The logical child segment

When defining a logical child in its physical database, the length specified for it must be large enough to contain the concatenated key of the logical parent.

Any length greater than that required for the concatenated key of the logical parent can be used to store intersection data, a type of data that is unique to a particular logical relationship.

To identify which logical parent is pointed to by a logical child, the concatenated key of the logical parent must be present. Each logical child segment must be present in the application program's I/O area when the logical child is initially presented for loading into the database. However, if the logical parent is in an HD database, its concatenated key might not be written to storage when the logical child is loaded. If the logical parent is in a HISAM database, a logical child in storage must contain the concatenated key of its logical parent.

For logical child segments, you can define a special operand on the PARENT= parameter of the SEGM statement. This operand determines whether a symbolic pointer to the logical parent is stored as part of the logical child segment on the storage device. If PHYSICAL is specified, the concatenated key of the logical parent is stored with each logical child segment. If VIRTUAL is specified, only the intersection data portion of each logical child segment is stored.

When a concatenated segment is retrieved through a logical database, it contains the logical child segment, which consists of the concatenated key of the destination parent, followed by any intersection data. In turn, this is followed by data in the destination parent. The following figure shows the format of a retrieved concatenated segment in the I/O area. The concatenated key of the destination parent is returned with each concatenated segment to identify which destination parent was retrieved. IMS™ gets the concatenated key from the logical child in the concatenated segment or by constructing the concatenated key. If the destination parent is the logical parent and its concatenated key has not been stored with the logical child, IMS constructs the concatenated key and presents it to the application program. If the destination parent is the physical parent, IMS must always construct its concatenated key.