iX V6.5 (30391-90011)

42 Chapter2

Database Structure and Protection

Data Set Types and Relationships

Because a detail data set can contain as many as 16 search items, it can be related to at

most 16 master data sets. Each master-to-detail relationship must be relative to a different

detail search item. The SALES data set is related to the CUSTOMER, PRODUCT, and

DATE-MASTER data sets.

A detail data set can be multi-indexed by a single master data set. For example, SALES is

indexed twice by DATE-MASTER. The DATE search item forms one path with the

PURCH-DATE search item and one path with the DELIV-DATE search item. Each master

data set can serve as an index to one or more detail data sets. No master data set can be

related to more than 16 detail data sets. For each such relationship, TurboIMAGE/XL

keeps independent chain information with each master entry. This information consists of

pointers to the ﬁrst and last entries of the chain whose search item value matches the

master data set entry's key item value and a count of the number of entries in the chain.

This is called a chain head. The format of chain heads is given in chapter 10. For

example, the DATE-MASTER data entries each contain two sets of pointers, one for

PURCH-DATE chains and one for DELIV-DATE chains. TurboIMAGE/XL automatically

maintain the chain heads.

Primary Paths

One of the paths of each detail data set can be designated by the database designer as the

primary path. The main reason for designating a primary path is to maintain the entries

of each chain of the path in contiguous storage locations. To maintain contiguous locations,

occasionally use the DBUNLOAD utility program to copy the database to tape, the DBUTIL

utility program to erase the database, and the DBLOAD program to reload the database from

the tape. When the database is reloaded, contiguous storage locations are assigned to

entries of each primary path chain. Therefore, the database designer should designate the

path most frequently accessed in chained order as the primary path. This type of access is

discussed in chapter 5.

A primary path also serves as the default path when accessing a detail data set if no path

is speciﬁed by the calling program. This characteristic of primary paths is described with

the DBGET procedure in chapter 5.

Sort Items

For any path, it is possible to designate a data item other than the search item as a sort

item. If a sort item is speciﬁed, each of the chains of the path are maintained in ascending

sorted order based on the values of the sort item. Different paths can have different sort

items, and one path's sort item can be another path's search item. Only data items of type

logical or character can be designated as sort items.

For example, chains in the SALES data set composed of entries with identical ACCOUNT

values are maintained in sorted order by PURCH-DATE. When information about sales to

a particular customer is required, the SALES data entries for that customer's account can

be retrieved in sorted order according to purchase date. PURCH-DATE is a meaningful

sort item because the dates are stored in a properly form for collating (year-month-day).

The sorted order of entries is maintained by logical pointers rather than physical

placement of entries in consecutive records. Figure 2-4. illustrates the way that

TurboIMAGE/XL maintains sorted paths. When an entry is added to a detail data set, it is

added to or inserted in a chain. If the path does not have a sort item deﬁned, the entry