Join slows down sql [closed]

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We have a discussion over SQL Server 2008 and join. One half says the more joins the slower you sql runs. The开发者_运维技巧 other half says ihat it does not matter because SQL server takes care of business so you wil not notice any performance loss. What is true?

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Instead of asking the question the way you have, consider instead:

• Can I get the data I want without the join?
  • No => You need the join, end of discussion.

It is also a matter of degree. It is impossible for a join not to add additional processing. Even if the Query Optimizer takes it out (e.g. left join with nothing used from the join) - it still costs CPU cycles to parse it.

Now if the question is about comparing joins to another technique, such as one special case of LEFT JOIN + IS NULL vs NOT EXISTS for a record in X not in Y scenario, then let's discuss specifics - table sizes (X vs Y), indexes etc.

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It will slow it down: the more complicated a query, the more work the database server has to do to execute it.

But about that "performance loss": over what? Is there another way to get at the same data? If so, then you can profile the various options against each other to see which is fastest.