Thanks for this post.Its very easy to understand.Good post.

I’m confused why the best recommendation isn’t to create a clustered index. In our situation we have a 5 Billion row table without a clustered index. The RID lookups are killing us. The need for a lot of extra non-clustered indexes is also killing performance because this is on a replication server.

Finally the writes to a clustered index are dramatically less than to a non-clustered index with a HEAP.

See: http://msdn.microsoft.com/en-us/library/cc917672.aspx#EFAA

How can anyone justify a large table without a clustered index?

Nick

Nice post, very impressive and understandable…Thank You.

We have a requirement to Merge six 12millions data tables into single Table.All the tables are having duplicate Emails.So We put the Primary key for email column using EnterPrise Manager and We r trying to merge by writing the following insert query, But it’s taking more than 20hrs…:-(

Insert into Total( Email,FirstName,LastName,Address,City,State,Zipcode,Gender,DOB ,
Phone,WebAddress1,Webaddress2,IPAddress,DateTime,Interest )
Select Email,FirstName,LastName,Address,City,State,Zipcode,Gender,DOB ,
Phone,WebAddress1,Webaddress2,IPAddress,DateTime,Interest
From [Set4]
Where
(not exists(select email from Total
where Total.email=[Set4].email))

How can we do this most efficiently??? Please Help Us……

Thanks in advance…

Please read my follow up post here :
SQL SERVER – Query Optimization – Remove Bookmark Lookup – Remove RID Lookup – Remove Key Lookup – Part 3
http://blog.sqlauthority.com/2009/10/12/sql-server-query-optimization-remove-bookmark-lookup-remove-rid-lookup-remove-key-lookup-part-3

Kind Regards,
Pinal

I’m curious to know why you have included the ID column in the covering index, as well as in the nc index with the includes. I thought all columns in the Clustered index were already included in the non clustered index.

Is there a reason why you did that?

Thanks,

If we do not need those columns in query for SURE they need to be removed. I agree with you there.

I will send you other details soon.

Kind Regards,
Pinal

We can not remove the columns which are needed in query from SELECT statement.

That would not be best practices.

Why not just remove, from the query, the column that’s causing the key lookup. This is really much simpler I’m sure you’ll agree. You’ve written about best practices for database design in the past, and taught me that fewer columns per table are always better. So if you’re getting lots of key lookups, it follows that we should split up our tables and make them smaller. This is the query optimizer’s way of telling us that we have messed up our design!

Thank you for continuing to educate us on how best to use our database.

Thank you

You are correct, however, I am going to cover that particular concept in different blog post.

This post is written to show the concept of covering Index. In next blog post, I am going to show as there is clustered index, your non clustered index will not require that key and it can be still covering Index.

Kind Regards,
Pinal