Logical relationship pointer types

In all logical relationships the logical child establishes a path between two segment types. The path is established by use of pointers.

For HALDB databases, consider the following:

Logical relationships are not allowed between HALDB databases and non-HALDB databases.
Direct pointers and indirect pointers are used.
Unidirectional relationships and bidirectional, physically paired relationships are supported for HALDB databases.
Physical parent pointers are always present in PHDAM and PHIDAM segments.

Logical parent pointer

The pointer from the logical child to its logical parent is called a logical parent (LP) pointer. This pointer must be a symbolic pointer when it is pointing into a HISAM database. It can be either a direct or a symbolic pointer when it is pointing into an HDAM or a HIDAM database. PHDAM or PHIDAM databases require direct pointers.

A direct pointer consists of the direct address of the segment being pointed to, and it can only be used to point into a database where a segment, once stored, is not moved. This means the logical parent segment must be in an HD (HDAM, PHDAM, HIDAM, or PHIDAM) database, since the logical child points to the logical parent segment. The logical child segment, which contains the pointer, can be in a HISAM or an HD database except in the case of HALDB. In the HALDB case, the logical child segment must be in an HD (PHDAM or PHIDAM) database. A direct LP pointer is stored in the logical child's prefix, along with any other pointers, and is four bytes long. The following figure shows the use of a direct LP pointer. In a HISAM database, pointers are not required between segments because they are stored physically adjacent to each other in hierarchical sequence. Therefore, the only time direct pointers will exist in a HISAM database is when there is a logical relationship using direct pointers pointing into an HD database.

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In the preceding figure, the direct LP pointer points from the logical child ORDITEM to the logical parent ITEM. Because it is direct, the LP pointer can only point to an HD database. However, the LP pointer can exist in a HISAM or an HD database. The LP pointer is in the prefix of the logical child and consists of the 4-byte direct address of the logical parent.

A symbolic LP pointer, which consists of the logical parent's concatenated key (LPCK), can be used to point into a HISAM or HD database. The following figure illustrates how to use a symbolic LP pointer. The logical child ORDITEM points to the ITEM segment for BOLT. BOLT is therefore stored in ORDITEM in the LPCK. A symbolic LP pointer is stored in the first part of the data portion in the logical child segment.

Note: The LPCK part of the logical child segment is considered non-replaceable and is not checked to see whether the I/O area is changed. When the LPCK is virtual, checking for a change in the I/O area causes a performance problem. Changing the LPCK in the I/O area does not cause the REPL call to fail. However, the LPCK is not changed in the logical child segment.

With symbolic pointers, if the database the logical parent is in is HISAM or HIDAM, IMS uses the symbolic pointer to access the index to find the correct logical parent segment. If the database containing the logical parent is HDAM, the symbolic pointer must be changed by the randomizing module into a block and RAP address to find the logical parent segment. IMS accesses a logical parent faster when direct pointing is used.

Although the figures show the LP pointer in a unidirectional relationship, it works exactly the same way in all three types of logical relationships.

The following figure shows an example of a symbolic logical parent pointer.

An arrow representing a symbolic pointer points from logical child ORDITEM in the ORDER database, to an index or randomizing module, and then to logical parent ITEM in the ITEM database.

In the preceding figure, the symbolic LP pointer points from the logical child ORDITEM to the logical parent ITEM. With symbolic pointing, the ORDER and ITEM databases can be either HISAM or HD. The LPCK, which is in the first part of the data portion of the logical child, functions as a pointer from the logical child to the logical parent, and is the pointer used in the logical child.

Logical child pointer

Logical child pointers are only used in logical relationships with virtual pairing. When virtual pairing is used, there is only one logical child on DASD, called the real logical child. This logical child has an LP pointer. The LP pointer can be symbolic or direct. In the ORDER and ITEM databases you have seen, the LP pointer allows you to go from the database containing the logical child to the database containing the logical parent. To enter either database and cross to the other with virtual pairing, you use a logical child pointer in the logical parent. Two types of logical child pointers can be used:

Logical child first (LCF) pointers, or
The combination of logical child first (LCF) and logical child last (LCL) pointers

The LCF pointer points from a logical parent to the first occurrence of each of its logical child types. The LCL pointer points to the last occurrence of the logical child segment type for which it is specified. A LCL pointer can only be specified in conjunction with a LCF pointer. The following figure shows the use of the LCF pointer. These pointers allow you to cross from the ITEM database to the logical child ORDITEM in the ORDER database. However, although you are able to cross databases using the logical child pointer, you have only gone from ITEM to the logical child ORDITEM. To go to the ORDER segment, use the physical parent pointer.

LCF and LCL pointers are direct pointers. They contain the 4-byte direct address of the segment to which they point. This means the logical child segment, the segment being pointed to, must be in an HD database. The logical parent can be in a HISAM or HD database. If the logical parent is in a HISAM database, the logical child segment must point to it using a symbolic pointer. LCF and LCL pointers are stored in the logical parent's prefix, along with any other pointers. The following figure shows an LCF pointer.

In the preceding figure, the LCF pointer points from the logical parent ITEM to the logical child ORDITEM. Because it is a direct pointer, it can only point to an HD database, although, it can exist in a HISAM or an HD database. The LCF pointer is in the prefix of the logical parent and consists of the 4-byte RBA of the logical child.

Physical parent pointer

Physical parent (PP) pointers point from a segment to its physical parent. They are generated automatically by IMS for all HD databases involved in logical relationships. PP pointers are put in the prefix of all logical child and logical parent segments. They are also put in the prefix of all segments on which a logical child or logical parent segment is dependent in its physical database. This creates a path from a logical child or its logical parent back up to the root segment on which it is dependent. Because all segments on which a logical child or logical parent is dependent are chained together with PP pointers to a root, access to these segments is possible in reverse of the usual order.

In the preceding figure, you saw that you could cross from the ITEM to the ORDER database when virtual pairing was used, and this was done using logical child pointers. However, the logical child pointer only got you from ITEM to the logical child ORDITEM. The following figure shows how to get to ORDER. The PP pointer in ORDITEM points to its physical parent ORDER. If ORDER and ITEM are in an HD database but are not root segments, they (and all other segments in the path of the root) would also contain PP pointers to their physical parents.

PP pointers are direct pointers. They contain the 4-byte direct address of the segment to which they point. PP pointers are stored in a logical child or logical parent's prefix, along with any other pointers.

ORDITEM contains a PP pointer to its physical parent ORDER in its prefix and an LP pointer to its logical parent in its data part.

In the preceding figure, the PP pointer points from the logical child ORDITEM to its physical parent ORDER. It is generated automatically by IMS for all logical child and logical parent segments in HD databases. In addition, it is in the prefix of the segment that contains it and consists of the 4-byte direct address of its physical parent. PP pointers are generated in all segments from the logical child or logical parent back up to the root.

Logical twin pointer

Logical twin pointers are used only in logical relationships with virtual pairing. Logical twins are multiple logical child segments that point to the same occurrence of a logical parent. Two types of logical twin pointers can be used:

Logical twin forward (LTF) pointers, or
The combination of logical twin forward (LTF) and logical twin backward (LTB) pointers

An LTF pointer points from a specific logical twin to the logical twin stored after it. An LTB pointer can only be specified in conjunction with an LTF pointer. When specified, an LTB points from a given logical twin to the logical twin stored before it. Logical twin pointers work in a similar way to the physical twin pointers used in HD databases. As with physical twin backward pointers, LTB pointers improve performance on delete operations. They do this when the delete that causes DASD space release is a delete from the physical access path. Similarly, PTB pointers improve performance when the delete that causes DASD space release is a delete from the logical access path.

The following figure shows use of the LTF pointer. In this example, ORDER 123 has two items: bolt and washer. The ITEMORD segments beneath the two ITEM segments use LTF pointers. If the ORDER database is entered, it can be crossed to the ITEMORD segment for bolts in the ITEM database. Then, to retrieve all items for ORDER 123, the LTF pointers in the ITEMORD segment can be followed. In the following figure only one other ITEMORD segment exists, and it is for washers. The LTF pointer in this segment, because it is the last twin in the chain, contains zeros.

LTB pointers on dependent segments improve performance when deleting a real logical child in a virtually paired logical relationship. This improvement occurs when the delete is along the physical path.

LTF and LTB pointers are direct pointers. They contain the 4-byte direct address of the segment to which they point. This means LTF and LTB pointers can only exist in HD databases. The following figure shows a LTF pointer.

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In the preceding figure, the LTF pointer points from a specific logical twin to the logical twin stored after it. In this example, it points from the ITEMORD segment for bolts to the ITEMORD segment for washers. Because it is a direct pointer, the LTF pointer can only point to an HD database. The LTF pointer is in the prefix of a logical child segment and consists of the 4-byte RBA of the logical twin stored after it.

Indirect pointers

HALDB databases (PHDAM, PHIDAM, and PSINDEX databases) use direct and indirect pointers for pointing from one database record to another database record. The following figure shows how indirect pointers are used.

An EPS of a secondary index or logically related segment points to an ILK of a target segment. An ILE in an ILDS also points to the ILK. After a reorganization the EPS is corrected by the ILE.

The use of indirect pointers prevents the problem of misdirected pointers that would otherwise occur when a database is reorganized.

The repository for the indirect pointers is the indirect list data set. The misdirected pointers after reorganization are self-healing using indirect pointers.