@Table vs #Table

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@Table vs #Table

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gerald.green

Posted Thursday, March 21, 2013 9:45 AM

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I'll start by saying that I am a C# developer, not a DBA. When it comes to performance tweaks and making the Procs I write run the fastest they can, I trust the DBAs.

My last job, I was told by more that one DBA that table variables run in RAM and do not access the hard drive. I worked with the DBAs for over six years and came to trust & value what they told me. I used Table Variables when ever possible and didn't use Temp Tables.

While reworking a (messy) stored procedure at my current job the DBA told me "That is a common misconception. Table Variables and Temp Tables both access TempDB. Therefore, IO hard drive access."

Which DBA is correct?

Post #1433875

Lynn Pettis

Posted Thursday, March 21, 2013 9:51 AM

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The one you work with now. However, it is possible that one or both will access tempdb or that one or both will reside completely in memory. It all depends on the amount of data and the query.

The biggest thing to remember is the table variables do not have statistics and will be treated by the optimizer as if they have one row, regardless of the actual number of rows they have. I have heard differently at times, but my experience seems to hold this to be true. Best to test for yourself.

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Lynn Pettis

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Post #1433881

Mark-101232

Posted Thursday, March 21, 2013 10:42 AM

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There's an excellent article here

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Post #1433906

GilaMonster

Posted Thursday, March 21, 2013 11:42 AM

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http://sqlinthewild.co.za/index.php/2010/10/12/a-trio-of-table-variables/

The 'table variables are memory only' is an all-too-common misconception. Both temp tables and table variables are allocated space in TempDB and added to the tempDB system tables. They may however not actually be on disk, SQL will try to keep them both in memory as much as possible because it's very likely that those pages will be needed again shortly.

Gail Shaw
Microsoft Certified Master: SQL Server 2008, MVP
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

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Post #1433938

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