Indexed (Indexed Sequential) File Organisation
It organises the file like a big dictionary, i.e., records are kept in order of the key but an index is stored which also allows a type of direct access. The records are kept sequentially by primary key values and there is an index built over the primary key field.
The retrieval of a data from a sequential file, on average, needs access to half the records in the file, making such inquiries not only useless but very time consuming for big files. To improve the query response time of a sequential file, a type of indexing method can be added.
An index is a set of address pairs, index value. Indexing associates a set of objects to a set of orderable quantities, which are generally smaller in number or their properties. Therefore, an index is a mechanism for faster search. While the indices and the data blocks are stored together physically, they are logically distinct. Let us use the word an index file to defines the indexes and let us refer to data files as data records. An index can be small sufficient to be read into the main memory.
A sequential file that is indexed on its primary key is known as an index sequential file. The index permits for random access to records, as the sequential storage of the records of the file gives easy access to the sequential records. An additional feature of this file system is the over flow area. The overflow area gives additional space for record addition without the need to make.