presto-main/src/main/java/io/prestosql/sql/planner/optimizations/HashGenerationOptimizer.java/Rewriter/canSkipHashGeneration
class Rewriter:        private boolean canSkipHashGeneration(List<Symbol> partitionSymbols)
        {
            // HACK: bigint grouped aggregation has special operators that do not use precomputed hash, so we can skip hash generation
            return partitionSymbols.isEmpty() || (partitionSymbols.size() == 1 && types.get(Iterables.getOnlyElement(partitionSymbols)).equals(BIGINT));
        }