1. A computer-implemented method comprising: providing in a database layer, an in-memory database comprising data of a first type and data of a second type; causing an in-memory database engine to receive from a query engine of an overlying application layer, a query in a database language defining a transient field, wherein the database language is extended to include an entity of an entity-relationship data model (ERM), and the transient field is defined by the entity; causing the in-memory database engine to communicate with the in-memory database to generate a query result from the query, wherein the in-memory database engine fills in a value for the transient field derived at least in part from the data of the second type; and causing the in-memory database engine to communicate the query result to the query engine for display to a user; wherein the value is materialized in a persistent field of the in-memory database based upon a hint from the in-memory database engine, the hint comprising a directive in a declaration of the transient field in the ERM.