[buckbova]

Query 1 is an index seek only. It does not access the table data.

Query 2 will perform the same index seek but will need to do a key lookup on each row and filter.

It's not negligible. The 100 results are not comprised of a lot more information in this case, regardless of the grouping, because the 1st query does not access the table.

Edit:

I happen to have a table laying around with a little over a million rows and set up a similar set of queries.

The query optimizer suggested the index seek taking 6% of total operation time while the key lookup taking up the other 94%. The rest was negligible.