presto-main/src/main/java/io/prestosql/cost/AggregationStatsRule.java/AggregationStatsRule/estimateAggregationStats
class AggregationStatsRule:    private static SymbolStatsEstimate estimateAggregationStats(Aggregation aggregation, PlanNodeStatsEstimate sourceStats)
    {
        requireNonNull(aggregation, "aggregation is null");
        requireNonNull(sourceStats, "sourceStats is null");

        // TODO implement simple aggregations like: min, max, count, sum
        return SymbolStatsEstimate.unknown();
    }