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Commit8693ebe

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Avoid some zero-divide hazards in the planner.
Although I think on all modern machines floating division by zeroresults in Infinity not SIGFPE, we still don't want infinitiesrunning around in the planner's costing estimates; too much riskof that leading to insane behavior.grouping_planner() failed to consider the possibility that final_relmight be known dummy and hence have zero rowcount. (I wonder if itwould be better to set a rows estimate of 1 for dummy relations?But at least in the back branches, changing this convention seemslike 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 andsmall ntuples; we did not prevent the result from rounding to zero.For good luck I applied clamp_row_est() to all the nonconstant returnvalues.In ExecChooseHashTableSize(), Assert that we compute positive nbucketsand nbatch. I know of no reason to think this isn't the case, but itseems like a good safety check.Per reports from Piotr Stefaniak. Back-patch to all active branches.
1 parent5515ec0 commit8693ebe

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3 files changed

+12
-7
lines changed

3 files changed

+12
-7
lines changed

‎src/backend/executor/nodeHash.c

Lines changed: 3 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -542,6 +542,9 @@ ExecChooseHashTableSize(double ntuples, int tupwidth, bool useskew,
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nbatch <<=1;
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}
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545+
Assert(nbuckets>0);
546+
Assert(nbatch>0);
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545548
*numbuckets=nbuckets;
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*numbatches=nbatch;
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}

‎src/backend/optimizer/plan/planner.c

Lines changed: 4 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -1536,9 +1536,11 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
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standard_qp_callback,&qp_extra);
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/*
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* Extract rowcount and width estimates for use below.
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* Extract rowcount and width estimates for use below. If final_rel
1540+
* has been proven dummy, its rows estimate will be zero; clamp it to
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* one to avoid zero-divide in subsequent calculations.
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*/
1541-
path_rows=final_rel->rows;
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path_rows=clamp_row_est(final_rel->rows);
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path_width=final_rel->width;
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/*

‎src/backend/utils/adt/selfuncs.c

Lines changed: 5 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -4632,8 +4632,8 @@ examine_simple_variable(PlannerInfo *root, Var *var,
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* *isdefault: set to TRUE if the result is a default rather than based on
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* anything meaningful.
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*
4635-
* NB: be careful to produceanintegral result, since callers may compare
4636-
* the result to exact integer counts.
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* NB: be careful to producea positiveintegral result, since callers may
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*comparethe result to exact integer counts, or might divide by it.
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*/
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double
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get_variable_numdistinct(VariableStatData*vardata,bool*isdefault)
@@ -4709,7 +4709,7 @@ get_variable_numdistinct(VariableStatData *vardata, bool *isdefault)
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* If we had an absolute estimate, use that.
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*/
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if (stadistinct>0.0)
4712-
returnstadistinct;
4712+
returnclamp_row_est(stadistinct);
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/*
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* Otherwise we need to get the relation size; punt if not available.
@@ -4730,15 +4730,15 @@ get_variable_numdistinct(VariableStatData *vardata, bool *isdefault)
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* If we had a relative estimate, use that.
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*/
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if (stadistinct<0.0)
4733-
returnfloor((-stadistinct*ntuples)+0.5);
4733+
returnclamp_row_est(-stadistinct*ntuples);
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/*
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* With no data, estimate ndistinct = ntuples if the table is small, else
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* use default. We use DEFAULT_NUM_DISTINCT as the cutoff for "small" so
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* that the behavior isn't discontinuous.
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*/
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if (ntuples<DEFAULT_NUM_DISTINCT)
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returnntuples;
4741+
returnclamp_row_est(ntuples);
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47434743
*isdefault= true;
47444744
returnDEFAULT_NUM_DISTINCT;

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