functions that map similar objects to the same hash codes with
higher probability than dissimilar objects. Plenty of solutions have
been proposed. Recent development focuses on supporting various
similarity measures [29] and space-ecient indexing [22, 28].
Learning to Hash
. Learning to hash (L2H) a data-dependent ap-
proach that maps original data to another (often Hamming) space
by exploiting the data distribution. The underlying principle is to
preserve the similarity information within an appropriate neighbor-
hood. Additional heuristics and optimizations are often added to fur-
ther reduce the information loss caused by the mapping or increase
generalization to unseen data. Recent advancements feature deep
learning in both supervised and unsupervised manner [
3
,
8
,
13
,
21
].
Another line of methods is based on product quantization [
11
], with
the unique ability to handle billions of objects.
Partition-based Methods
. Methods in this category can be deemed
as dividing the high-dimensional space into multiple disjoint re-
gions. Partition is often carried out in a recursive way, so the index
is represented by a tree or a forest. Notable methods are based on
pivoting [
27
], hyperplane [
2
,
20
], or compact partitioning such as
cluster [7] or Voronoi diagram [16].
Neighborhood-based Metho ds
. These methods construct a prox-
imity graph where nodes represent objects and edges connect
nearby objects. The main idea is to perform a search for similar ob-
jects atop the proximity graph. These methods achieve top accuracy
and speed trade-o in empirical evaluations [
12
]. Notable methods
include
𝑘
-NN graph [
5
], hierarchical navigable small world [
14
],
and navigating spreading-out graph [6].
1.4 Selectivity Estimation
Selectivity estimation outputs the approximate number of data
objects that satisfy a selection criterion. Due to its use in density
estimation, outlier detection, image retrieval, and query optimiza-
tion, selectivity estimation for high-dimensional data has received
considerable attention recently. Representative solutions are sam-
pling [
26
] and kernel density estimation [
15
]. A recent trend is to
formalize it as a regression task and utilize ML methods [23, 24].
1.5 Future Opportunities
We highlight a number of promising directions for future research:
(1) It is interesting to explore ML models as approximate solutions
(e.g., learning to index or learning to sample). (2) Answering com-
posite queries (e.g., conjunctive queries) over multiple attributes
will receive more attention. (3) Another direction is to develop e-
cient algorithms for query processing in data management systems,
where ML, CV, and NLP techniques can help improve the quality.
2 PREVIOUS EDITIONS
The previous edition of this tutorial appeared at VLDB 2020 [
18
].
The new edition focuses on data science related applications (e.g.,
classication, regression, anomaly detection, and recommender
systems). In addition, the new edition features the following new
materials: (1) a thorough discussion on the use of similarity query
processing in various application scenarios (e.g., the role of similar-
ity queries in the entire workow and the selection of algorithms),
(2) more data models and a broader range of related works, and
(3) more recent technical advancements and future trends.
Acknowledgements
This work was supported by Guangdong
Basic and Applied Basic Research Foundation 2020B1515120028,
Guangdong Pearl River Recruitment Program of Talents 2019ZT08X603,
Shenzhen Continuous Support Grant 20200811104054002, HKUST
Red Bird Visting Scholar Program, JSPS Kakenhi 17H06099, 18H04093,
and 19K11979, and ARC FT170100128 and DP210101393.
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