The physical or internal level organization of a database system is concerned with the efficient storage of information in the secondary storage devices. At this level we are no longer concerned with the application programmers views of the database. The physical to conceptual level mapping must provide the necessary shield to the user. The basic problem in physical database representation is to select a suitable file system to store the desired information. The file consists of records and records may consist of several fields.
The typical operations that may be performed on the information stored in the file are as follows.
- Retrieve: To find a record (or sets of records) having a particular value in a particular field or the field value satisfies a particular condition.
- Insert: Inserts a record or sets of record at some specific locations.
- Update: Modifies the field values of a record (or sets of records)
- Delete: Deletes a particular record or sets of records
The efficiency of a file system depends on how efficiently these operations can be performed on the file. However, the efficiency of these operations on a particular file may depend on the type of queries the file system needs to process. For instance a sequential file system turns out to be quiet efficient for an application where most of the storage and retrieval of record are performed sequentially. The same file system turns out to be inefficient in an environment where records are to be retrieved or updated randomly.
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