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T-SQL Merge Statement with Output Clause Performance Issues

I'm running a fairly complex T-SQL Merge statement which updates a slowly changing dimension from staging tables.

I'm noticing something weird in regards to performance.

When I run the query without an output clause, the query runs very quickly because there haven't been any changes in Staging since the last time I ran the query.

However, when I add the following:

OUTPUT $action, Inserted.*, Deleted.*

The performance is abysmal. This is strange because the returned result set is empty anyway.

Any ideas as to why the output clause is causing this?

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asked Nov 02, 2011 at 08:47 AM in Default

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DavidStein
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Any difference in the execution plans?

Nov 02, 2011 at 12:43 PM Kev Riley ♦♦
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asked: Nov 02, 2011 at 08:47 AM

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Last Updated: Nov 02, 2011 at 08:47 AM

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