An Introduction to Clustered and Non-Clustered Index Data Structures

The data will be displayed in the order it was entered in the table. This is how SQL stores the data in pages.  Actually, 26255 & 26409 both display the data page.

I have displayed the data page information for page 26255 only. This is how MS SQL stores the contents in data pages as such column name with its respective value.  

Record Type = PRIMARY_RECORD                        

EmpId          = 4

EmpName    = ddddddddddddddddddddddddddddddddddddddddddddddddddd
ddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd
ddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd
ddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd
ddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd

Record Type = PRIMARY_RECORD                       

EmpId            = 6

EmpName      = ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff

Record Type = PRIMARY_RECORD                       

EmpId           = 1

EmpName     = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

This displays the exact data storage in SQL, without any index on table. Now, let’s go and create a unique non-clustered index on the EmpID column.

Creating a Non-Clustered Index

Now, we will create a unique non-clustered index on the empid column to see how it affects the data, and how the data is stored in SQL Server.

CREATE UNIQUE NONCLUSTERED INDEX DummyTable1_empid
ON DummyTable1 (empid)
GO

Now, execute the DBCC ind (dbid, tabid, -1)

DBCC TRACEON (3604)
GO

Declare @DBID Int, @TableID Int
Select @DBID = db_id(), @TableID = object_id(‘DummyTable1′)
DBCC ind(@DBID, @TableID, -1)
GO

Here are the results:

PagePID

IndexID

PageType

26408

0

10

26255

0

1

26409

0

1

26411

2

10

26410

2

2

Now, we see two more rows than before, which now contains index page details. Page 26408 displays the page allocation details, and pages 26255 and 26409 display the data page details, as before.

In regard to the new pages, page 26411 displays the page allocation details of an index page and page 26410 displays the index page details.

MS SQL generates a page (pagetype = 10) for an index and explains the page allocation details for an index. It shows the number of index page have been occupied for an index.

Let us see what would be the output for page 26411, that is page type = 10

IAM: Single Page Allocations @0x308A608E

—————————————–

Slot 0 = (1:26410)

Let us view page 26410 to see the index page details.

DBCC TRACEON (3604)
GO

DBCC page(10, 1, 26410, 3)
GO

SQL populates the index column data in order. The last column (?) is pointed to the row locator.

Here are the results, using two different methods:

Method I

FileID

PageID

EMPID

?

1

26410

1

0x8F66000001000200

1

26410

3

0×2967000001000000

1

26410

4

0x8F66000001000000

1

26410

6

0x8F66000001000100

The row location display in one of two ways:

  • If the table does not have a clustered index, the row locator will be combination of fileno, pageno and the no of rows in a page. 
  • If the table does have clustered index, the row location will be clustered index key value.

Non-clustered indexes are particularly handy when we want to return a single row from a table.

For example, to search for employee ID (empid = 3) in a table that has a non-clustered index on the empid column, SQL Server looks through the index to find an entry that lists the exact page and row in the table where the matching empid can be found, and then goes directly to that page and row. This greatly speeds up accessing the record in question.

Select EmpID, EmpName From DummyTable1 WHERE EMPID = 3

Now, let’s insert some more rows in our table and view the data page storage of our non-clustered index.

Insert Into DummyTable1 Values (10, Replicate (‘j’,2000))
GO

Insert Into DummyTable1 Values (2, Replicate (‘b’,2000))
GO

Insert Into DummyTable1 Values (5, Replicate (‘e’,2000))
GO

Insert Into DummyTable1 Values (8, Replicate (‘h’,2000))
GO

Insert Into DummyTable1 Values (9, Replicate (‘i’,2000))
GO

Insert Into DummyTable1 Values (7, Replicate (‘g’,2000))
GO

Now, let’s view the data in our table.

Continues…

Pages: 1 2 3 4 5 6




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4 Responses to “An Introduction to Clustered and Non-Clustered Index Data Structures”

  1. Where is the SQL code to add the unique clustered index?

    RE: “Now, let’s add a unique clustered index on the empindex column and then view the differences in page 26410.”

  2. Hi Kevin,

    It is on page 5 of this article:

    CREATE UNIQUE CLUSTERED INDEX DummyTable2_EmpIndex
    ON DummyTable2 (EmpID)
    GO

    Regards, Matt

  3. Great article! I am wondering now what the result will be when adding two clustered indexes on a table for two different rows that you will often query a group of rows on. I’m not sure if I can have multiple clustered indexes on a single table. It seems I would have to have an additional copy of the data to have it physically arranged in two different orders if it’s possible taking up more space and making writes more expensive to the table.

  4. CREATE UNIQUE CLUSTERED INDEX DummyTable1_empindex
    ON DummyTable1 (EmpIndex);
    GO

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