Page 114 - Accounting Information Systems

CHAPT E R 2 Introduction to Transaction Processing 85

FI G U R E
2-37 SEQUENTIAL STORAGE AND ACCESS METHOD

Records Are Read Sequentially

Key Key Key
1874 Other Data 1875 Other Data 1876 Other Data

Keys Are in Sequence
(in this case, ascending order)

DIRECT ACCESS STRUCTURES
Direct access structures store data at a unique location, known as an address, on a hard disk or floppy
disk. The disk address is a numeric value that represents the cylinder, surface, and block location on the
4
disk. The operating system uses this address to store and retrieve the data record. Using our music anal-
ogy again, the direct access approach is similar to the way songs are stored on a compact disc. If the lis-
tener chooses, he or she can select a specific song directly without searching through all the other songs.
An important part of the direct access approach is in determining the disk address, which is based
on the record’s primary key. Bank account numbers, social security numbers, credit card numbers, and
license plate numbers are examples of primary keys that are translated into addresses to store and retrieve
data by different business applications. The following techniques are examples of data structures that
have direct access capability.

INDEXED STRUCTURE
An indexed structure is so named because, in addition to the actual data file, there exists a separate index that
is itself a file of record addresses. This index contains the numeric value of the physical disk storage location
(cylinder, surface, and record block) for each record in the associated data file. The data file itself may be
organized either sequentially or randomly. Figure 2-38 presents an example of an indexed random file.
Records in an indexed random file are dispersed throughout a disk without regard for their physical
proximity to other related records. In fact, records belonging to the same file may reside on different
disks. A record’s physical location is unimportant as long as the operating system software can find it
when needed. Searching the index for the desired key value, reading the corresponding storage location
(address), and then moving the disk read/write head to the address location accomplish this. When a new
record is added to the file, the data management software selects a vacant disk location, stores the record,
and adds the new address to the index.
The physical organization of the index itself may be either sequential (by key value) or random.
Random indexes are easier to maintain, in terms of adding records, because new key records are simply
added to the end of the index without regard to their sequence. Indexes in sequential order are more diffi-
cult to maintain because new record keys must be inserted between existing keys. One advantage of a se-
quential index is that it can be searched rapidly. Because of its logical arrangement, algorithms can be

4 For further explanation about disk addresses, see Section A of the Appendix to this chapter.