So OWL2 EL trades some expressive power for more efficient reasoning (processing of the knowledge representation language).
[4] The design of languages and formalisms involves a trade-off between expressive power and analyzability.
Each instance of a formalism, e.g. each grammar and each regular expression, describes a particular set of strings.
An important yardstick for describing the relative expressive power of formalisms in this area is the Chomsky hierarchy.
In the predominant relational database paradigm, the contents of a database are described as a finite set of finite mathematical relations; Boolean queries, that always yield true or false, are formulated in first-order logic.
[6] However, adding expressive power must be done with care: it must still remain possible to evaluate queries with reasonable efficiency, which is not the case, e.g., for second-order logic.