Lack of Impedance Mismatch
Object DBMSs allow you to store objects directly without any mapping to different data structures. Relational DBMSs require mapping from objects to tables. This mapping to different data structures is called "impedance mismatch." The figure below shows direct storage at the left and impedance mismatch at the right.
This lack of impedance mismatch in Object DBMSs gives them a performance advantage over Relational DBMSs, especially on complex data. Impedance mismatch slows down performance on complex data because of processing needed map from one data structure (tables) to another (objects). For more information see impedance mismatch . Also see complex data.
More on the general topic: Object-Oriented Database Management System (OODBMS) Definition