1. A computer-based method of automatically and adaptively determining query execution plans for queries having parameter markers, said method comprising: generating, by a computing system, a first classifier trained by an initial set of training points; dynamically updating, by a computing system at a first runtime thereof, at least one of a workload of queries processed by a database of said computing system and database statistics collected by said database for computing a plurality of selectivities; collecting, by a computing system in an off-line phase thereof, said off-line phase being subsequent to said first runtime, a new set of training points, said collecting responsive to a detection of said dynamically updating; modifying, by said computing system in said off-line phase, said first classifier into a second classifier, said modifying including utilizing said new set of training points; receiving, by said computing system at a second runtime thereof, said second runtime being subsequent to said off-line phase, a query for said database, said query including one or more predicates, each predicate including one or more parameter markers bound to one or more actual values, and said one or more predicates associated with one or more selectivities of said plurality of selectivities in a one-to-one correspondence; and automatically determining a query execution plan by said computing system, said automatically determining including mapping, by said second classifier, said one or more selectivities into said query execution plan, wherein said query execution plan is included in an augmented set of training points, said augmented set including said initial set and said new set.