Searching On Partial Keys
Go Up to Using Indexes to Search for Records
If the dataset has more than one key column, and you want to search for values in a subset of that key, set KeyFieldCount to the number of columns on which you are searching. For example, if the dataset's current index has three columns, and you want to search for values using just the first column, set KeyFieldCount to 1.
For table type datasets with multiple-column keys, you can search only for values in contiguous columns, beginning with the first. For example, for a three-column key you can search for values in the first column, the first and second, or the first, second, and third, but not just the first and third.