Page 113 - Accounting Information Systems

84 PART I Overview of Accounting Information Systems

TAB L E
2-2 TYPICAL FILE PROCESSING OPERATIONS

1. Retrieve a record from the file based on its primary key value.
2. Insert a record into a file.
3. Update a record in the file.
4. Read a complete file of records.
5. Find the next record in a file.
6. Scan a file for records with common secondary keys.
7. Delete a record from a file.

structures have two fundamental components: organization and access method. Organization refers to the
way records are physically arranged on the secondary storage device (for example, a disk). This may be
either sequential or random. The records in sequential files are stored in contiguous locations that occupy
a specified area of disk space. Records in random files are stored without regard for their physical relation-
ship to other records of the same file. In fact, random files may have records distributed throughout a disk.
The access method is the technique used to locate records and to navigate through the database or file.
No single structure is best for all processing tasks, and selecting a structure involves a trade-off
between desirable features. The file operation requirements that influence the selection of the data struc-
ture are listed in Table 2-2.
In the following section, we examine several data structures that are used in flat-file systems. Recall
from Chapter 1 that the flat-file model describes an environment in which individual data files are not
integrated with other files. End users in this environment own their data files rather than share them with
other users. Data processing is thus performed by stand-alone applications rather than integrated systems.
The flat-file approach is a single-view model that characterizes legacy systems in which data files are
structured, formatted, and arranged to suit the specific needs of the owner or primary user of the system.
Such structuring, however, may omit or corrupt data attributes that are essential to other users, thus pre-
venting successful integration of systems across the organization.

SEQUENTIAL STRUCTURE
Figure 2-37 illustrates the sequential structure, which is typically called the sequential access
method. Under this arrangement, for example, the record with key value 1875 is placed in the physical
storage space immediately following the record with key value 1874. Thus, all records in the file lie
in contiguous storage spaces in a specified sequence (ascending or descending) arranged by their
primary key.
Sequential files are simple and easy to process. The application starts at the beginning of the file
and processes each record in sequence. Of the file processing operations in Table 2-2, this approach is ef-
ficient for Operations 4 and 5, which are, respectively, reading an entire file and finding the next record
in the file. Also, when a large portion of the file (perhaps 20 percent or more) is to be processed in one
operation, the sequential structure is efficient for record updating (Operation 3 in Table 2-2). Sequential
files are not efficient when the user is interested in locating only one or a few records on a file. A simple
analogy can be made with an audiocassette. If you want to listen to only the tenth song on the tape, you
must fast-forward to the point at which you think the song is located and then press the play button. Some
searching is usually required to find the beginning of the song. However, if you are interested in hearing
all the songs on the tape, you can simply play it from the beginning. An example of a sequential file
application is payroll processing, whereby 100 percent of the employee records on the payroll file are
processed each payroll period. Magnetic tape is a cheap, effective, and commonly used storage medium
for sequential files. Sequential files may also be stored on magnetic disks.
The sequential access method does not permit accessing a record directly. Applications that require
direct access operations need a different data structure. The techniques described next address this need.