An Introduction to Clustered and Non-Clustered Index Data Structures

When I first started using SQL Server as a novice, I was initially confused as to the differences between clustered and non-clustered indexes. As a developer, and new DBA, I took it upon myself to learn everything I could about these index types, and when they should be used. This article is a result of my learning and experience, and explains the differences between clustered and non-clustered index data structures for the DBA or developer new to SQL Server. If you are new to SQL Server, I hope you find this article useful.

As you read this article, if you choose, you can cut and paste the code I have provided in order to more fully understand and appreciate the differences between clustered and non-clustered indexes.

Part I: Non-Clustered Index

To better explain SQL Server non-clustered indexes; let’s start by creating a new table and populating it with some sample data using the following scripts. I assume you have a database you can use for this. If not, you will want to create one for these examples.

Create Table DummyTable1
(
EmpId Int,
EmpName Varchar(8000)
)

When you first create a new table, there is no index created by default. In technical terms, a table without an index is called a “heap”. We can confirm the fact that this new table doesn’t have an index by taking a look at the sysindexes system table, which contains one for this table with an of indid = 0. The sysindexes table, which exists in every database, tracks table and index information. “Indid” refers to Index ID, and is used to identify indexes. An indid of 0 means that a table does not have an index, and is stored by SQL Server as a heap.

Now let’s add a few records in this table using this script:

Insert Into DummyTable1 Values (4, Replicate ('d',2000))
GO

Insert Into DummyTable1 Values (6, Replicate ('f',2000))
GO

Insert Into DummyTable1 Values (1, Replicate ('a',2000))
GO

Insert Into DummyTable1 Values (3, Replicate ('c',2000))
GO

Now, let’s view the contests of the table by executing the following command in Query Analyzer for our new table.

Select EmpID From DummyTable1
GO

As you would expect, the data we inserted earlier has been displayed. Note that the order of the results is in the same order that I inserted them in, which is in no order at all.

Now, let’s execute the following commands to display the actual page information for the table we created and is now stored in SQL Server.

dbcc ind(dbid, tabid, -1) – This is an undocumented command.

DBCC TRACEON (3604)
GO

Declare @DBID Int, @TableID Int
Select @DBID = db_id(), @TableID = object_id('DummyTable1')

DBCC ind(@DBID, @TableID, -1)
GO

This script will display many columns, but we are only interested in three of them, as shown below.

Here’s what the information displayed means:

PagePID is the physical page numbers used to store the table. In this case, three pages are currently used to store the data.

IndexID is the type of index,

Where:

0 – Datapage

1 – Clustered Index

2 – Greater and equal to 2 is an Index page (Non-Clustered Index and ordinary index),

PageType tells you what kind of data is stored in each database,

Where:

10 – IAM (Index Allocation MAP)

1 – Datapage

2 – Index page

Now, let us execute DBCC PAGE command. This is an undocumented command.

DBCC page(dbid, fileno, pageno, option)

Where:

dbid = database id.

Fileno = fileno of the page.  Usually it will be 1, unless we use more than one file for a database.

Pageno = we can take the output of the dbcc ind page no.

Option = it can be 0, 1, 2, 3. I use 3 to get a display of the data.  You can try yourself for the other options.

Run this script to execute the command:

DBCC TRACEON (3604)
GO

DBCC page(@DBID, 1, 26408, 3)
GO

The output will be page allocation details.

DBCC TRACEON (3604)
GO

dbcc page(@DBID, 1, 26255, 3)
GO