Avoid some zero-divide hazards in the planner.

Although I think on all modern machines floating division by zero
results in Infinity not SIGFPE, we still don't want infinities
running around in the planner's costing estimates; too much risk
of that leading to insane behavior.

grouping_planner() failed to consider the possibility that final_rel
might be known dummy and hence have zero rowcount.  (I wonder if it
would be better to set a rows estimate of 1 for dummy relations?
But at least in the back branches, changing this convention seems
like a bad idea, so I'll leave that for another day.)

Make certain that get_variable_numdistinct() produces a nonzero result.
The case that can be shown to be broken is with stadistinct < 0.0 and
small ntuples; we did not prevent the result from rounding to zero.
For good luck I applied clamp_row_est() to all the nonconstant return
values.

In ExecChooseHashTableSize(), Assert that we compute positive nbuckets
and nbatch.  I know of no reason to think this isn't the case, but it
seems like a good safety check.

Per reports from Piotr Stefaniak.  Back-patch to all active branches.

diff --git a/src/backend/executor/nodeHash.c b/src/backend/executor/nodeHash.c
index 8cb5611671b2c1c81d59c67f2982993daf53dc20..26eff5eb8a59a9dcf076622532ecc03fc9771e8c 100644 (file)
--- a/src/backend/executor/nodeHash.c
+++ b/src/backend/executor/nodeHash.c
@@ -508,6 +508,9 @@ ExecChooseHashTableSize(double ntuples, int tupwidth, bool useskew,
        i++;
    nbuckets = (1 << i);
 
+   Assert(nbuckets > 0);
+   Assert(nbatch > 0);
+
    *numbuckets = nbuckets;
    *numbatches = nbatch;
 }

diff --git a/src/backend/optimizer/plan/planmain.c b/src/backend/optimizer/plan/planmain.c
index bb163d0cab2b2d730b85adaebb6877801a79df9e..6df8ff6d5f3b622d75cfa1d3bf33cf0393ce117e 100644 (file)
--- a/src/backend/optimizer/plan/planmain.c
+++ b/src/backend/optimizer/plan/planmain.c
@@ -345,7 +345,7 @@ query_planner(PlannerInfo *root, List *tlist,
         * can be divided by the number of tuples.
         */
        if (tuple_fraction >= 1.0)
-           tuple_fraction /= final_rel->rows;
+           tuple_fraction /= clamp_row_est(final_rel->rows);
    }
 
    /*

diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index e64de3aa67ae467bf3b78709992357b4211631f4..8e90e193bffc7e59af7eae3b4957607ed261e5d4 100644 (file)
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -1247,11 +1247,14 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
        /*
         * Extract rowcount and width estimates for possible use in grouping
         * decisions.  Beware here of the possibility that
-        * cheapest_path->parent is NULL (ie, there is no FROM clause).
+        * cheapest_path->parent is NULL (ie, there is no FROM clause).  Also,
+        * if the final rel has been proven dummy, its rows estimate will be
+        * zero; clamp it to one to avoid zero-divide in subsequent
+        * calculations.
         */
        if (cheapest_path->parent)
        {
-           path_rows = cheapest_path->parent->rows;
+           path_rows = clamp_row_est(cheapest_path->parent->rows);
            path_width = cheapest_path->parent->width;
        }
        else

diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 94a2372ecaa9d90350ac102e6ee71a5efba65a85..9b5e5ab9c251020442c2efc09d85473a4ec9804b 100644 (file)
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -4616,8 +4616,8 @@ examine_simple_variable(PlannerInfo *root, Var *var,
  * *isdefault: set to TRUE if the result is a default rather than based on
  * anything meaningful.
  *
- * NB: be careful to produce an integral result, since callers may compare
- * the result to exact integer counts.
+ * NB: be careful to produce a positive integral result, since callers may
+ * compare the result to exact integer counts, or might divide by it.
  */
 double
 get_variable_numdistinct(VariableStatData *vardata, bool *isdefault)
@@ -4693,7 +4693,7 @@ get_variable_numdistinct(VariableStatData *vardata, bool *isdefault)
     * If we had an absolute estimate, use that.
     */
    if (stadistinct > 0.0)
-       return stadistinct;
+       return clamp_row_est(stadistinct);
 
    /*
     * Otherwise we need to get the relation size; punt if not available.
@@ -4714,7 +4714,7 @@ get_variable_numdistinct(VariableStatData *vardata, bool *isdefault)
     * If we had a relative estimate, use that.
     */
    if (stadistinct < 0.0)
-       return floor((-stadistinct * ntuples) + 0.5);
+       return clamp_row_est(-stadistinct * ntuples);
 
    /*
     * With no data, estimate ndistinct = ntuples if the table is small, else
@@ -4722,7 +4722,7 @@ get_variable_numdistinct(VariableStatData *vardata, bool *isdefault)
     * that the behavior isn't discontinuous.
     */
    if (ntuples < DEFAULT_NUM_DISTINCT)
-       return ntuples;
+       return clamp_row_est(ntuples);
 
    *isdefault = true;
    return DEFAULT_NUM_DISTINCT;