Skip to main content

Clustered Index on an UniqueIdentifier column is costly

Generally having a clustered index on a UniqueIdentifier column is going to be costly. Even if we add some 5000 or 10000 records in the table the fragmentation level would be surely around 95+ % which means we have to very frequently REORGANIZE or REBUILD that index. So always i would suggest lets double check and be very sure that we need a GUID column and not an INT or BIGINT column.

Fragmentation Level would be too heavy

Lets create a sample to see how data is getting fragmented for different datatypes.


-----------------------------------------------------
--Demo Table Creation Scripts
-----------------------------------------------------

--Table with UniqueIdentifier as Clusetered Index
CREATE TABLE dbo.TblUID
(
Sno Uniqueidentifier NOT NULL DEFAULT NEWID(),
FirstName VARCHAR(100) NOT NULL,
DOB DATETIME NOT NULL,
CONSTRAINT pk_tblUid PRIMARY KEY CLUSTERED(sno asc)
);

--Table with UniqueIdentifier as Clusetered Index
CREATE TABLE dbo.TblSEQUID
(
Sno Uniqueidentifier NOT NULL DEFAULT NEWSEQUENTIALID(),
FirstName VARCHAR(100) NOT NULL,
DOB DATETIME NOT NULL,
CONSTRAINT pk_tblSEQUid PRIMARY KEY CLUSTERED(sno asc)
);

--Table with INTeger column as Clusetered Index
CREATE TABLE dbo.TblInt
(
Sno INT IDENTITY(1,1) NOT NULL,
FirstName VARCHAR(100) NOT NULL,
DOB DATETIME NOT NULL,
CONSTRAINT pk_tblInt PRIMARY KEY CLUSTERED(sno asc)
);

--Table with BIGINTeger as Clusetered Index
CREATE TABLE dbo.TblBigInt
(
Sno BIGINT IDENTITY(1,1) NOT NULL,
FirstName VARCHAR(100) NOT NULL,
DOB DATETIME NOT NULL,
CONSTRAINT pk_tblBigInt PRIMARY KEY CLUSTERED(sno asc)
);


-----------------------------------------------------
--Create NON Clustered Index on DOB Column
-----------------------------------------------------
CREATE NONCLUSTERED INDEX nx_tbluid_dob on dbo.tbluid(DOB);
CREATE NONCLUSTERED INDEX nx_tblsequid_dob on dbo.tblSEQUid(DOB);
CREATE NONCLUSTERED INDEX nx_tblInt_dob on dbo.tblInt(DOB);
CREATE NONCLUSTERED INDEX nx_tblBigInt_dob on dbo.tblBigInt(DOB);


/*
Insert dummy records in each of the table. 
Change the number near GO for capturing data for more records
*/

SET NOCOUNT ON;

INSERT INTO dbo.TblUID (FirstName, DOB)
SELECT REPLICATE('x',100),Getdate();
GO 10000

INSERT INTO dbo.TblSEQUID (FirstName, DOB)
SELECT REPLICATE('x',100),Getdate();
GO 10000

INSERT INTO dbo.TblInt(FirstName, DOB)
SELECT REPLICATE('x',100),Getdate();
GO 10000

INSERT INTO dbo.TblBigInt(FirstName, DOB)
SELECT REPLICATE('x',100),Getdate();
GO 10000


---------------------------------------------------
--Check the Space Used and Fragmentation Level
---------------------------------------------------
EXEC sp_spaceused tblUID, True;
SELECT index_type_desc, avg_fragmentation_in_percent, page_count, record_count
FROM sys.dm_db_index_physical_stats (DB_ID(), OBJECT_ID(N'tblUID'), NULL, NULL, NULL)
OPTION (MAXDOP 1);

EXEC sp_spaceused tblSEQUID, True;
SELECT index_type_desc, avg_fragmentation_in_percent, page_count, record_count
FROM sys.dm_db_index_physical_stats (DB_ID(), OBJECT_ID(N'tblSEQUID'), NULL, NULL, NULL)          
OPTION (MAXDOP 1);
  
EXEC sp_spaceused tblINT, True;
SELECT index_type_desc, avg_fragmentation_in_percent, page_count, record_count
FROM sys.dm_db_index_physical_stats (DB_ID(), OBJECT_ID(N'tblINT'), NULL, NULL, NULL)
OPTION (MAXDOP 1);

EXEC sp_spaceused tblBIGINT, True;
SELECT index_type_desc, avg_fragmentation_in_percent, page_count, record_count
FROM sys.dm_db_index_physical_stats (DB_ID(), OBJECT_ID(N'tblBIGINT'), NULL, NULL, NULL)
OPTION (MAXDOP 1);

Inspired by this blog post of Kimberlyhttp://www.sqlskills.com/BLOGS/KIMBERLY/post/GUIDs-as-PRIMARY-KEYs-andor-the-clustering-key.aspx



Comments

Popular posts from this blog

[Non Tech] Want to know the recipe for Omelette :)

Fed up with Bread - Jam and Curd Rice, today i wanted to eat Omelette. Interesting part is I wanted to cook it myself :) So in the first picture you see all the items which are needed for preparing an Omelette. When I had a closer look at the eggs I see that almost all the eggs are broken. But believe me when I bought it couple of days back it was in perfect condition! I was wondering whether the eggs have become rotten or pretty old to consume! I tried taking an egg and break it but couldn't break it at all :) Since I have kept in the freezer all the eggs have frozen and looked like a iron ball :) After trying for few minutes of trying i removed the shell of the egg and then kept that iron ball :) into a bowl and placed it within Oven. I heated it for 1 minute and checked. It melted only to a limit. So i just set it for another 2 minutes and checked it later. It has melted but the part of the egg white has become a Omelette :( I didn't leave it there. I took the bowl out of ...

Script table as - ALTER TO is greyed out - SQL SERVER

One of my office colleague recently asked me why we are not able to generate ALTER Table script from SSMS. If we right click on the table and choose "Script Table As"  ALTER To option would be disabled or Greyed out. Is it a bug? No it isn't a bug. ALTER To is there to be used for generating modified script of Stored Procedure, Functions, Views, Triggers etc., and NOT for Tables. For generating ALTER Table script there is an work around. Right click on the table, choose "Modify" and enter into the design mode. Make what ever changes you want to make and WITHOUT saving it right click anywhere on the top half of the window (above Column properties) and choose "Generate Change Script". Please be advised that SQL Server would drop actually create a new table with modifications, move the data from the old table into it and then drop the old table. Sounds simple but assume you have a very large table for which you want to do this! Then it woul...

My Wedding Anniversary :)

Six years back on the same day I married Sai Lakshmi (12-July-2000). I know Sai for almost 13 years now :) I fell in love with her during my 12th standard. I know @ 17 yrs any person wouldn't be matured enough to make a big decision like this. But thank God my choice was perfect :) Even now, very often we used to think about the past and laugh at our behaviors/actions then. My love story would be really interesting (at least for me and Sai :)) and I am sure none of you guys would be interested in reading about it so lemme not get into it in-depth. But one thing which I want to share is "Without Sai, I wouldn't have entered into the IT field at all". She was instrumental in convincing me to study my Master's degree in Computer Application. That's the move that changed my career. Till my schooling, my dream was to either become a "big" sportsman (Cricket and Badminton were my favorites at that time.) or an Aeronautics engineer. Unfortunately, my l...