A Learned Query Rewrite System

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Abstract

Query rewriting is a challenging task that transforms a SQL query to improve its performance while maintaining its result set. However, it is difficult to rewrite SQL queries, which often involve complex logical structures, and there are numerous candidate rewrite strategies for such queries, making it an NP-hard problem. Existing databases or query optimization engines adopt heuristics to rewrite queries, but these approaches may not be able to judiciously and adaptively apply the rewrite rules and may cause significant performance regression in some cases (e.g., correlated subqueries may not be eliminated). To address these limitations, we introduce LearnedRewrite, a query rewrite system that combines traditional and learned algorithms (i.e., Monte Carlo tree search + hybrid estimator) to rewrite queries. We have implemented the system in Calcite, and experimental results demonstrate LearnedRewrite achieves superior performance on three real datasets.