Captured and virtual sources.

47// VirtualConnection returns the one external execution context for the entire48// relation set. Mixing physical stores is an explicit unsupported plan, never49// a reason to scan or materialize another source implicitly.50func (r *Registry) VirtualConnection(datasets []string) (*ConnectionSpec, bool, error) {51	var selected *ConnectionSpec52	var captured bool53	for _, name := range datasets {54		d, ok := r.Dataset(name)55		if !ok {56			return nil, false, fmt.Errorf("source dataset %s does not exist", name)57		}58		if !d.IsVirtual {59			captured = true60			continue61		}62		connection, ok := r.Connection(d.Connection)63		if !ok {64			return nil, true, fmt.Errorf("virtual source connection is unavailable")65		}66		if selected != nil && selected.Name != connection.Name {67			return nil, true, BadRequest("cross-source virtual queries are unsupported")68		}69		selected = connection70	}71	if selected != nil && captured {72		return nil, true, BadRequest("joining captured and virtual sources is unsupported")73	}74	return selected, selected != nil, nil75}76