In cost_size.c, there is this comment block: + * Note: in this cost model, AGG_SORTED and AGG_HASHED have exactly the + * same total CPU cost, but AGG_SORTED has lower startup cost. If the + * input path is already sorted appropriately, AGG_SORTED should be + * preferred (since it has no risk of memory overflow). This will happen + * as long as the computed total costs are indeed exactly equal --- but + * if there's roundoff error we might do the wrong thing. So be sure + * that the computations below form the same intermediate values in the + * same order.
But, why should they have the same cost in the first place? A sorted aggregation just has to do an equality comparison on each adjacent pair, which is costed as (cpu_operator_cost * numGroupCols) * input_tuples. A hash aggregation has to do a hashcode computation for each input, which apparently is also costed at (cpu_operator_cost * numGroupCols) * input_tuples. But it also has to do the equality comparison between the input tuple and any aggregation it is about to be aggregated into, to make sure the hashcode is not a false collision. That should be another (cpu_operator_cost * numGroupCols) * (input_tuples - numGroups), shouldn't it? Currently, that is apparently assumed to be free. Is there something here I am overlooking? Cheers, Jeff
