presto-main/src/main/java/io/prestosql/operator/aggregation/multimapagg/MultimapAggregationStateSerializer.java/MultimapAggregationStateSerializer/deserialize
class MultimapAggregationStateSerializer:    @Override
    public void deserialize(Block block, int index, MultimapAggregationState state)
    {
        state.reset();
        ColumnarRow columnarRow = toColumnarRow(arrayType.getObject(block, index));
        Block keys = columnarRow.getField(KEY_CHANNEL);
        Block values = columnarRow.getField(VALUE_CHANNEL);
        for (int i = 0; i < columnarRow.getPositionCount(); i++) {
            state.add(keys, values, i);
        }
    }