But in the query planner, at that point, Postgresql 9.6.1 seems not to use any index (single on dalle / alle field and combindex index on dalle+alle) but it use seqscan:
Given a query with expected "rows=290" I am not surprised that it would simply scan the entire relation. Especially since you have four columns in your where clause and so any one index would be insufficient.
You sound as if you believe that any query that doesn't use an index is flawed. That is not the case.