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

Posted on 2010-11-16
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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
);

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Question by:ZuZuPetals
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EvilPostIt earned 250 total points
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Yes you are correct. You will effectivly be running through the index from begining to end and pulling the relevant information.
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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
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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?
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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.
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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.
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