Depth estimation for ranking query optimization |
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Authors: | Karl Schnaitter Joshua Spiegel Neoklis Polyzotis |
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Affiliation: | (1) UC Santa Cruz, Santa Cruz, CA, USA;(2) Oracle, St. Louis, MO, USA |
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Abstract: | A relational ranking query uses a scoring function to limit the results of a conventional query to a small number of the most
relevant answers. The increasing popularity of this query paradigm has led to the introduction of specialized rank join operators
that integrate the selection of top tuples with join processing. These operators access just “enough” of the input in order
to generate just “enough” output and can offer significant speed-ups for query evaluation. The number of input tuples that
an operator accesses is called the input depth of the operator, and this is the driving cost factor in rank join processing. This introduces the important problem of depth estimation, which is crucial for the costing of rank join operators during query compilation and thus for their integration in optimized
physical plans. We introduce an estimation methodology, termed deep, for approximating the input depths of rank join operators in a physical execution plan. At the core of deep lies a general, principled framework that formalizes depth computation in terms of the joint distribution of scores in the
base tables. This framework results in a systematic estimation methodology that takes the characteristics of the data directly
into account and thus enables more accurate estimates. We develop novel estimation algorithms that provide an efficient realization
of the formal deep framework, and describe their integration on top of the statistics module of an existing query optimizer. We validate the
performance of deep with an extensive experimental study on data sets of varying characteristics. The results verify the effectiveness of deep as an estimation method and demonstrate its advantages over previously proposed techniques. |
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Keywords: | Data statistics Top-k DEEP Query optimization Depth estimation Relational ranking query |
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