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Guard against input_rows == 0 in estimate_num_groups().

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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.
This commit is contained in:
Tom Lane 2013-05-10 17:15:30 -04:00
parent 477b5a0e24
commit 69cc60dcfd
1 changed files with 8 additions and 0 deletions
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8
src/backend/utils/adt/selfuncs.c
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@ -3209,6 +3209,14 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows)
double numdistinct; double numdistinct;
ListCell *l; 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 * 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 * for normal cases with GROUP BY or DISTINCT, but it is possible for