presto-main/src/main/java/io/prestosql/operator/unnest/MapUnnester.java/MapUnnester/getElementsBlock
class MapUnnester:    @Override
    protected Block getElementsBlock(int channel)
    {
        checkState(channel == 0 || channel == 1, "index is not 0 or 1");
        if (channel == 0) {
            return columnarMap.getKeysBlock();
        }
        return columnarMap.getValuesBlock();
    }