Adjust cost model for HashAgg that spills to disk.
Tomas Vondra observed that the IO behavior for HashAgg tends to be
worse than for Sort. Penalize HashAgg IO costs accordingly.
Also, account for the CPU effort of spilling the tuples and reading
them back.
Discussion:
Backpatch-through: 13
@@ -2416,6 +2416,7 @@ cost_agg(Path *path, PlannerInfo *root,
double pages;
double pages_written = 0.0;
double pages_read = 0.0;
+ double spill_cost;
double hashentrysize;
double nbatches;
Size mem_limit;
@@ -2453,9 +2454,21 @@ cost_agg(Path *path, PlannerInfo *root,
pages = relation_byte_size(input_tuples, input_width) / BLCKSZ;
pages_written = pages_read = pages * depth;
+ /*
+ * HashAgg has somewhat worse IO behavior than Sort on typical
+ * hardware/OS combinations. Account for this with a generic penalty.
+ */
+ pages_read *= 2.0;
+ pages_written *= 2.0;
+
startup_cost += pages_written * random_page_cost;
total_cost += pages_written * random_page_cost;
total_cost += pages_read * seq_page_cost;
+
+ /* account for CPU cost of spilling a tuple and reading it back */
+ spill_cost = depth * input_tuples * 2.0 * cpu_tuple_cost;
+ startup_cost += spill_cost;
+ total_cost += spill_cost;
}
/*
