Local vs. Global Optimization: Operator Placement Strategies in Heterogeneous Environments

In several parts of query optimization, like join enumeration or physical operator selection, there is always the question of how much optimization is needed and how large the performance benefits are. In particular, a decision for either global optimization (e.g., during query optimization) or local optimization (during query execution) has to be taken. In this way, heterogeneity in the hardware environment is adding a further optimization aspect while it is yet unknown, how much optimization is actually required for that aspect. Generally, several papers have shown that heterogeneous hardware environments can be used e ciently by applying operator placement for OLAP queries. However, whether it is better to apply this placement in a local or global optimization strategy is still an open question. To tackle this challenge, we examine both strategies for a column-store database system in this paper. Aside from describing local and global placement in detail, we conduct an exhaustive evaluation to draw some conclusions. For the global placement strategy, we also propose a novel approach to address the challenge of an exploding search space together with discussing wellknown solutions for improving cardinality estimation.