What's the necessity of the HepRelVertex used in HepPlanner

I'm studying Calcite recently and I found that RelNode Tree would be converted to a DAG composed by HepRelVertex and Edge before the RBO rules execution, and I don't understand why this step is necessary since RelNode tree is a DAG essentially.

Re: What's the necessity of the HepRelVertex used in HepPlanner

Ayaka, Per my understanding, HepRelVertex is used to make RelNode changeable easily, you can see more about HepRelVertex#replaceRel, it is used to change one RelNode to another after some rules applied and new equal node produced. In this way, we can easily change subgraphs of the original DAG aft

Re: What's the necessity of the HepRelVertex used in HepPlanner

Hi Ayaka, I agree with Benchao, you can look at this article to get a sense on why a data structure allowing node substitution is useful in query planning: https://www.querifylabs.com/blog/memoization-in-cost-based-optimizers Best regards, Alessandro On Wed, 21 Feb 2024 at 05:55, Benchao Li wrot

Re: What's the necessity of the HepRelVertex used in HepPlanner

Let’s suppose that the information in HepRelVertex could somehow be moved into the RelNode. (Since the data structure is a DAG it seems plausible.) Now each RelNode would contain mutable state that relates to the HEP planner’s algorithm. That state needs to be managed (e.g. reinitialized when st