Which index is used in select and why?

Posted by Lukasz Lysik on Stack Overflow See other posts from Stack Overflow or by Lukasz Lysik
Published on 2010-05-27T08:29:49Z Indexed on 2010/05/27 8:31 UTC
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sql-server

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query

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select

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index

I have the table with zip codes with following columns:

id - PRIMARY KEY
code - NONCLUSTERED INDEX
city

When I execute query

SELECT TOP 10 * FROM ZIPCodes

I get the results sorted by id column. But when I change the query to:

SELECT TOP 10 id FROM ZIPCodes

I get the results sorted by code column. Again, when I change the query to:

SELECT TOP 10 code FROM ZIPCodes

I get the results sorted by code column again. And finally when I change to:

SELECT TOP 10 id,code FROM ZIPCodes

I get the results sorted by id column.

My question is in the title of the question. I know which indexes are used in the queries, but my question is, why those indexes are used? I the second query (SELECT TOP 10 id FROM ZIPCodes) wouldn't it be faster if the clusteder index was used? How the query engine chooses which index to use?

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