From: Tom Lane Date: Fri, 10 May 2013 21:15:35 +0000 (-0400) Subject: Guard against input_rows == 0 in estimate_num_groups(). X-Git-Tag: REL9_2_5~101 X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=4ebbbf3c483a47e1627fae50c41f650b2a1d549f;p=postgresql 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. --- diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c index 4b1229484e..de2d7f98b2 100644 --- a/src/backend/utils/adt/selfuncs.c +++ b/src/backend/utils/adt/selfuncs.c @@ -3207,6 +3207,14 @@ 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