THE CLASSIFICATION OF TYPES OF DTDS

Before we choose a database selection method to locate the most appropriate databases to search for a given user query, it is necessary to know how many types of DTDs exist and which kinds of selection cases may appear in each type of DTD. In this section, we will discuss the classification of types of DTDs based on the relationships of the indexing methods and on the term weight schemes of DTDs. The definition of four different types of DTDs are shown as follows :

Definition 12

If all of the databases in a DTD have the same indexing method and the same term weight scheme, the DTD is called a homogeneous DTD. This type of DTD can be defined as:

 

Definition 13

If all of the databases in a DTD have the same indexing method, but at least one database has a different term weight scheme, the DTD is called a partially homogeneous DTD. This type of DTD can be defined as:

 

Definition 14

If at least one database in a DTD has a different indexing method from other databases, but all of the databases have the same term weight scheme, the DTD is called a partially heterogeneous DTD. This type of DTD can be defined as:

 

Definition 15

If at least one database in a DTD has a different indexing method from other databases, and at least one database has a different term weight scheme from the other databases, the DTD is called a heterogeneous DTD. This type of DTD can be defined as: