postgres.git / summary / log / commit / refs

commit    8693ebe37d65ec1bc1ebeab36f60b38f18fb25d6
Author:   Tom Lane <tgl@sss.pgh.pa.us>
Date:     Thu Jul 30 16:11:23 2015 +0000

    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.


 src/backend/executor/nodeHash.c      |  3 +++
 src/backend/optimizer/plan/planner.c |  6 ++++--
 src/backend/utils/adt/selfuncs.c     | 10 +++++-----
 3 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/src/backend/executor/nodeHash.c b/src/backend/executor/nodeHash.c
index 906cb46b658..ee9298a157f 100644
--- 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 6ee411eec87..09d4ea12e87 100644
--- 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 64b6ae4838f..14b8c2ff545 100644
--- 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;


[parent: 5515ec0b3a94]