What is indexed sequential file organization? What are the applications of this organization?

 Ans: An index file can be employed to effectively overcome the problem of storing and to speed up the key search also. The very simple indexing structure is the single-level one: a file whose records are pair's key-pointer, in which the pointer is the position in the data file of the record along with the given key. Just only a subset of data records, consistently spaced along the data file, are indexed, thus to mark intervals of data records. 

 A key search after that proceeds as follows: the search key is compared with the index ones to find out the highest index key preceding the search one, and a linear search is carried out from the record the index key points onward, till the search key is matched or until the record pointed by the next index entry is reached. In place of the double file access (index + data) needed by this type of search, the decrease in access time along with respect to a sequential file is important.