since you said the PartNumber is the first field, then yes; but it also depends on how selective that single column is.
if there's a lot of common values, SQL might just decide to do a clustered index scan instead.
so to confirm, if PartNumber is the leading column in your index that has three fields, would it be posible for a query to use the index.
so if the index is (PartNumber,SecondKey,ThirdKey) then yes.
if the index was created as (FirstKey,SecondKey,PartNumber) then no, you'd need an extra idnex.
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