As you put the word Reasoning in quotation marks, I might misunderstand your bottom line here (I am Autistic, so please do not get quirky on natural language semantics), but the bare statement: "Reasoning can be added to any conventional database" is just not right. Reasoning is a well-defined notion from logic, that is based on formal languages, semantics and a relation called entailment (inference in proof theory) respectively. None of that does natively exist in a database. In the literature, there are two well-known ways for integrating a notion of reasoning into a database. Firstly, Datalogic was invented to create recursive queries. Datalogic's relation to reasoning was a side-effect, and it only covers a fragment based on horn clauses. On the other hand there's OWL-DL a (limited) fragment of OWL, that encodes some kind of reasoning via query expansion on vanilla SQL-Queries. So maybe you can elaborate on the notion of "using views, and sometimes custom indices to add reasoning to a conventional database".