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POSQL Server Query Tuning: why CPU Time is higher than Elapsed Time ? Are they relevant to set operation?
 

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  1. POSQL Server Query Tuning: why CPU Time is higher than Elapsed Time ? Are they relevant to set operation?
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    <p>I have two query to filter some userid depend on question and its answers.</p> <h2>Scenario</h2> <p>Query A is (the original version):</p> <pre><code>SELECT userid FROM mem..ProfileResult WHERE ( ( QuestionID = 4 AND QuestionLabelID = 0 AND AnswerGroupID = 4 AND ResultValue = 1 ) OR ( QuestionID = 14 AND QuestionLabelID = 0 AND AnswerGroupID = 19 AND ResultValue = 3 ) OR ( QuestionID = 23 AND QuestionLabelID = 0 AND AnswerGroupID = 28 AND ( ResultValue &amp; 16384 &gt; 0 ) ) OR ( QuestionID = 17 AND QuestionLabelID = 0 AND AnswerGroupID = 22 AND ( ResultValue = 6 OR ResultValue = 19 OR ResultValue = 21 ) ) OR ( QuestionID = 50 AND QuestionLabelID = 0 AND AnswerGroupID = 51 AND ( ResultValue = 10 OR ResultValue = 41 ) ) ) GROUP BY userid HAVING COUNT(*) = 5 </code></pre> <p>I use 'set statistics time on' and 'set statistic io on' to check the cpu time and io performance.</p> <p>the result is:</p> <pre><code>CPU time = 47206 ms, elapsed time = 20655 ms. </code></pre> <p>I rewrote Query A via using Set Operation, let me name it Query B:</p> <pre><code>SELECT userid FROM ( SELECT userid FROM mem..ProfileResult WHERE QuestionID = 4 AND QuestionLabelID = 0 AND AnswerGroupID = 4 AND ResultValue = 1 INTERSECT SELECT userid FROM mem..ProfileResult WHERE QuestionID = 14 AND QuestionLabelID = 0 AND AnswerGroupID = 19 AND ResultValue = 3 INTERSECT SELECT userid FROM mem..ProfileResult WHERE QuestionID = 23 AND QuestionLabelID = 0 AND AnswerGroupID = 28 AND ( ResultValue &amp; 16384 &gt; 0 ) INTERSECT SELECT userid FROM mem..ProfileResult WHERE QuestionID = 17 AND QuestionLabelID = 0 AND AnswerGroupID = 22 AND ( ResultValue = 6 OR ResultValue = 19 OR ResultValue = 21 ) INTERSECT SELECT userid FROM mem..ProfileResult WHERE QuestionID = 50 AND QuestionLabelID = 0 AND AnswerGroupID = 51 AND ( ResultValue = 10 OR ResultValue = 41 ) ) vv; </code></pre> <p>the CPU Time and Elapsed Time is:</p> <pre><code>CPU time = 8480 ms, elapsed time = 18509 ms </code></pre> <h2>My Simple Analysis</h2> <p>As you can see from up result, Query A have CPU Time more than 2 times of Elapsed time</p> <p>I search for this case, mostly people say CPU time should less than Elapsed time, because CPU time is how long the CPU running this task. And the Elapsed time include I/O time and other sort of time cost. But one special case is when the Server has multiple Core CPU. However, I just checked the development db server and it has one single core CPU.</p> <h2>Question 1</h2> <p>How to explain that CPU time more than Elapsed time in Query A in a single core CPU environment?</p> <h2>Question 2</h2> <p>After, using set operation, Is the performance really improved?</p> <p>I have this question because logical reads of Query B is 280627 which is higher than Query A's 241885</p> <p><a href="http://www.bradmcgehee.com/" rel="nofollow">Brad McGehee</a> said in his article that <a href="http://www.sql-server-performance.com/2002/statistics-io-time/3/" rel="nofollow">'The fewer the logical reads performed by a query, the more efficient it is, and the faster it will perform, <strong>assuming all other things are held equal</strong>.'</a></p> <p>Than, does it correctly say that even Query B have higher logical reads than Query A, but CPU time is significantly less than Query A, Query B should have a better performance.</p>
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      1. POSQL Server Query Tuning: why CPU Time is higher than Elapsed Time ? Are they relevant to set operation?
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    1. COYour CPU is equipped with a flux capacitor and can run faster than time. Is there no room for humor here? It's obviously a bug in the CPU task scheduler or the database program. Does the 1-core CPU appear to be 2 virtual CPUs?
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      1. USDaniel Teichman
    2. CONo, only 1 single core CPU, and no virtual CPU.
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    3. COThen that shouldn't be possible. The server must think it has multiple CPUs.
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