Does a dual key or single key lookup perform better on a table with a lot of data? We have table where rows unique between 3 columns. GroupA, GroupB, ProductID.

GroupA &amp; GroupB are used in combinations to Group sets of ProductIDs and will always be used to lookup sets of productIDs. Sometimes a ProductID will be used in in the query also. 

I wasnt sure if there would be any performance gains from combining GroupA and GroupB into the same column so that the index is clustered.

   GroupA | GroupB | ProductID
   ----------------------------
   A1     | B1     | 1
   A1     | B1     | 99
   A1     | B2     | 1
   A1     | B2     | 99
   A2     | B1     | 1
   A2     | B1     | 99
   A2     | B2     | 1
   A2     | B2     | 99


Dual group method above OR Single group method below.

   GroupAB | ProductID
   --------------------
   A1-B1   | 1
   A1-B1   | 99
   A1-B2   | 1
   A1-B2   | 99
   A2-B1   | 1
   A2-B1   | 99
   A2-B2   | 1
   A2-B2   | 99


   GroupA | GroupB | GroupAB
   ----------------------------
   A1     | B1     | A1-B1
   A1     | B2     | A1-B2
   A2     | B1     | A2-B1
   A2     | B2     | A2-B2
I don't think it would really matter.  You can cluster on both without concatenating them:

CREATE CLUSTERED INDEX ixc_MyIndexName ON MyTable (GroupA, GroupB)


Which will accomplish the same thing but not require you to modify your data.