Andrew Gierth <andrew@tao11.riddles.org.uk> writes:
> The model (assuming I understand it rightly) is that what we're actually
> tracking is a startup cost and a per-output-row cost, but for comparison
> purposes we actually store the rows and the computed total, rather than
> just the per-row cost:
> rows
> startup_cost
> total_cost = startup_cost + (rows * per_row_cost)
Right. I tend to think of it differently: the cost to retrieve the
first K rows out of a total of N is
startup_cost + (total_cost - startup_cost) * K/N
but that comes out to the same thing as what Andrew said.
regards, tom lane