Describe the storage structure of indexed sequential files and their access method. 

Ans:   Storage structure of Indexed Sequential files and their access: To achieve fast random access to records in a file, we can utilize an index structure. An index record contains a search key value and pointers to data records that is related with a specific search key. An ordered index stores the values of the search keys in sorted order. A file may have various indices on different search keys. If the files consisting of the records are sequentially ordered, a primary index is an index whose search key also describes the sequential order of the file. Such types of files are termed as index sequential files. There are two sorts of ordered indices: dense and sparse. In dense index, and index record appears only for a few of the search-key in the files as displayed below.

To access a specific record with search key value, K, by using dense index we search index record along with search key value, K, by which reach the first entry of data record along with search key value K. After that the data records are searched linearly to acquire the required record. If either the index record is not there or the linear search arrive the data record with different search key value, then the record is not there. Here now for sparse index, we search index record along with search key value, K or the index record with highest search key value less as compared to K, from which arrives the first entry of data record with search key value K or highest value less than K. After that the data records are searched linearly to acquire the required record. If the linear searches the data record with search key value greater as compared to K, then the record is not there.