisons on the column(s) in which it is created. The index will improve the performance of data retrieval and adds some overhead on data modification such create, delete and Modify.  So it depends on how much data retrieval can be performed on the table versus how much of DML (Insert, Delete and Update) Operations.

In this article, we will see creating the Index. The below two sections are taken from my previous article as it is required here. If your database already has changed for next two sections, you can directly go to the section 5.

3. First, Create two tables

To explain these constraints we need two tables. First, let us create these tables. Run the below scripts to create the tables. Copy paste the code on the new Query Editor window then execute it.

CREATE TABLE Student(StudId smallint, StudName varchar(50), Class tinyint);
CREATE TABLE TotalMarks(StudentId smallint, TotalMarks smallint);

Note that there are no constraints at present on these tables. We will add the constraints one by one.

4. Primary Key Constraint

A table column with this constraint is called as the "key column" for the table. This constraint helps the table to make sure that the column value is not repeated and also no null entries. We will mark the StudId column of the Student table as a primary key. To do so, follow these steps:

  1. Right click the student table and click on the modify button
  2. From the displayed layout select the StudId row by clicking the Small Square like button on the left side of the row.
  3. Click on the "Set Primary Key" toolbar button to set the StudId column as a primary key column.

Primary Key Index Creation
Fig 1. Creating Primary key from UI

Now this column does not allow null values and duplicate values. You can try inserting values to violate these conditions and see what happens. A table can have only one Primary key. But, multiple columns can participate on the primary key. In such case, the uniqueness is considered among all the participant columns by combining their values. The primary key with combined columns is known as "Composite Primary Key".

5. Clustered Index

The primary key created for the StudId column will create a clustered index for the "Studid" column. A table can have only one clustered index on it.

When creating the clustered index, SQL server 2005 reads the "Studid" column and forms a Binary tree like structure on it. This binary tree information is then stored separately in the disc. From Management studio, expand the table Student and then expand the Indexes. You will see the following index created for you when the primary key is created:

Fig 2. Clustered Index

With the use of the binary tree, now the search for the student based on the "studid" decreases the number of comparisons to a large amount. Let us assume that you had entered the following data in the table student:

Table with Clustered Index
Fig 3. Table with Clustered Index

The index will form the below specified binary tree (Or a tree similar to this). Note that for a given parent, there is only one or two Child. The left side will always have a lesser value and right side will always have a greater value when compared to the parent. The tree can be constructed in a reverse way also. That is, left side higher and right side lower.

Tree Structure of Clustered Index
Fig 4. Tree Structure of Clustered-Index

Now let us assume that we had written a query like below:

Select * from student where studid = 103;
Select * from student where studid = 107;

  • Execution without index will return value for the first query after the third comparison.
  • Execution without index will return value for the second query at eighths comparison.

Execution of the first query with index will return a value at the first comparison.
Execution of the Second query with index will return the value at the third comparison. 

Look below:
1) Compare 107 vs 103 : Move to right node
2) Compare 107 vs 106 : Move to right node
3) Compare 107 vs 107 : Matched, return the record

If numbers of records are less, you cannot see a different. Now apply this technique with a Yahoo email user accounts stored in a table called say YahooLogin. Let us assume there 33 millions of users around the world have yahoo email id and that is stored in the YahooLogin. When a user logs in by giving the username and password, the comparison required is minimum 1 to maximum 25, with the binary tree that is clustered index. Look at the above picture and guess yourself how fast you will reach the level 25. Without Clustered index, the comparison required is 1 to 33 million(s).

Got the usage of Clustered index? Let us move to a Non-Clustered index.

6. Non-Clustered Index

A "non-clustered Index" is useful for columns that have some repeated values. Say fox example AccountType column of a bank database may have 10 million rows. But, the distinct values of account type may be 10-15. A clustered index is automatically created when we create the primary key for the table. We need to take care of the creation of the non-clustered index.

Follow the steps below to create a Non-clustered index on our table Student based on the column class.

1) After expanding the Student table, right click on the Indexes. And click on the New Index.

Index DB Objects
Fig 5. Indexes as Database Objects

2) From the displayed dialog type the index name as shown below and then click on the Add button to select the column(s) that participate in the index. Make sure the Index type is Non-Clustered.

Non-Clustered Index creation Dialog
Fig 6. Creating Non-Clustered Index

3) In the "select column dialog", place a check mark for the column class. This tells that we need a non-clustered index for the column Student.Class. You can also combine more than one column to create the Index. Once the column is selected click on the OK button. You will return the dialog shown above with the selected column marked in blue. Our index has only one column. If you selected more than one column, using the MoveUp and MoveDown button you can change the order of the indexed columns. When you are using the combination of columns, Always use the highly repeated data column first and comparatively unique data columns down in the list. For example, let use assume the correct order for creating the Non-clustered index is:  Class, DateOfBirth, PlaceOfBirth

Pick column for the index
Fig 7. Selecting table column for Non-Clustered Index

4) Click on the Index folder on the right side and you will notice the non-clustered index created for you and the index created for the data column class.

Summary of the indexes
Fig 8. Index summary dialog

7. How does a Non-Clustered Index work?

A table can have more than one Non-Clustered index. But, it should have only one clustered index that works based on the Binary tree concept. Non-Clustered column always depends on the Clustered column in the database.

This can easily be explained with the concept a book and their index page at the end. Let us assume that you are going to a bookshop and found a big 1500 pages of C# book that says "all about C#".  When you glanced the book, it has all beautiful colour pages and shiny papers. But, that is not only the eligibility for a good book right? Once you impressed with the book, you want to see your favourite topic Regular Expression and how it is explained in the book. What will you do? I just peeped at you from the behind and recorded what you did as below:

  1. You went to the Index page (It has total 25 pages). It is already sorted and hence you easily picked up Regular Expression that comes on the page Number 17.
  2. Next, you noted down the number displayed next to it which is 407, 816, 1200-1220
  3. Your first target is Page 407. You opened a page in the middle, the page is greater than 500.
  4. Then you moved to a somewhat lower page. But it still reads 310.
  5. Then you moved to a higher page. You are very lucky you exactly got page 407. [Yes man you got it. Otherwise, I need to write more. OK?]
  6. That’s all, you started exploring what is written about Regular expression on that page, keeping in mind that you need to find page 816 and the Range 1200-1220 also.

In the above Scenario, the Index page is Non-Clustered index and the page numbers are clustered index arranged in a binary tree. See how you came to the page 407 very quickly. Your mind actually traversed the binary tree way left and right to reach the page 407 quickly.

Here, the class column with distinct values 1,2,3..12 will store the clustered index columns value along with it. Say for example; Let us take the only class value of 1. The Index goes like this:

1: 100, 104, 105

So here, you can easily get all the records that have value for class = 1. Map this with the Book index example now. See you all in the next articles.


  1. hi Siva,
    simply superb article. I haven't read anything this simple that explains such a complex stuff. keep up the good work.


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