Last time I built an ECS it was effectively a bare bones relational database. A system just queries the information that it needs. The code basically looked like this. There was no need for explicit where or select queries because all I really needed to check was the presence of a component. Here is a contrived example. ecs has an inner join function that accepts all components (stored in an array) that you want to join and everything gets passed to a callback that receives all the components.

    void autoheal_system() {
         ecs.inner_join(ecs.status, ecs.inventory, ..., [](status, inventory, ...){
             if(status.health < (0.2 * status.max_health) {
                inventory.find("potion").use()
             }
         })
    }

One big advantage with this system based implementation is that you are processing all entities in batches compared to the naive OOP style where executing entity.foo() may run Dog.foo or Cat.foo and thrash the instruction cache and cause branch misprediction because of the virtual function dispatch that is constantly switching between functions. I haven't done parallelism with this pattern yet but as long as you do not cross reference other entities it shouldn't be too difficult to just slap on an OpenMP pragma and have a parallel_inner_join function that runs everything on multiple cores.