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POWhy would using a temp table be faster than a nested query?
 

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  1. POWhy would using a temp table be faster than a nested query?
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    <p>We are trying to optimize some of our queries. </p> <p>One query is doing the following:</p> <pre><code>SELECT t.TaskID, t.Name as Task, '' as Tracker, t.ClientID, (&lt;complex subquery&gt;) Date, INTO [#Gadget] FROM task t SELECT TOP 500 TaskID, Task, Tracker, ClientID, dbo.GetClientDisplayName(ClientID) as Client FROM [#Gadget] order by CASE WHEN Date IS NULL THEN 1 ELSE 0 END , Date ASC DROP TABLE [#Gadget] </code></pre> <p>(I have removed the complex subquery. I don't think it's relevant other than to explain why this query has been done as a two stage process.)</p> <p>I <em>thought</em> it would be far more efficient to merge this down into a single query using subqueries as:</p> <pre><code>SELECT TOP 500 TaskID, Task, Tracker, ClientID, dbo.GetClientDisplayName(ClientID) FROM ( SELECT t.TaskID, t.Name as Task, '' as Tracker, t.ClientID, (&lt;complex subquery&gt;) Date, FROM task t ) as sub order by CASE WHEN Date IS NULL THEN 1 ELSE 0 END , Date ASC </code></pre> <p>This would give the optimizer better information to work out what was going on and avoid any temporary tables. I assumed it should be faster.</p> <p>But it turns out it is a lot slower. 8 seconds vs. under 5 seconds.</p> <p>I can't work out why this would be the case, as all my knowledge of databases imply that subqueries would always be faster than using temporary tables.</p> <p>What am I missing?</p> <p><strong>Edit</strong> --</p> <p>From what I have been able to see from the query plans, both are largely identical, except for the temporary table which has an extra "Table Insert" operation with a cost of 18%. </p> <p>Obviously as it has two queries the cost of the Sort Top N is a lot higher in the second query than the cost of the Sort in the Subquery method, so it is difficult to make a direct comparison of the costs.</p> <p>Everything I can see from the plans would indicate that the subquery method would be faster.</p>
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    1. COHave you compared query plans?
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    2. CO@Oded, see my edit.
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    3. CODid you clear the data cache between each test? If not, that could skew your comparison
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