As far as I understand it this is about the ECS equivalent of an INNER JOIN, that is, some variation on "iterate over all entities that possess instances of com

As far as I understand it this is about the ECS equivalent of an INNER JOIN, that is, some variation on "iterate over all entities that possess instances of component A and B". Iterating over single component types in a system is trivial if they are separated into pools by types. When you have a system requiring more than one component, it isn't that easy to do better than iterating over all entities and checking whether they possess each required component. You can decide to maintain back references from components to their entities, but this means that you incur that management overhead pretty much globally. This essay says that doing away with entities altogether and just keeping direct links between your components is better. And it probably is when it comes to handling related components. However, I believe (without having actually tried it) that this proposal has its own drawbacks. it's harder to thoroughly clean up all components that form a single entity. My suspicion is that it's easier to produce bugs like keeping e.g. a stray collision shape or sound source around while the other parts of an entity are removed. All you need for that to happen is an incomplete graph traversal.