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 Performance impact of adding XML column?		  New Topic  Reply to Topic
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malachi151
Posting Yak Master

142 Posts
Posted - 10/08/2012 :  15:37:09  Show Profile  Visit malachi151's Homepage  Reply with Quote
What are the performance impacts, if any, of adding an XML column to a table, in relation to queries against the non-XML columns?

In other words, if an add an XML column to a table, will it impact queries against the non-XML columns?

I know it has an impact on distributed queries, i.e. if you try to query a table across a Linked Server that has an XML column in it you can't do that, even if you don't include the XML column in the query, etc.

So let's say that I added an XML column, and its mostly NULL, how would this impact performance, how exactly is it stored on the pages, etc?

Thanks

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My blog: http://practicaltsql.net/
Twitter: @RGPSoftware

chadmat
The Chadinator

USA
1958 Posts
Posted - 10/08/2012 :  15:57:38  Show Profile  Visit chadmat's Homepage  Reply with Quote
This behavior is somewhat configureable, but by default, I think XML is stored In Row. So it would affect your page density (Less rows per page). If they are mostly null, then the effect would be minimal.

-Chad
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jackv
Flowing Fount of Yak Knowledge

United Kingdom
1770 Posts
Posted - 10/09/2012 :  01:47:27  Show Profile  Visit jackv's Homepage  Reply with Quote
There are ways of configuring XML to be stored off-row check http://www.sqlserver-dba.com/2012/09/sql-server-large-value-types-out-of-row-and-performance.html, which replaces the data row with a 16 byte text pointer. Your potential performance hit may be large scans being forced therefore high IO. It depends on what portion of your queries are referencing the XM column.

Jack Vamvas
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http://www.sqlserver-dba.com
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