presto-spi/src/main/java/io/prestosql/spi/Page.java/Page/getRegion
class Page:    public Page getRegion(int positionOffset, int length)
    {
        if (positionOffset < 0 || length < 0 || positionOffset + length > positionCount) {
            throw new IndexOutOfBoundsException(format("Invalid position %s and length %s in page with %s positions", positionOffset, length, positionCount));
        }

        int channelCount = getChannelCount();
        Block[] slicedBlocks = new Block[channelCount];
        for (int i = 0; i < channelCount; i++) {
            slicedBlocks[i] = blocks[i].getRegion(positionOffset, length);
        }
        return new Page(length, slicedBlocks);
    }