Alternative for distinct pluck in hasMorph of QueriesRelationships trait

[monove]

We are using morph relationships in multiple tables, some of which have a few million rows.

In the QueriesRelationships trait, this snippet in the hasMorph function has brough our database to its knees:

if ($types === ['*']) {
    $types = $this->model->newModelQuery()->distinct()->pluck($relation->getMorphType())->filter()->all();
}

Of course we could add more indexes, but it would be nice if there was an alternative where the distinct morphTypes could be defined in the model.

Thoughts?