Representing list of polymophic objects in db

Let's suppose I have a list of derived objects from some base:

class Circle : public Shape;
class Rectangle : public Shape;

There is object, which has vector of Shape * as member variable:

class Screen {
    std::vector<Shape *> shapes_;
}

But now I need to persist this in database (PostgreSQL to be precise). Only way I thought of is to have table per class, something like this:

table screen ();

table circle (
    int screen_id;
);

table rectangle (
    int screen_id;
);

And after retrieving from object of type Screen do query for Circles and Rectangles with screen.id. Should be doable.

However I fear that when number of types grows (and it will), it would mean many db queries for just one object retrieval/persist.

Right know we'll have about 10 types I think? That's 11 db queries just to retrieve one Screen object.

So my questing is, is there better way to achieve my goal? I'm fine with redesigning my code if needed, I just would like some O(1) solution in correlation to number of types (my solution is O(n) ).

Thank you guys ^_^

NOTE: based on comment I'm adding that tables of Circles and Rectangles etc. will be usedonly to put together Screen object. I don't access (look-up, retrieve, persist, ...) to them individually, only as parts of Screen(s). They don't even have to be in their own table. Just didn't find a way to nicely put it all together :/