... functions can be used in the same manner as Oracle-provided functions, such as SUBSTR and TO_ CHAR Custom functions cannot be used in CHECK or DEFAULT constraints and cannot manipulate any database values ... details on the use of abstract datatypes, first introduced in Chapter This chapter covers topics such as security administration for abstract datatypes and the indexing of abstract datatype attributes ... Elements of each of these topics play a part in the implementation of an object-relational database The first section of this chapter provides a high-level review of the abstract datatypes shown in Chapter...