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: https://postgr.es/m/20200906212112.nzoy5ytrzjjodpfh@development
Reviewed-by: Tomas Vondra
Backpatch-through: 13

diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index 104e779f6accd2a2d1d3a0ee26dde9f6acdba807..f39e6a9f80d81ac262b5b3e7ddf49cb33f2d9b58 100644 (file)
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -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;
        }
 
        /*