From ea6e286491210e4ec2ddd542faccfe9227eb890f Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Thu, 30 Jul 2015 12:11:23 -0400 Subject: Avoid some zero-divide hazards in the planner. Although I think on all modern machines floating division by zero results in Infinity not SIGFPE, we still don't want infinities running around in the planner's costing estimates; too much risk of that leading to insane behavior. grouping_planner() failed to consider the possibility that final_rel might be known dummy and hence have zero rowcount. (I wonder if it would be better to set a rows estimate of 1 for dummy relations? But at least in the back branches, changing this convention seems like a bad idea, so I'll leave that for another day.) Make certain that get_variable_numdistinct() produces a nonzero result. The case that can be shown to be broken is with stadistinct < 0.0 and small ntuples; we did not prevent the result from rounding to zero. For good luck I applied clamp_row_est() to all the nonconstant return values. In ExecChooseHashTableSize(), Assert that we compute positive nbuckets and nbatch. I know of no reason to think this isn't the case, but it seems like a good safety check. Per reports from Piotr Stefaniak. Back-patch to all active branches. --- src/backend/optimizer/plan/planner.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) (limited to 'src/backend/optimizer/plan/planner.c') diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c index 3a812877d62..e062cabf7a2 100644 --- a/src/backend/optimizer/plan/planner.c +++ b/src/backend/optimizer/plan/planner.c @@ -1134,11 +1134,14 @@ grouping_planner(PlannerInfo *root, double tuple_fraction) /* * Extract rowcount and width estimates for possible use in grouping * decisions. Beware here of the possibility that - * cheapest_path->parent is NULL (ie, there is no FROM clause). + * cheapest_path->parent is NULL (ie, there is no FROM clause). Also, + * if the final rel has been proven dummy, its rows estimate will be + * zero; clamp it to one to avoid zero-divide in subsequent + * calculations. */ if (cheapest_path->parent) { - path_rows = cheapest_path->parent->rows; + path_rows = clamp_row_est(cheapest_path->parent->rows); path_width = cheapest_path->parent->width; } else -- cgit v1.2.3