Are Multi-way Joins Actually Useful?

Multi-way joins improve performance by avoiding extra I/Os from multiple partitioning steps. There are several multi-way join algorithms proposed, and the research results are encouraging. However, commercial database systems are not currently using multi-way joins. Practical issues include modifying the optimizer and execution system to support multi-way operators and ensuring robust and reliable performance. The contribution of this work is an implementation and experimental evaluation of multi-way joins in PostgreSQL. We provide algorithms that modify the optimizer to cost multi-way joins and create and execute query plans that have more than two input operators. Experimental results show that multi-way joins are beneficial for several queries in a production database system and can be effectively exploited by the optimizer, however there are implementation issues that must be resolved to guarantee robust performance.