Nested Loops Revisited Again.

Hash joins and sort-merge joins have been considered the algorithms of choice for analytical relational queries in most parallel database systems because of their performance robustness and ease of parallelization. On the other hand, nested loop joins have been considered less attractive and are conservatively used. In this paper, we revisit the potential of nested loop joins in a cluster environment. We focus on exploring the parallelism aspect of nested loop joins because there could still be space for improvement by fully exploiting the parallelism of current commodity hardware, which could handle more than thousands of concurrent IOs. We also introduce scalable massively-parallel execution as one of the approaches for achieving massive parallelism in nested loop joins to explore how it widens the potential benefit of nested loop joins. Finally, we discuss future research directions based on our exploration.