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

Chapter 4 109
Using the Database
Reading the Data
program can use the chained access method of retrieving data. You can use both forward
and backward chained access. In either case, if there are no more entries in the chain when
you request the next one, DBGET returns an exceptional condition, beginning-of-chain or
end-of-chain for backward and forward access, respectively.
Chained access to master data sets retrieves the next entry in the current synonym
chain. The use of synonym chains applies to only a limited number of special situations.
They are discussed in chapter 10.
Chained access to detail data sets is particularly useful when you want to retrieve
information about related events such as all inventory records for the H&S Surplus
supplier in the ORDERS database. When a B-Tree index is created on the key item of the
master set, chained access following a DBFIND can also be done on the master data set.
More detailed information on B-trees is given in chapter 11.
Locking. If concurrent users are allowed to modify data entries in the chain you are
currently accessing, you can use locking to ensure data consistency. For example, suppose a
chain consists of several data entries, each containing a line item from a particular order.
If user A is performing a series of chained reads while user B is canceling the order by
deleting data entries one by one, user A could retrieve an incomplete order. To prevent this
from happening, a lock can be established covering the group of data entries to be retrieved
(the chain, in this case). This can usually be done with a single DBLOCK call. Refer to the
discussion of the locking facility later in this chapter.
Sorted Sequential Access
The sorted sequential access is used for B-Tree indices and can be used both for master
and detail data sets. It is used for retrieving records in a sorted sequential order of the key
item or search item value. For masters, each key item value is different and the records
retrieved will be in a sorted sequential order of the key item value. The order can be
ascending when DBGET mode 5 is used, and it is descending when DBGET mode 6 is used
following a B-Tree DBFIND. For detail data sets, the B-Tree search can result into
traversing multiple chains of the detail data set. Each chain has the same search item
value. The subsequent chain will have a different search item value in a sorted sequential
order. Therefore, even for detail data sets, the records retrieved will be in a sorted
sequential order of the search item value. Note that within each such chain of the detail
data set, the order of the records is chronological.
For example, when a B-Tree index is created on the key item SUPPLIER of
SUP-MASTER, DBFIND can be used with SUP-MASTER in the
dset parameter and the
key item SUPPLIER in the
item parameter. If the argument is equivalent to "Greater than
BAY PAPER CO.", the order of records retrieved will be record number 5, the entry
pertaining to the supplier H & S SURPLUS and record number 1 pertaining to JAKE'S
SHOP in Figure 4-2. IF the DBFIND is for the detail data set, INVENTORY, and the same
argument, the order of records retrieved will be 9, 2, 5, and 6 in Figure 4-2. The record
numbers 9, 2, and 5 are pertinent to the supplier H & S SURPLUS (one chain) and 6 is
pertinent to JAKE'S SHOP (second chain). The second chain has only 1 record. The chain
head for H & S SURPLUS in SUP-MASTER contains record number 9 for beginning entry
and 5 as ending entry for the path related to detail data set INVENTORY.