presto-orc/src/main/java/io/prestosql/orc/writer/MapColumnWriter.java/MapColumnWriter/writeBlock
class MapColumnWriter:    @Override
    public void writeBlock(Block block)
    {
        checkState(!closed);
        checkArgument(block.getPositionCount() > 0, "Block is empty");

        ColumnarMap columnarMap = toColumnarMap(block);
        writeColumnarMap(columnarMap);
    }