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Obviously, SQL Server is choosing the wrong query plan. Yes, that can happen, I've had exactly the same scenario as you a few times. The problem is that optimizing a query (you mention a "complex subquery") is a non-trivial task: If you have n tables, there are roughly n! possible join orders -- and that's just the beginning. So, it's quite plausible that doing (a) first your inner query and (b) then your outer query is a good way to go, but SQL Server cannot deduce this information in reasonable time. What you can do is to help SQL Server. As Dan Tow writes in his great book "<a href="http://rads.stackoverflow.com/amzn/click/0596005733" rel="nofollow noreferrer">SQL Tuning</a>", the key is usually the join order, going from the most selective to the least selective table. Using common sense (or the method described in his book, which is a lot better), you could determine which join order would be most appropriate and then use the <a href="http://msdn.microsoft.com/en-us/library/ms181714.aspx" rel="nofollow noreferrer"><code>FORCE ORDER</code></a> query hint. Anyway, every query is unique, there is no "magic button" to make SQL Server faster. If you really want to find out what is going on, you need to look at (or show us) the query plans of your queries. Other interesting data is shown by <a href="http://msdn.microsoft.com/en-us/library/ms184361.aspx" rel="nofollow noreferrer">SET STATISTICS IO</a>, which will tell you how much (costly) HDD access your query produces.
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1. POWhy would using a temp table be faster than a nested query?
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