presto-main/src/main/java/io/prestosql/operator/aggregation/CountAggregation.java/CountAggregation/output
class CountAggregation:    @OutputFunction(StandardTypes.BIGINT)
    public static void output(@AggregationState LongState state, BlockBuilder out)
    {
        BIGINT.writeLong(out, state.getLong());
    }