presto-spi/src/main/java/io/prestosql/spi/block/AbstractMapBlock.java/AbstractMapBlock/getRegion
class AbstractMapBlock:    @Override
    public Block getRegion(int position, int length)
    {
        int positionCount = getPositionCount();
        checkValidRegion(positionCount, position, length);

        return createMapBlockInternal(
                mapType,
                position + getOffsetBase(),
                length,
                Optional.ofNullable(getMapIsNull()),
                getOffsets(),
                getRawKeyBlock(),
                getRawValueBlock(),
                getHashTables());
    }