Synonyms
XML cardinality estimation
Definition
Selectivity estimation in database systems refers to the task of estimating the number of results that will be output for a given query. Selectivity estimates are crucial in query optimization, since they enable optimizers to select efficient query plans. They are also employed in interactive data exploration as timely feedback about the expected outcome of user queries, and can even serve as approximate answers for count queries.
Selectivity estimators apply an estimation procedure on a synopsis of the data. Due to the stringent time and space constraints of query optimization, of which selectivity estimation is only one of the steps, selectivity estimators are faced with two, often conflicting, requirements: they have to accurately and efficiently estimate the cardinality of queries while keeping the synopsis size to a minimum.
While there is a large body of literature on selectivity estimation in the context of relational databases, the...
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Ramanath, M., Freire, J., Polyzotis, N. (2018). XML Selectivity Estimation. In: Liu, L., Özsu, M.T. (eds) Encyclopedia of Database Systems. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-8265-9_801
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