Indexed Sequential Organization

Like sequential organization, keyed sequential organizati6nstores data in physically contiguous blocks. The difference is in the use of indexes to locate records. To understand this method, we' need to distinguish among three areas in disk storage: prime area, overflow area, and index area. The prime area contains file records stored by key or ID numbers. All records are initially stored in the prime area. The overflow area contains records added o the file that cannot be placed in logical sequence in the prime area. The index area is more like a data dictionary. It contains keys of records and their locations on the disk. A pointer associated with each key is an address that tells the system where to find a record.

  The index area contains pointers to the Chicago and Houston flights. The Chicago flight points to the Chicago flight information stored in the prime area. The Houston fight points to the Houston flight information in the prime area. Lack of pace to store the Huntsville flight in sequential order made it necessary to load it in the overflow area. The overflow pointer places it logically in sequential order in the prime area. The same arrangement applies to the Louisville flight.

Indexed-sequential organization reduces the magnitude of the sequential search and provides quick access for sequential and direct processing. The primary drawback is the extra storage space required for the index. It also takes longer to search the index for data access or retrieval.