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Avoid some zero-divide hazards in the planner.

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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.
This commit is contained in:
Tom Lane
2015-07-30 12:11:23 -04:00
parent 2d8c136e74
commit 54833c5e77
4 changed files with 14 additions and 8 deletions
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10
src/backend/utils/adt/selfuncs.c
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@ -4337,8 +4337,8 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
*
* vardata: results of examine_variable
*
* 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)
@ -4412,7 +4412,7 @@ get_variable_numdistinct(VariableStatData *vardata)
* 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.
@ -4427,14 +4427,14 @@ get_variable_numdistinct(VariableStatData *vardata)
* 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
* use default.
*/
if (ntuples < DEFAULT_NUM_DISTINCT)
return ntuples;
return clamp_row_est(ntuples);
return DEFAULT_NUM_DISTINCT;
}