PHYSICAL DATABASE DESIGN - Fundamentals of Database Management Systems - page 237

In the CUSTOMER table, some large corporate customers' records are
accessed much more frequently than the rest of the customer records. To help
ease this performance bottleneck and to gather these customer records together in
one disk area to further enhance performance, a subset table of copies of just these
records can be created and stored elsewhere on the disk or on a different disk. Again,
the issue of simultaneous updates of the duplicate data must be considered. Note
the difference between creating a subset table and creating a horizontal partition.
In the case of subset tables, a copy of the records is left behind in the original table;
in the case of horizontal partitioning, no copy is left behind.
SUMMARY
Data is all around us but we normally don't think about it unless we have to use it to
keep track of objects that are important to us. The objects and events we come into
contact with and their attributes can be noted in structures as simple as lists, which,
by extension, we can think of as files and their records.
Moving on to storing data in computers, four basic operations have to be
performed: retrieving stored data, inserting new data, deleting stored data, and
updating stored data. Applications requiring these operations, in particular the
operation of retrieving stored data, may require data to be accessed sequentially
while other applications—most of the applications we deal with today—may require
data to be accessed on a direct basis.
Disk devices are the predominant secondary memory devices in use today.
They are capable of providing both sequential and direct access to data. Disk
devices consist of one or more platters on which data can be stored magnetically,
mounted on a central spindle. The data is stored on each platter surface in a pattern
of concentric circles called tracks. Tracks located one above another on successive
surfaces comprise a cylinder.
The arrangement of data on disks is based on a file organization that in turn
allows data to be retrieved using an access method. Two such methods for direct
access are indexes and hashing. A simple linear index consists of two columns: an
ordered list of the identifiers of the records being indexed, each of which is associated
in the second column with its physical location on the disk. A more practical arrange-
ment and the one in common use in today's computers is the B + -tree, in which the
index is constructed in a hierarchical arrangement. Hashing is a way of arranging the
records on the disk based on a mathematical calculation on each record's identifier;
retrieval is accomplished using the same mathematical calculation.
Physical database design is the modification of the database structure to
improve performance. A variety of factors involving the database structure or its
use can adversely affect system performance. In addition to the logical design
results, inputs to the physical design process include response time requirements,
throughput requirements, and a variety of other data and application characteristics
and operational requirements.
Physical database design techniques fall into two categories: techniques that
do not change the logical design and techniques that do change the logical design.
The former include adding external features such as indexes, reorganizing stored
data on the disk, and splitting a table into multiple tables. The latter include adding
attributes to a table or changing attributes in a table, combining tables, and adding
new tables.