TurboIMAGE/XL Database Management System Reference Manual MPE/iX V6.5 (30391-90011)

Chapter 3 77
Defining a Database
Set Part
expanded. The number of disk extents used for the data set file may also impact the
performance of TurboIMAGE/XL.
NOTE For existing databases, use DBChange Plus or a third-party utility to specify
the expansion parameters.
Master Key and Detail Search Items
The master key items and detail search items (also called critical items) that define a path
between two data sets must have identical type designators and simple sub-item lengths
when they are defined in the item part. The sub-item lengths must be simple because the
key and search items cannot be compound items. Because the same data item name can
appear in more than one data set, you can use the same data item name and definition for
both the master key items and detail search items. For example, the data item ACCOUNT
is used as the key item in the CUSTOMER master and as the search item in the SALES
detail data sets. If you want to make a distinction between the search items, they can be
defined separately. An example of this technique is found in the ORDERS database. The
key item DATE links the DATE-MASTER data set to the SALES data set through two
paths and two search items, PURCH-DATE and DELIV-DATE. These three data items
look like this in the item part:
DATE, X6;
DELIV-DATE, X6 (/14);
PURCH-DATE, X6 (11/14);
Each data item has type designator X and sub-item length 6. However, the item names,
read class lists, and write class lists differ. Figure 3-5. at the end of this chapter contains
the listing printed by the Schema Processor when the ORDERS database schema is
processed. Refer to this figure for examples of the schema parts.
Data Set Identifiers
Similar to data items, data sets can be referenced by name or number. The data set
number is determined by the set's position in the set part of the schema. It is more flexible
to use data set names in order to maintain program file independence.