+ /* Add MergeAppend node overhead like we do it for the Append node */ + run_cost += cpu_tuple_cost * DEFAULT_APPEND_COST_FACTOR * tuples; + path->startup_cost = startup_cost + input_startup_cost; path->total_cost = startup_cost + run_cost + input_total_cost; }
You're doing that right after a comment that says we don't do that. It also does look like the "run_cost += cpu_operator_cost * tuples" is trying to do the same thing, so perhaps it's worth just replacing that, which by default will double that additional cost, although doing so would have the planner slightly prefer a MergeAppend to an Append than previously.
I think we can remove that code block entirely. I have added relevant comments around DEFAULT_APPEND_COST_FACTOR already.
However, I am not sure of doing this as you correctly said it may prefer MergeAppend to an Append. Will it be fine we remove that code block?
+#define DEFAULT_APPEND_COST_FACTOR 0.5
I don't really think the DEFAULT_APPEND_COST_FACTOR adds much. it means very little by itself. It also seems that most of the other cost functions just use the magic number.
Agree, but those magic numbers used only once at that place. But here there are two places. So if someone wants to update it, (s)he needs to make sure to update that at two places. To minimize that risk, having a #define seems better.
