Steve Baldwin <steve.baldwin@gmail.com> writes: > I guess the difference doesn't have a huge bearing (as far as I can tell) > on the result, but it just seems odd that the inner-most 'Output' step > outputs the entire row in the case of the copy and only the required field > in the case of the original table. What triggers that behaviour?
The plan with the full output row is actually slightly cheaper, or at least so the planner thinks, because it saves a projection step. I imagine the reason you're not getting that with the original table is that there are some dropped column(s) in the original table, forcing the projection to be done to get rid of them.