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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 d20327a4fd
commit ea6e286491
4 changed files with 14 additions and 8 deletions
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2
src/backend/optimizer/plan/planmain.c
View File

@@ -365,7 +365,7 @@ query_planner(PlannerInfo *root, List *tlist,
* can be divided by the number of tuples.
*/
if (tuple_fraction >= 1.0)
tuple_fraction /= final_rel->rows;
tuple_fraction /= clamp_row_est(final_rel->rows);
}
/*

7
src/backend/optimizer/plan/planner.c
View File

@@ -1134,11 +1134,14 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
/*
* Extract rowcount and width estimates for possible use in grouping
* decisions. Beware here of the possibility that
* cheapest_path->parent is NULL (ie, there is no FROM clause).
* cheapest_path->parent is NULL (ie, there is no FROM clause). Also,
* if the final rel has been proven dummy, its rows estimate will be
* zero; clamp it to one to avoid zero-divide in subsequent
* calculations.
*/
if (cheapest_path->parent)
{
path_rows = cheapest_path->parent->rows;
path_rows = clamp_row_est(cheapest_path->parent->rows);
path_width = cheapest_path->parent->width;
}
else