presto-main/src/main/java/io/prestosql/sql/planner/iterative/rule/AddExchangesBelowPartialAggregationOverGroupIdRuleSet.java/BaseAddExchangesBelowExchangePartialAggregationGroupId/estimatedGroupCount
class BaseAddExchangesBelowExchangePartialAggregationGroupId:        private double estimatedGroupCount(List<Symbol> symbols, PlanNodeStatsEstimate statsEstimate)
        {
            return symbols.stream()
                    .map(statsEstimate::getSymbolStatistics)
                    .mapToDouble(this::ndvIncludingNull)
                    // This assumes no correlation, maximum number of aggregation keys
                    .reduce(1, (a, b) -> a * b);
        }