Parametric Plan Caching Using Density-Based Clustering

Query plan caching eliminates the need for repeated query optimization, hence, it has strong practical implications for relational database management systems (RDBMSs). Unfortunately, existing approaches consider only the query plan generated at the expected values of parameters that characterize the query, data and the current state of the system, while these parameters may take different values during the lifetime of a cached plan. A better alternative is to harvest the optimizer's plan choice for different parameter values, populate the cache with promising query plans, and select a cached plan based upon current parameter values. To address this challenge, we propose a parametric plan caching (PPC) framework that uses an online plan space clustering algorithm. The clustering algorithm is density-based, and it exploits locality-sensitive hashing as a pre-processing step so that clusters in the plan spaces can be efficiently stored in database histograms and queried in constant time. We experimentally validate that our approach is precise, efficient in space-and-time and adaptive, requiring no eager exploration of the plan spaces of the optimizer.