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POCommon Table Expressions slow when using a table variable
 

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  1. POCommon Table Expressions slow when using a table variable
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    <p>I have been experimenting with the following (simplified) CTE. When using a table variable () the query runs for minutes before I cancel it. Any of the other commented out methods return in less than a second. </p> <p>If I replace the whole WHERE clause with an INNER JOIN it is fast as well. </p> <p>Any ideas why using a table variable would run so slowly?</p> <p>FWIW: The database contains 2.5 million records and the inner query returns 2 records. </p> <pre class="lang-sql prettyprint-override"><code>CREATE TABLE #rootTempTable (RootID int PRIMARY KEY) INSERT INTO #rootTempTable VALUES (1360); DECLARE @rootTableVar TABLE (RootID int PRIMARY KEY); INSERT INTO @rootTableVar VALUES (1360); WITH My_CTE AS ( SELECT ROW_NUMBER() OVER(ORDER BY d.DocumentID) rownum, d.DocumentID, d.Title FROM [Document] d WHERE d.LocationID IN ( SELECT LocationID FROM Location JOIN @rootTableVar rtv ON Location.RootID = rtv.RootID -- VERY SLOW! --JOIN #rootTempTable tt ON Location.RootID = tt.RootID -- Fast --JOIN (SELECT 1360 as RootID) AS rt ON Location.RootID = rt.RootID -- Fast --WHERE RootID = 1360 -- Fast ) ) SELECT * FROM My_CTE WHERE (rownum &gt; 0) AND (rownum &lt;= 100) ORDER BY rownum </code></pre> <p>This is from when using the table variable. The query took over 17 minutes to run: <img src="https://i.stack.imgur.com/ZJXzP.png" alt="enter image description here"></p> <p><strong>Execution plans in XML format</strong></p> <p>Temp table: <a href="https://docs.google.com/open?id=0B66I-fxlyEtEZEthV3ZaWlNLWXM" rel="nofollow noreferrer">https://docs.google.com/open?id=0B66I-fxlyEtEZEthV3ZaWlNLWXM</a></p> <p>Table variable: <a href="https://docs.google.com/open?id=0B66I-fxlyEtEbUFZa3RJejFCTkk" rel="nofollow noreferrer">https://docs.google.com/open?id=0B66I-fxlyEtEbUFZa3RJejFCTkk</a></p>
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    1. PTQuestion
    1. USPhil Haselden
    1. USPhil Haselden
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      1. POCommon Table Expressions slow when using a table variable
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      1. VTUpMod
    2. VO
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    1. COPost the query plans for the table variable and the temp table versions.
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      1. USRBarryYoung
    2. COthis is actually the first time i've seen a primary key inside a table variable. i'm curious - what does the execution plan have to say ?
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      1. POCommon Table Expressions slow when using a table variable
      1. USYS.
    3. COI let the table variable version run to completion: 17m 41s. Any particular format you would like for the execution plan? BTW - the primary key was just something I was playing around with to see if it would make a difference. It didn't seem to.
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      1. POCommon Table Expressions slow when using a table variable
      1. USPhil Haselden
 

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