From a547e6867527ca16628a3fb1cf3ef6f785210a31 Mon Sep 17 00:00:00 2001 From: Jeff Davis Date: Mon, 7 Sep 2020 13:31:59 -0700 Subject: [PATCH] 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 --- src/backend/optimizer/path/costsize.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c index fda4b2c6e87..cd3716d494f 100644 --- 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; } /*