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Commitdbd68df

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Guard against input_rows == 0 in estimate_num_groups().
This case doesn't normally happen, because the planner usually clampsall row estimates to at least one row; but I found that it can arisewhen dealing with relations excluded by constraints. Without a defense,estimate_num_groups() can return zero, which leads to divisions by zeroinside the planner as well as assertion failures in the executor.An alternative fix would be to change set_dummy_rel_pathlist() to makethe size estimate for a dummy relation 1 row instead of 0, but that seemedpretty ugly; and probably someday we'll want to drop the convention thatthe minimum rowcount estimate is 1 row.Back-patch to 8.4, as the problem can be demonstrated that far back.
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‎src/backend/utils/adt/selfuncs.c

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@@ -3055,6 +3055,14 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows)
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doublenumdistinct;
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ListCell*l;
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/*
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* We don't ever want to return an estimate of zero groups, as that tends
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* to lead to division-by-zero and other unpleasantness. The input_rows
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* estimate is usually already at least 1, but clamp it just in case it
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* isn't.
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*/
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input_rows=clamp_row_est(input_rows);
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/*
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* If no grouping columns, there's exactly one group. (This can't happen
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* for normal cases with GROUP BY or DISTINCT, but it is possible for

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