Solved

T-SQL: Order By ColA, ColB --means--> Create Index ... (ColA, ColB), right? [SQL Server 2005]

Posted on 2010-11-16
5
448 Views
Last Modified: 2012-05-10
Using the table below if I wish to have the most efficient two-level Order By as such:
select * from Simple order by ColA, ColB
The composite index should be ColA, ColB, right?  Or is it ColB, ColA?
CREATE INDEX SimpleIndex ON Simple (ColA,ColB);
create table Simple (
  PK int,
  ColA int,
  ColB int
);

Open in new window

0
Comment
Question by:ZuZuPetals
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 2
  • 2
5 Comments
 
LVL 16

Accepted Solution

by:
EvilPostIt earned 250 total points
ID: 34144898
Yes you are correct. You will effectivly be running through the index from begining to end and pulling the relevant information.
0
 
LVL 57

Assisted Solution

by:Raja Jegan R
Raja Jegan R earned 250 total points
ID: 34144904
Simply depends upon the usage and Cardinality of the columns.
If ColB is used most in WHERE clauses to uniquely identify records, then order should be ColB, ColA else you can have it as ColA, ColB
0
 
LVL 2

Author Comment

by:ZuZuPetals
ID: 34144931
@rrjegan17: So, for Order By ONLY, is the index ordering irrelevant? Let's say I never use the columns in a WHERE clause... is the index ColA,ColB the exact same as ColB, ColA?  Or, does index order matter for the Order By?
0
 
LVL 16

Assisted Solution

by:EvilPostIt
EvilPostIt earned 250 total points
ID: 34144991
No if you were to test this you would see in the query plan that an index scan is used on both clustered and nonclustered indexes.

If you were to use a where clause it would be a seek.
0
 
LVL 57

Assisted Solution

by:Raja Jegan R
Raja Jegan R earned 250 total points
ID: 34145008
>> So, for Order By ONLY, is the index ordering irrelevant?

Yes, it would be used in ORDER BY along with columns involved in WHERE clause.
Say if you have WHERE ColB = ? referenced in your query, then Index with order ColB, ColA would be used whereas ColA, ColB would not be used at all.

>> Let's say I never use the columns in a WHERE clause... is the index ColA,ColB the exact same as ColB, ColA?  

Yes, if it is not used in both ORDER BY and WHERE conditions, then it is exactly the same.
Hope that clarifies.
0

Featured Post

Ransomware: The New Cyber Threat & How to Stop It

This infographic explains ransomware, type of malware that blocks access to your files or your systems and holds them hostage until a ransom is paid. It also examines the different types of ransomware and explains what you can do to thwart this sinister online threat.  

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

This article will describe one method to parse a delimited string into a table of data.   Why would I do that you ask?  Let's say that you need to pass multiple parameters into a stored procedure to search for.  For our sake, we'll say that we wa…
In this article I will describe the Backup & Restore method as one possible migration process and I will add the extra tasks needed for an upgrade when and where is applied so it will cover all.

752 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question