[SigmundA]

B-trees have little to do with it, if the table has many columns its cheaper to scan the index for the value because it occupies less pages, thats all, less I/O more cache hits etc, goes from top to bottom on the index scanning for the result. This is the distinction between scan and seek.

I just ran a common one I see and yep MS SQL is still doing a index scan then key lookup to get result with a select * from table where col LIKE '%abc' type query.