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| author | Tom Lane <tgl@sss.pgh.pa.us> | 2013-05-10 17:15:51 -0400 |
|---|---|---|
| committer | Tom Lane <tgl@sss.pgh.pa.us> | 2013-05-10 17:15:51 -0400 |
| commit | dbd68df9b636d4b36469dab7281a2e7d38863570 (patch) | |
| tree | f13d14894e3f0e7292f03f82cf8bc98ae7c62edc | |
| parent | 53cdc3e9f061dde0e13726f3964812446834a6bf (diff) | |
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.
| -rw-r--r-- | src/backend/utils/adt/selfuncs.c | 8 |
1 files changed, 8 insertions, 0 deletions
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c index e383533fe71..0cbb73d670e 100644 --- a/src/backend/utils/adt/selfuncs.c +++ b/src/backend/utils/adt/selfuncs.c @@ -3056,6 +3056,14 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows) 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 * corner cases with set operations.) |