What characterises logical databases in relation to text databases is that logical databases need the concept of logical equality and the subsumption of individual by class, so the data for which a logical database is to be used must support these concepts. Text databases do not make these assumptions. This is the reason text database systems suffer from problems of limited precision and limited recall.

For an object to be represented in a logical database, it must be completely characterised by the classes of which it is an instance. Letter P of the previous section is completely characterised by its membership in the class order and its membership in associations between the class order and the classes product, customer and so on. To the university student record system, a person is completely characterised by membership in the class student and membership in associations between student and the classes enrolment, program and so on. This is why we can expect 100% precision and 100% recall.

In a text database, we can’t even reliably identify a document as a member of a class, much less characterise its content by class and association.

The ability to completely characterise an object by the class in which it is an instance is the basis for logical equality, which in turn is necessary for the computations performed in logical databases. The number of students enrolled in a course can be computed because the class list defines the enrolment, and all students’ enrolments are equivalent. A grade point average can be computed because a student’s performance in a course is completely characterised by the grade awarded, and the same grade awarded in different courses is logically the same.

So the first answer to the question as to what problems a logical database is a solution for is those applications where the assumptions hold that class and association membership completely characterise the objects. This might be somewhat less circular, but is still not satisfactory. What sort of world produces records that are completely characterised by class and association membership?