From 8693ebe37d65ec1bc1ebeab36f60b38f18fb25d6 Mon Sep 17 00:00:00 2001 From: Tom Lane <tgl@sss.pgh.pa.us> Date: Thu, 30 Jul 2015 12:11:23 -0400 Subject: [PATCH] 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; -- GitLab