Indexed sequential files

We are all familiar with the concept of an index. For example, the directory in a large multi storied building is an index that helps us locate a particular person's room within the building. For instance, to find the room of Dr. Sam within the building, we would look up his name in the directory (index) and read the corresponding floor number and room number. this idea of scanning a logically sequenced table is preferable to searching door by door for a particular name. Indexed sequential files use exactly the same principle. The records in this type of file are organized in a sequence and an index table is used to speed up access to the records without requiring a search of the entire file. The records of the file can be stored in random sequence but the index table is in stored sequence on the key value. This provides the user with a very powerful tool. Not only can the file be processed randomly, but it can also be processed sequentially. Since the index table is in a stored sequence on the key value, the file management system simply accesses the data records in the order of the index values. Thus indexed sequential files provide the user sequential access, even though the file management system is accessing the data records in a physically random order. This technique of file management is randomly referred to as the Indexed Sequential Access Method (ISAM). Files of this type are called ISAM files.

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