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Don Burleson Blog 







Multi-Dimensional Pointers

Oracle Database Tips by Donald Burleson

To understand multi-dimensional pointers, let's begin with a simple example of a natural hierarchy (Figure 6.7).  In this example, we see that each university has many departments, each department offers many courses, and that each course offers many sections.  This is a natural descending hierarchy of one-to-many relationships.

Figure 6.7   A Natural Hierarchy of data relationships

Let's take a look at how we could create a data structure that would embed pointers to establish these relationships.

 Department table

 (1-20) *course

 Course table

 course number
 (0-10) *section
 (1) *department

 Section table

 (1) *course

From this definition, we see that the department table consists of the department name, followed by from one to 20 pointers to courses.  The course table contains the course number and course name followed by from zero to ten pointers to sections and one "owner" pointer, pointing to the department's row. The section table consists mostly of data items, except for the pointer to the course row.

As we know there are many options for modeling this type of descending one-to-many data relationship.  In a ?vanilla? relational database, each entity would exist as a table, with the primary key of the owner table copied into the member table.  But there is an alternative to modeling this structure in the object/relational model.  Let's take a look at how a hierarchical data structure might be implemented in Oracle:

CREATE TYPE full_name (
        first_name   varchar(20),
        MI           char(1),
        last_name    varchar(20)); 

 CREATE TYPE section_type (
    section_number   number(5),
    instructor_name  full_name,
    semester         char(3),
    building         varchar(20),
    room             char(4),
    days_met         char(5),
    time_met         char(20));   

CREATE TABLE section OF section_type;    

CREATE TYPE section_array AS VARRAY(10) OF section_type; 

CREATE TYPE course_type (
    course_ID        number(5),
    course_name      varchar(20),
    credit_hours     number(2),
    section_list     section_array); 

CREATE TABLE course OF course_type; 

CREATE TYPE course_array as VARRAY(20) OF course_type; 

CREATE TYPE dept_type (
    dept_name              varchar(20),
    chairperson_name   full_name,
    course_list               course_array); 

CREATE TABLE department OF dept_type;

This is what this data structure would look like (Figure 6.8).

Figure 6.8  An implementation of multidimensional row pointers

Here we see that the pointers allow fast access from owner to member in the hierarchy.  But where are the "owner" pointers?  As it turns out, we must first define the hierarchy before we have the necessary definitions to include the pointers.  Let's add them using the ALTER TYPE statement:

ALTER TYPE section_type
   ADD COLUMN course_owner_pointer     course_type; 

ALTER TYPE course_type
   ADD COLUMN department_owner_pointer department_type;

We have now created a two-way pointer structure, such that all owner rows in the hierarchy point to their member rows, while all member rows will point up to their owners.  However, we must bear in mind that these are only data structures;  it is up to the programmer, to assign these pointers when the rows are created.

In a sense, this data structure is the object/relational equivalent to the **char data structure.  Essentially, a **char data structure is a structure where we have a pointer to an array of pointers to characters.  In Oracle, the department has an array of pointers to curses, which, in turn, contain arrays of pointers to sections.

But how do we query these pointers with SQL?  In order to accommodate the new object features, most object/relational vendors are implementing the CAST and the MULTISET extensions to SQL.  For example, here is what the query to populate the student_list internal table:

                 GRADE, STUDENT
                 GRADE.course_name = ?CS101?
                 GRADE.student_name = STUDENT.student_name


As we can see, the new SQL extensions are rather foreign to those who are accustomed to pure relational syntax.




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