Adding more columns slows down the SELECT query

Question

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Imagine Foo table has non-clustered indexes on ColA and ColB and NO Indexes on ColC, ColD

SQL

SELECT colA, colB
FROM Foo

takes about 30 seconds.

SQL

SELECT colA, colB, colC, colD
FROM Foo

takes about 2 minutes.
Foo table has more than 5 million rows.
Question: Is it possible that including columns that are not part of the indexes can slow down the query? If yes, WHY? -Are not they part of the already read PAGEs?

Posted 27-Jun-13 18:41pm

Thomas Bovee

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ArunRajendra · Answer 1 · 2013-06-27T18:51:00

You can improve the performace by creating multiple index with different combinations of columns as per the need. The other option is if you want to fetch the columns let's say colB, colC, colD only based on colA index then you can use include while creating the index as shown in the example.

SQL

CREATE NONCLUSTERED INDEX IX_INDEX1    ON Foo (colA)
    INCLUDE (colB, colC, colD);

And also look at the execution plan to identify the bottelnecks. For more details about creating index try this link http://msdn.microsoft.com/en-us/library/ms188783.aspx[^]

Hope this is useful.

Bogdan Ionesku · Answer 2 · 2013-06-27T18:59:00

Solution 2

Clustered indexes are stored along with data while non-clustered indexes are not. Mixing them in SELECT statement is never a smart choice.

Posted 27-Jun-13 18:59pm

Bogdan Ionesku