presto-spi/src/main/java/io/prestosql/spi/block/LazyBlock.java/LazyBlock/getRegion
class LazyBlock:    @Override
    public Block getRegion(int positionOffset, int length)
    {
        if (isLoaded()) {
            return getBlock().getRegion(positionOffset, length);
        }
        checkValidRegion(getPositionCount(), positionOffset, length);
        return new LazyBlock(length, new RegionLazyBlockLoader(lazyData, positionOffset, length));
    }