Metadata for Approximate Query Answering Systems

Table 2

OL2AP metamodel classes.

Metamodel	Class	CWM Description
Core	ModelElement	A model element is an element that is an abstraction drawn from the system being modelled.
	TaggedValue	A tagged value allows information to be attached to any model element in the form of a “tagged value” pair; that is, name = value.
Relational	Catalog	A catalog is the unit of logon and identification. It also identifies the scope of SQL statements; the tables contained in a catalog can be used in a single SQL statement.
	Schema	A schema is a named collection of tables and collects all the tables of the same relational schema (i.e., a logical database).
	Table	Table is a data structure representing a relation.
	Column	Column is a data structure representing the field of a relation.
OL2AP	Reduction	This class represents a process of reduction of the data stored in a relational database.
	RSchema	This class represents a relational schema chosen for the data reduction.
	RTable	This class represents a table chosen for the data reduction.
	RColumn	This class represents a column whose data have been reduced.
	Descriptor	A descriptor is a tag that can be attached to any element of the model.