The problem of c-Approximate Nearest Neighbor (c-ANN) search in high-dimensional space is fundamentally important in many applications, such as image database and data mining. Locality-Sensitive Hashing (LSH) and its variants are the well-known indexing schemes to tackle the c-ANN search problem. Traditionally, LSH functions are constructed in a query-oblivious manner, in the sense that buckets are partitioned before any query arrives. However, objects closer to a query may be partitioned into different buckets, which is undesirable. Due to the use of query-oblivious bucket partition, the state-of-the-art LSH schemes for external memory, namely C2LSH and LSB-Forest, only work with approximation ratio of integer c≥2\documentclass[12pt]{minimal}
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\begin{document}$$c \ge 2$$\end{document}. In this paper, we introduce a novel concept of query-aware bucket partition which uses a given query as the “anchor” for bucket partition. Accordingly, a query-aware LSH function under a specific lp\documentclass[12pt]{minimal}
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\begin{document}$$l_p$$\end{document} norm with p∈(0,2]\documentclass[12pt]{minimal}
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\begin{document}$$p \in (0,2]$$\end{document} is a random projection coupled with query-aware bucket partition, which removes random shift required by traditional query-oblivious LSH functions. The query-aware bucket partitioning strategy can be easily implemented so that query performance is guaranteed. For each lp\documentclass[12pt]{minimal}
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\begin{document}$$l_p$$\end{document} norm (p∈(0,2])\documentclass[12pt]{minimal}
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\begin{document}$$(p \in (0,2])$$\end{document}, based on the corresponding p-stable distribution, we propose a novel LSH scheme named query-aware LSH (QALSH) for c-ANN search over external memory. Our theoretical studies show that QALSH enjoys a guarantee on query quality. The use of query-aware LSH function enables QALSH to work with any approximation ratio c>1\documentclass[12pt]{minimal}
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\begin{document}$$c > 1$$\end{document}. In addition, we propose a heuristic variant named QALSH+\documentclass[12pt]{minimal}
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\begin{document}$$^+$$\end{document} to improve the scalability of QALSH. Extensive experiments show that QALSH and QALSH+\documentclass[12pt]{minimal}
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\begin{document}$$^+$$\end{document} outperform the state-of-the-art schemes, especially in high-dimensional space. Specifically, by using a ratio c<2\documentclass[12pt]{minimal}
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\begin{document}$$c < 2$$\end{document}, QALSH can achieve much better query quality.
