This is an oversimplification. You will have to read the index to figure out where the parts you need to read are. And if your columns don't align well with t

This is an oversimplification. You will have to read the index to figure out where the parts you need to read are. And if your columns don't align well with the blocks / pages of the storage you are using, this "optimization" may turn out to be worthless, or, perhaps even harmful, if your system is starving for memory.