A Bayesian Approach to Estimating the Selectivity of Conjunctive Predicates

: Cost-based optimizers in relational databases make use of data statistics to estimate intermediate result cardinalities. Those cardinalities are needed to estimate access plan costs in order to choose the cheapest plan for executing a query. Since statistics are usually collected on single attributes only, the optimizer can not directly estimate result cardinalities of conjunctive predicates over multiple attributes. To avoid having to fall back to assuming statistical independence, modern relational database systems offer the possibility to additionally collect joint statistics over multiple attributes. These statistics allow a direct cardinality estimate for conjunctive predicates. A widely used approach is collecting the number of distinct value combinations as a joint statistic. This can be used for a uniformity based estimate, which assumes each value combination to occur equally often. Although this estimate is likely an improvement, it is still inaccurate, since “real worldâ...

• Type: Conference
• Year: 2009
• Where: BTW
• Authors: M. Heimel, Volker Markl, Keshava Murthy
• Keywords: BTW 2009, Conjunctive Predicates, Direct Cardinality Estimate, Multiple Attributes