From 69cc60dcfd0fb643cd2fe3ce66d4389858bfdeb5 Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Fri, 10 May 2013 17:15:30 -0400 Subject: [PATCH] Guard against input_rows == 0 in estimate_num_groups(). This case doesn't normally happen, because the planner usually clamps all row estimates to at least one row; but I found that it can arise when dealing with relations excluded by constraints. Without a defense, estimate_num_groups() can return zero, which leads to divisions by zero inside the planner as well as assertion failures in the executor. An alternative fix would be to change set_dummy_rel_pathlist() to make the size estimate for a dummy relation 1 row instead of 0, but that seemed pretty ugly; and probably someday we'll want to drop the convention that the minimum rowcount estimate is 1 row. Back-patch to 8.4, as the problem can be demonstrated that far back. --- src/backend/utils/adt/selfuncs.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c index 656d03b69a..0d5cafba96 100644 --- a/src/backend/utils/adt/selfuncs.c +++ b/src/backend/utils/adt/selfuncs.c @@ -3209,6 +3209,14 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows) double numdistinct; ListCell *l; + /* + * We don't ever want to return an estimate of zero groups, as that tends + * to lead to division-by-zero and other unpleasantness. The input_rows + * estimate is usually already at least 1, but clamp it just in case it + * isn't. + */ + input_rows = clamp_row_est(input_rows); + /* * If no grouping columns, there's exactly one group. (This can't happen * for normal cases with GROUP BY or DISTINCT, but it is possible for -- GitLab