A new look at generating multi-join continuous query plans: A qualified plan generation problem

State-of-the-art relational and continuous algorithms alike have focused on producing optimal or near-optimal query plans by minimizing a single cost function. However, ensuring accurate yet real-time responses for stream processing applications necessitates that the system identifies qualified rather than optimal query plans with the former guaranteeing that their utilization of both the CPU and the memory resources stays within their respective system capacities. In such scenarios, being optimal in one resource usage while out-of-bound in the other is not viable. Our experimental study illustrates that to be effective a qualified plan optimizer must explore an extended plan search space called the jtree space composed not only of the standard mjoin and binary join plans, but also of general join trees with mixed operator types. While our proposed dynamic programming-based JTree-Finder algorithm is guaranteed to generate a qualified query plan if such a plan exists in the search space, its exponential time complexity makes it not viable for continuous stream environments. To facilitate run-time optimization, we thus propose an efficient yet effective two-layer plan generation framework. The proposed framework first exploits the positive correlation between the CPU and memory usages to obtain plans that are minimal in at least one of the two resource usages. In our second layer we propose two alternative polynomial-time algorithms to explore the negative correlation between the resource usages to successfully generate query plans that adhere to both CPU and memory resource constraints. Effectiveness and efficiency of the proposed algorithms are experimentally evaluated by comparing them to each other as well as state-of-the-art techniques.