Cache-conscious radix-decluster projections

As CPUs become more powerful with Moore's law and memory latencies stay constant, the impact of the memory access performance bottleneck continues to grow on relational operators like join, which can exhibit random access on a memory region larger than the hardware caches. While cache-conscious variants for various relational algorithms have been described, previous work has mostly ignored (the cost of) projection columns. However, real-life joins almost always come with projections, such that proper projection column manipulation should be an integral part of any generic join algorithm. In this paper, we analyze cache-conscious hash-join algorithms including projections on two storage schemes: N-ary Storage Model (NSM) and Decomposition Storage Model (DSM). It turns out, that the strategy of first executing the join and only afterwards dealing with the projection columns (i.e., post-projection) on DSM, in combination with a new finely tunable algorithm called Radix-Decluster, outperforms all previously reported projection strategies. To make this result generally applicable, we also outline how DSM Radix-Decluster can be integrated in a NSM-based RDBMS using projection indices.