presto-spi/src/main/java/io/prestosql/spi/Page.java/Page/getPositions
class Page:    public Page getPositions(int[] retainedPositions, int offset, int length)
    {
        requireNonNull(retainedPositions, "retainedPositions is null");

        Block[] blocks = new Block[this.blocks.length];
        Arrays.setAll(blocks, i -> this.blocks[i].getPositions(retainedPositions, offset, length));
        return new Page(length, blocks);
    }