Paths in logical relationships

The relationship between physical parent and logical child in a physical database and the LP pointer in each logical child creates a physical parent to logical parent path.

To define use of the path, the logical child and logical parent are defined as a concatenated segment type that is a physical child of the physical parent, as shown in the following figure. Definition of the path and the concatenated segment type is done in what is called a logical database.

Figure 1. Defining a physical parent to logical parent path in a logical database
Part 1: a line passes from a phys. parent, to a log. child, to a separate log. parent. Part 2: a line connects the phys. parent to a single segment created by joining log. child with log. parent.

In addition, when LC pointers are used in the logical parent and logical twin and PP pointers are used in the logical child, a logical parent to physical parent path is created. To define use of the path, the logical child and physical parent are defined as one concatenated segment type that is a physical child of the logical parent, as shown in the following figure. Again, definition of the path is done in a logical database.

Figure 2. Defining a logical parent to physical parent path in a logical database
Part 1: a double line passes from phys. parent, to log. child, to log. parent and back again. Part 2: a line connects the log. parent to a single segment created by joining log. child with phys. parent.

When use of a physical parent to logical parent path is defined, the physical parent is the parent of the concatenated segment type. When an application program retrieves an occurrence of the concatenated segment type from a physical parent, the logical child and its logical parent are concatenated and presented to the application program as one segment. When use of a logical parent to physical parent path is defined, the logical parent is the parent of the concatenated segment type. When an application program retrieves an occurrence of the concatenated segment type from a logical parent, an occurrence of the logical child and its physical parent are concatenated and presented to the application program as one segment.

In both cases, the physical parent or logical parent segment included in the concatenated segment is called the destination parent. For a physical parent to logical parent path, the logical parent is the destination parent in the concatenated segment. For a logical parent to physical parent path, the physical parent is the destination parent in the concatenated segment.