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Indexed Sequential Access Method (ISAM) is a method for creating, maintaining, and manipulating computer files of data so that records can be retrieved sequentially or randomly by one or more keys. Indexes of key fields are maintained to achieve fast retrieval of required file records in indexed files .
Sequential access is a term describing a group of elements (such as data in a memory array or a disk file or on magnetic-tape data storage) being accessed in a predetermined, ordered sequence. It is the opposite of random access , the ability to access an arbitrary element of a sequence as easily and efficiently as any other at any time.
The indexes are created with the file and maintained by the system. IBM supports indexed files with the Indexed Sequential Access Method on OS/360 and successors. IBM virtual storage operating systems added VSAM, which supports indexed files as Key Sequenced Data Sets (KSDS), with more options.
Index is a full index so data file does not have to be ordered; Pros and cons versatile data structure – sequential as well as random access; access is fast; supports exact, range, part key and pattern matches efficiently. volatile files are handled efficiently because index is dynamic – expands and contracts as table grows and shrinks
An index is a persisted ordering of records in a table. Indexes are used for both sequential access to rows in the order defined, and for direct record navigation based on indexed column values. The order defined by an index is described in terms of an array of columns, in precedence order. This array of columns is also called the index key.
Virtual Storage Access Method (VSAM) [1] is an IBM direct-access storage device (DASD) file storage access method, first used in the OS/VS1, OS/VS2 Release 1 (SVS) and Release 2 (MVS) operating systems, later used throughout the Multiple Virtual Storage (MVS) architecture and now in z/OS.
A sparse index in databases is a file with pairs of keys and pointers for every block in the data file. Every key in this file is associated with a particular pointer to the block in the sorted data file. In clustered indices with duplicate keys, the sparse index points to the lowest search key in each block.
The index can stay the same, and the record can just be marked as deleted. The database remains in sorted order. If there are a large number of lazy deletions, then searching and storage become less efficient. [19] Insertions can be very slow in a sorted sequential file because room for the inserted record must be made.