Blog Post

Clustered Index Seek or Key Lookup

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When I teach performance tuning I always remind students to be cautious about service packs because things you "know" may no longer be true after the service pack because of subtle or no so subtle changes made by MS. I was lucky enough to have an example of this show up during a recent class. I had SP1 installed on my laptop and SP2 just added to the image used for training machines. In SP1 when you examine a graphical query plan you will see one of two operatars when doing a bookmark lookup, either a Clustered Index Seek or a RID Lookup. In SP2 what used to be a Clustered Index Seek is now a Key Lookup. Nothing in the release notes that I can see about why the change was made, but you can see the MS confirmation at Key Lookup Info. Not a huge change, but seems like change for the sake of change.

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