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

108 Chapter4
Using the Database
Reading the Data
effectively allows you to use a data set in the same way you would use an MPE/iX file.
Thus, you have the advantages of TurboIMAGE/XL database organization and the
efficiency of serial access.
NOTE When using serial access with master data sets, you should be aware of
migrating secondaries. These are described in chapter 10.
Locking. If concurrent users are allowed to modify the data set (access mode 1), you may
wish to lock the data set or database before you begin the serial access sequence. Locking
prevents entries from being added, modified, moved, or removed by the other processes.
Calculated Access
The calculated access method allows you to retrieve an entry from a master data set by
specifying a particular key item value. For example, the SUP-MASTER data entry for the
supplier Acme shown in Figure 4-2. can be retrieved with this method because SUPPLIER
is a key item in the SUP-MASTER data set. TurboIMAGE/XL locates the entry in the data
set whose key item value matches the requested value. The exact technique used to
perform calculated access is described in chapter 10.
Calculated access can be used only with master data sets. It is very useful for retrieving a
single entry for some special purpose. For example, a program used infrequently to get
information about a particular customer or supplier could use calculated access to quickly
locate the information in the ORDERS database.
Chained Access
The chained access method is used to retrieve the next entry in the current chain. To
perform chained access of detail data set entries, you must first locate the beginning of the
chain you want to retrieve, and thus establish the current chain, by calling the DBFIND
procedure. The calling program specifies the name of the detail search item that defines
the path to which the chain belongs and a value for the item. TurboIMAGE/XL determines
which master set forms a path with the specified search item and locates the entry in that
master data set whose key item value matches the specified value. The entry it locates
contains pointers to the first and last entries in the desired chain and a count of the
number of entries in the chain. This information is maintained internally and defines the
attributes of the current path.
If a program uses chained access to read the INVENTORY data set entries pertaining to
the supplier H&S SURPLUS shown in Figure 4-2. it must first call the DBFIND procedure
to locate the chain head in the SUP-MASTER data set. The program specifies the
INVENTORY data set, the SUPPLIER search item in the INVENTORY data set, and the
value H&S SURPLUS for that item. TurboIMAGE/XL uses a calculated read to locate the
SUP-MASTER entry with a key item value of H&S SURPLUS. If the program then
requests a forward chained read using the DBGET procedure, the entry in record 9 of
INVENTORY, which is set at the beginning of the chain, is read. If a backward chained
read is requested, the entry in record 5 is read.
If the last call to DBGET used chained access to read the entry in record 9, the next forward
chained read reads the entry in record 2 of the INVENTORY data set.
Once a current path and chain have been established for a detail data set, the calling