presto-main/src/main/java/io/prestosql/operator/unnest/ArrayOfRowsUnnester.java/ArrayOfRowsUnnester/getElementsBlock
class ArrayOfRowsUnnester:    @Override
    protected Block getElementsBlock(int channel)
    {
        checkState(channel >= 0 && channel < fieldCount, "Invalid channel number");
        return columnarRow.getField(channel);
    }