For the latter part of your question: Yes, there is at least on RDBMS, Umbra, that implements wcoj's. I have a pointer to it in the blog post. Although, I'm convinced eventually many GDBMSs will integrate these algorithms, whether RDBMSs will broadly integrate these is less clear to me. The reason is not whether it's doable or not. Every DBMS, graph or relational, is relational at its core, in the sense that they compile high-level query languages to joins, groups by, filters, scans etc. In fact wcoj algorithms were invented assuming a relational system joining multiple relations. The reason is whether they care enough because wcojs are primarily for "cyclic joins" and they don't appear too often in traditional olap workloads, which are aggregation-heavy. Cyclic joins are more frequent on workloads on GDBMSs (e.g., finding a clique of users to develop a recommendation engine). So these algorithms are more critical for GDBMSs.