Other Primary File Organizations
1. Files of Mixed
Records
The file organizations we have studied so far assume that all records of
a particular file are of the same record type. The records could be of EMPLOYEEs, PROJECTs, STUDENTs, or
DEPARTMENTs, but each file contains records
of only one type. In most database applications, we
encounter situations in which numerous types of entities are interrelated in
various ways, as we saw in Chapter 7. Relationships among records in various
files can be represented by connecting
fields. For exam-ple, a STUDENT record can have a connecting
field Major_dept whose value gives the name of the DEPARTMENT in which
the student is majoring. This Major_dept field refers to a DEPARTMENT entity, which should be represented by a record of its own in the DEPARTMENT file. If
we want to retrieve field values from two related records, we must retrieve one
of the records first. Then we can use its connecting field value to retrieve
the related record in the other file. Hence, relationships are implemented by logical field references among the
records in distinct files.
File organizations in object DBMSs, as well as legacy systems such as
hierarchical and network DBMSs, often implement relationships among records as physical relationships realized by physical contiguity (or clustering) of related
records or by physical pointers.
These file organizations typically assign an area of the disk to hold records of more than one type so that
records of different types can be physically
clustered on disk. If a particular relationship is expected to be used
frequently, implementing the relationship physically can increase the system’s
efficiency at retrieving related records. For example, if the query to
retrieve a DEPARTMENT
record and all records for STUDENTs majoring in that department is frequent,
it would be desirable to place each DEPARTMENT record
and its cluster of STUDENT records contiguously on disk in a
mixed file. The concept of physical clustering of object types is used in object DBMSs to store related objects
together in a mixed file.
To distinguish the records in a mixed file, each record has—in addition
to its field values—a record type
field, which specifies the type of record. This is typically the first field in
each record and is used by the system software to determine the type of record
it is about to process. Using the catalog information, the DBMS can deter-mine
the fields of that record type and their sizes, in order to interpret the data
val-ues in the record.
2. B-Trees and Other Data Structures as Primary Organization
Other data structures can be used for primary file organizations. For
example, if both the record size and the number of records in a file are small,
some DBMSs offer the option of a B-tree data structure as the primary file
organization. We will describe B-trees in Section 18.3.1, when we discuss the
use of the B-tree data structure for indexing. In general, any data structure
that can be adapted to the characteristics of disk devices can be used as a
primary file organization for record placement on disk. Recently, column-based
storage of data has been proposed as a primary method for storage of relations
in relational databases. We will briefly introduce it in Chapter 18 as a
possible alternative storage scheme for relational databases.
Related Topics
Privacy Policy, Terms and Conditions, DMCA Policy and Compliant
Copyright © 2018-2024 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.