Table D (File architecture)

Ordered Index

Understanding the Ordered Index

Types of nodes

The Ordered Index can have the following types of tree nodes, which are shown in Understanding the Ordered Index and described in the following table:

Node type	Description
Root	Highest level node, which normally contains on one page all the pointers to the next level down-either an intermediate node or leaf node, if no intermediate nodes exist. Ordered Index searches begin at the root node.
Intermediate(s)	Level(s) below the root and above the leaf, which act as directory nodes and contain only pointers to leaf nodes or to lower intermediate nodes.
Leaf	Lowest level node, which contains a key field-name-equals-value pair, plus a record access pointer to each Table B record containing the pair, or to a Table D record list, or bit pattern that points to the Table B record. (Record lists and bit patterns are described in .) A small Ordered Index tree can fit on a single node, in which case the root is also a leaf node.

In general, each node that has pointers to a lower level node has a large number of such pointers, which are referred to as a large fanout in searching down the tree.

Ordered Index B-Tree diagram

Understanding the Ordered Index represents a portion of a typical Ordered Index B-Tree, depicting the relationship of the tree nodes. The figure does not show leaf-node pointers to Table B records or to Table D record lists or bit patterns. It also does not represent the likely case in which the tree nodes contain a few hundred pointers per page.

Logical Structure of Ordered Index B-Tree Processing

The Ordered Index B-Tree is processed as follows:

• When new Ordered Index entries are inserted in a full leaf node, a split of the node into two new nodes occurs, and an additional pointer is added to the node at the next higher level.
• If this node is also full, it is split into two new nodes, and a new pointer is added to an intermediate node at the next higher level.
• If this process reaches back to a full root node, the root is split into two intermediate nodes that are pointed to by a new root, one level higher.

This newly created level represents an increase by one of the tree's depth. Only root node splits increase the tree's depth. All leaf node entries are at the same depth.

An Ordered Index tree with a depth of three levels, with reasonable assumptions about entry sizes, can hold about 500 million entries. Model 204 automatically maintains the tree structure.

Ordered Index spacing parameters

Certain characteristics of the Ordered Index tree structure are controlled by the qualifying attributes called spacing parameters of the ORDERED attributes. Understanding the Ordered Index lists these spacing parameters. You specify spacing parameters in the DEFINE FIELD command, which is documented in the Model 204 Parameter and Command Reference.

Ordered Index spacing parameters

Parameter	Default	Range	Specifies...
LRESERVE	15	0 - 99	Percentage of left-hand leaf node space to leave free upon a node split during deferred update mode processing, including file loading or Ordered Index tree restructuring with the REORGANIZE OI command. Too low a setting can cause excessive splitting during subsequent minor modifications.
NRESERVE	15	0 - 99	Percentage of left-hand intermediate node space to leave free upon a split during deferred update mode processing, including file loading or Ordered Index tree restructuring with the REORGANIZE OI command. Too low a setting can cause excessive splitting during subsequent minor modifications.
SPLITPCT (or SPLT)	50	1 - 100	Percentage of node data placed on the left-hand node when a split occurs during non-deferred User Language or Host Language Interface requests. Note: Unlike the two previous parameters, SPLITPCT specifies data to go on the left node, rather than space. Too high or too low a setting can cause excessive splitting.
IMMED	1	0 and 255	Number of direct (immediate) pointers to Table B records per leaf page entry per segment in the Ordered Index tree. If the number of records to be pointed to in an entry is greater than the value of IMMED, all the record pointers are moved to a Table D list or bit map containing pointers to Table B. Storing information in Table D lists slows range retrieval but reduces Ordered Index tree depth.

Spacing parameter guidelines

The following guidelines, while not precise, might help you reduce the number of splits and increase update performance at your site. How these parameters function is closely dependent on the data and the application. Model 204 customer support strongly suggests that you try the default settings, as well as experiment with these parameter settings to see what works for your site.

Because updates that cause page splits consume more resources than updates to less-than-full pages, set the spacing parameters to allow sufficient space for subsequent updating. Allowing too much space, however, might increase the B-Tree's depth and thereby impede performance. Knowing the likely updating characteristics of the ORDERED fields is the best guide for setting these parameters.

The settings of LRESERVE, NRESERVE, SPLITPCT, and IMMED are recommended settings only. The actual movement of data during an update might not conform exactly to the value of the appropriate parameter. The spacing parameters can be changed by specifying the parameter and a new value with the REDEFINE command:

• Use the default setting for SPLITPCT (50), unless the field is updated Online and most values are added in strict ascending or descending order, such as dates or employee ID numbers. If field values are added in:
• Ascending order, set SPLITPCT higher than 50.
• Descending order, set SPLITPCT lower than 50.
• If you load records either via the File Load Utility (FLOD) or as deferred updates applied with the Z command, and the ordered field is heavily updated in Online mode, set LRESERVE and NRESERVE higher: up to 50. This causes more splits in the FLOD Z step (see : ), but, by not packing the tree as tightly during the Z step, it should decrease the number of Online splits. Of these two parameters, LRESERVE is more important for tuning. If the field is not heavily updated Online, use the defaults for LRESERVE and NRESERVE.
• If the field is updated Online with information that is generally not in order, Model 204 customer support suggests that you experiment with the IMMED parameter. This parameter is less important if most updates are entered during the FLOD Z step. With the IMMED parameter, there might be a trade-off between Table D LIST I/O and B-Tree I/O.

Record Map of Preallocated Fields

Also in Table D (and no more than one page) is a map of the preallocated fields defined within the file. This is used when ever a new record is added to the file, as space is reserved for these fields just after the extension pointer at the beginning of the record.

The DISPLAY RECORD command against the file shows the map. The fields appear in the map in the same order they are defined to the file, so, if you execute the following DEFINE FIELD commands:

DEFINE FIELD PRE.FIELD.2 (OCC 1 LEN 9) DEFINE FIELD PRE.FIELD.1 (OCC 1 BIN) DEFINE FIELD PRE.FIELD.3 (OCC 1 LEN 20)

The DISPLAY RECORD command produces:

LENGTH OCCURS PAD NAME 9 1 X'00' PRE.FIELD.2 BIN 1 X'00' PRE.FIELD.1 20 1 X'00' PRE.FIELD.3