What are the advantages of by using an index and what are its disadvantages ?

In difference, if the search key of a secondary index is not a candidate key, it is not sufficient to point to only the first record along with each search-key value. The remaining records along with the similar search - key value could be anywhere in the file, since the records are ordered through the search key of the primary index, rather than through the search key if the secondary index. Thus, a secondary index must contain pointers to all the records. Secondary indices improve the performance of queries in which use keys other than the search key of the primary index. Therefore, they impose an important overhead on modification of the database. The designer of a database decides that secondary indices are desirable on an estimate of the relative frequency of query's and modifications.

Some of the advantages of using an index are:

(i) Indexes speed up search on the indexed attributes(s). Without an index either a sequential search or a few sort of binary search would be required.
(ii) Indexes can also speed up chronological processing of the file while the file is not stored as a sequential file.

Some of the disadvantages of using an index are :

(i) An index needed additional storage. This additional storage can be significant while a number of indexes are being used on a file.
(ii) Insertion, deletion and updates on a file along with indexes takes more time than on a file without any indexes.