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| author | Tom Lane <tgl@sss.pgh.pa.us> | 2015-07-30 12:11:23 -0400 |
|---|---|---|
| committer | Tom Lane <tgl@sss.pgh.pa.us> | 2015-07-30 12:11:23 -0400 |
| commit | 8693ebe37d65ec1bc1ebeab36f60b38f18fb25d6 (patch) | |
| tree | 3bb7d4aa347ba2c23eae8f764d78dfbe9af5b96d /src/backend/utils/adt/selfuncs.c | |
| parent | 5515ec0b3a9425a0c0e36e26b5774c1568f22b3a (diff) | |
| download | postgresql-8693ebe37d65ec1bc1ebeab36f60b38f18fb25d6.tar.gz postgresql-8693ebe37d65ec1bc1ebeab36f60b38f18fb25d6.zip | |
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.
Diffstat (limited to 'src/backend/utils/adt/selfuncs.c')
| -rw-r--r-- | src/backend/utils/adt/selfuncs.c | 10 |
1 files changed, 5 insertions, 5 deletions
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; |