Before revision:  In this paper, we first systematically compare these two methods  . Extensive experiment results demonstrate that dense representation method significantly outperforms the sparse representation, but costs more time and storage occupation.
After revision:  In this paper, we first systematically compare these two methods from four aspects: (1) effectiveness; (2) index stoarge; (3) search time cost; (4) human evaluation . Extensive experiment results demonstrate that dense representation method significantly outperforms the sparse representation, but costs more time and storage occupation.