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 906cb46b65892c7b0fb325c8fb0057589de6d981..ee9298a157f564c6f52b3bc3afdb1ccc224d93be 100644 (file)
--- a/src/backend/executor/nodeHash.c
+++ b/src/backend/executor/nodeHash.c
@@ -542,6 +542,9 @@ ExecChooseHashTableSize(double ntuples, int tupwidth, bool useskew,
                        nbatch <<= 1;
        }
 
+       Assert(nbuckets > 0);
+       Assert(nbatch > 0);
+
        *numbuckets = nbuckets;
        *numbatches = nbatch;
 }

diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 6ee411eec870a95a3c42a198f79af15b52cf3464..09d4ea12e874c6ae29f1a96405e430e686a97e0d 100644 (file)
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -1536,9 +1536,11 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
                                                                  standard_qp_callback, &qp_extra);
 
                /*
-                * Extract rowcount and width estimates for use below.
+                * Extract rowcount and width estimates for use below.  If final_rel
+                * has been proven dummy, its rows estimate will be zero; clamp it to
+                * one to avoid zero-divide in subsequent calculations.
                 */
-               path_rows = final_rel->rows;
+               path_rows = clamp_row_est(final_rel->rows);
                path_width = final_rel->width;
 
                /*

diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 64b6ae4838f2438ec7a36c3a838180c60b0f35aa..14b8c2ff54591b355bf59a96d5cbc785a073da87 100644 (file)
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -4632,8 +4632,8 @@ 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)
@@ -4709,7 +4709,7 @@ 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.
@@ -4730,7 +4730,7 @@ 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
@@ -4738,7 +4738,7 @@ 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;