presto-main/src/main/java/io/prestosql/operator/aggregation/ArbitraryAggregationFunction.java/ArbitraryAggregationFunction/combine
class ArbitraryAggregationFunction:    public static void combine(BlockPositionState state, BlockPositionState otherState)
    {
        if (state.getBlock() != null) {
            return;
        }
        state.setBlock(otherState.getBlock());
        state.setPosition(otherState.getPosition());
    }