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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.