What is the Logical Layer?

The semantic model's logical layer defines the dimensional business model of the data and specifies the mapping between the business model and the physical layer schemas.

The logical layer determines the analytic behavior seen by users, and defines the superset of objects and relationships available to users. The logical layer hides the complexity of the source data models.

The logical layer can contain more than one business model, but ideally a single, integrated business model is preferred to provide common dimensions used to analyze data across subject areas. Business models are always dimensional, unlike objects in the physical layer, which reflect the organization of the data sources. Each business model contains logical tables, logical columns, logical table sources, and logical hierarchies.

Even though similar terminology is used for logical table and physical table objects, such as tables and joins, objects in the logical layer have their own set of rules that differ from those of relational models. For example, logical joins can represent many possible physical joins.

Tables, joins, mappings, and other objects in the logical layer are typically created automatically when you drag and drop objects from the physical layer to a business model. After these objects have been created, you can perform tasks like create additional logical joins, perform calculations and transformations on columns, and add and remove joins from dimension and fact tables.

Each column in the logical layer maps to one or more columns in the physical layer. At run time, the Oracle Analytics query engine evaluates Logical SQL requests against the logical layer, and then uses the mappings to determine the best set of physical tables and files for generating the necessary physical queries. The mappings often contain calculations and transformations, and might combine multiple physical tables.