Covering Index vs Included Columns

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Covering Index vs Included Columns

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tfifield

Posted Friday, May 30, 2008 12:39 PM

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Hi Guys!

Can anyone tell me if there is any functional or performance difference between designing a covering index and an index that includes columns that will be queried.

Let's say I wanted to sum up the total quantity ordered by product in a date range.

SELECT ProductID, SUM(QtySold) AS MarchQty
FROM OrderDetail
WHERE OrderDate >= '3/1/08'
AND OrderDate < '4/1/08
GROUP BY ProductID

Would there by any performance difference between:
CREATE NONCLUSTERED INDEX IX_OrderDetailDateProdSold ON dbo.OrderDetail
( OrderDate ASC, ProductID ASC, QtySold ASC );

and

CREATE NONCLUSTERED INDEX IX_OrderDetailDateWithProdSold ON dbo.OrderDeail
(OrderDate) ASC
INCLUDE (ProductID, QtySold);

Post #509318

Matt Miller (#4)

Posted Friday, May 30, 2008 12:53 PM

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There should be, although how much might be debatable. Ultimately - I think the "sweet spot" is the version you didn't include:

CREATE NONCLUSTERED INDEX IX_OrderDetailDateProdSold ON dbo.OrderDetail
( OrderDate ASC, ProductID ASC) INCLUDE (QtySold);

The short version as I understand it to make the "ideal" covering would be:
- columns appearing in the WHERE, FROM, GROUP BY and ORDER BY should appear in the "main part" of the index.
- columns not otherwise included that appear in the SELECT or HAVING go in the UNCLUDE clause.

Technically - since the "main part" is what the B-tree is based on - by keeping the main part as skinny as you can, you get better perf, while avoiding the bookmark lookup with the INCLUDE stuff.

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Post #509326

Jack Corbett

Posted Friday, May 30, 2008 1:06 PM

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I think Matt covered it very well. Another way to explain it that the included columns should be columns that you don't search on, but will return. I think a good example are middle_name and suffix columns. You probably return it but your searching, ordering, and grouping will usually be by last_name, first_name which would be your index with middle_name and suffix as the included columns.

Jack Corbett

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Post #509334

tfifield

Posted Saturday, May 31, 2008 11:01 AM

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Thanks guys. It makes sense I'll give it a whirl.
Todd Fifield

Post #509487

Hans Lindgren

Posted Tuesday, April 13, 2010 3:03 AM

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Matt Miller (#4) (5/30/2008)

CREATE NONCLUSTERED INDEX IX_OrderDetailDateProdSold ON dbo.OrderDetail
( OrderDate ASC, ProductID ASC) INCLUDE (QtySold);

Matt,

Any specific reasons you wouldn't go for:

CREATE NONCLUSTERED INDEX IX_OrderDetailDateProdSold ON dbo.OrderDetail
( ProductID ASC, OrderDate ASC) INCLUDE (QtySold);

which (quite probably.Here I assume more products then dates in your solution) would have a much higher cardinality and would be far more effictive?

Regards,
Hans

Post #902213

Hans Lindgren

Posted Tuesday, April 13, 2010 3:12 AM

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tfifield (5/30/2008)

Can anyone tell me if there is any functional or performance difference between designing a covering index and an index that includes columns that will be queried.

Hi,

I'd like to point out that a covered index contains all columns in every node of the index while a index with included columns only contains all columns on the leaf level.

I.e. a Covered index is wider in every node, hence takes more space and will be less performant for queries of the type
SELECT col1,col2,col3
FROM tab1
WHERE col1=... AND col2=...
(where col1 and col2 are in both types of indexes and col3 is in the covered index but Included in the 'Included Columns Index')

Regards,
Hanslindgren

Post #902217

SQLRNNR

Posted Tuesday, April 13, 2010 5:50 PM

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As for the covering index vs. include topic, I would recommend reading an article by Josef Richburg and the ensuing discussion on the topic. You can find the article here.

I would also recommend reading up on Gail Shaws articles on the topic. You can find her first article in the series [urlhttp://www.sqlservercentral.com/articles/Indexing/68439/]here[/url].

In the discussion on Josef's article, it is pointed out that the space saved is negligible between the covering index and the include. Also, Matt's answer pretty much nailed it otherwise.

Jason AKA CirqueDeSQLeil
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MCM SQL Server 2008

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Post #902815

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