Defining sequence fields for logical relationships

Logical parent sequence fields

To avoid potential problems in processing databases using logical relationships, unique sequence fields should be defined in all logical parent segments and in all segments a logical parent is dependent on in its physical database. When unique sequence fields are not defined in all segments on the path to and including a logical parent, multiple logical parents in a database can have the same concatenated key. When this happens, problems can arise during and after initial database load when symbolic logical parent pointers in logical child segments are used to establish position on a logical parent segment.

At initial database load time, if logical parents with non-unique concatenated keys exist in a database, the resolution utilities attach all logical children with the same concatenated key to the first logical parent in the database with that concatenated key.

When inserting or deleting a concatenated segment and position for the logical parent, part of the concatenated segment is determined by the logical parent's concatenated key. Positioning for the logical parent starts at the root and stops on the first segment at each level of the logical parent's database that satisfies the key equal condition for that level. If a segment is missing on the path to the logical parent being inserted, a GE status code is returned to the application program when using this method to establish position in the logical parent's database.

Real logical children sequence fields

If the sequence field of a real logical child consists of any part of the logical parent's concatenated key, PHYSICAL must be specified on the PARENT= parameter in the SEGM statement for the logical child. This will cause the concatenated key of the logical parent to be stored with the logical child segment.

Virtual logical children sequence fields

As a general rule, a segment can have only one sequence field. However, in the case of virtual pairing, multiple FIELD statements can be used to define a logical sequence field for the virtual logical child.

A sequence field must be specified for a virtual logical child if, when accessing it from its logical parent, you need real logical child segments retrieved in an order determined by data in a field of the virtual logical child as it could be seen in the application program I/O area. This sequence field can include any part of the segment as it appears when viewed from the logical parent (that is, the concatenated key of the real logical child's physical parent followed by any intersection data). Because it can be necessary to describe the sequence field of a logical child as accessed from its logical parent in non-contiguous pieces, multiple FIELD statements with the SEQ parameter present are permitted. Each statement must contain a unique fldname1 parameter.