Ranked Enumeration of Conjunctive Query Results
release_xan6qk6mqbaf7p274lfmhiblyi
by
Shaleen Deep, Paraschos Koutris
2021
Abstract
We investigate the enumeration of top-k answers for conjunctive queries
against relational databases according to a given ranking function. The task is
to design data structures and algorithms that allow for efficient enumeration
after a preprocessing phase. Our main contribution is a novel priority queue
based algorithm with near-optimal delay and non-trivial space guarantees that
are output sensitive and depend on structure of the query. In particular, we
exploit certain desirable properties of ranking functions that frequently occur
in practice and degree information in the database instance, allowing for
efficient enumeration. We introduce the notion of decomposable and compatible ranking functions in conjunction with query decomposition, a
property that allows for partial aggregation of tuple scores in order to
efficiently enumerate the ranked output. We complement the algorithmic results
with lower bounds justifying why certain assumptions about properties of
ranking functions are necessary and discuss popular conjectures providing
evidence for optimality of enumeration delay guarantees. Our results extend and
improve upon a long line of work that has studied ranked enumeration from both
theoretical and practical perspective.
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