It may be an excellent idea to keep data of the students in the order of the programme they have registered as most of the data file accesses may need student data of a particular programme only. An index that is formed on an ordered file whose records of a file are physically ordered on a non-key field (that is the field does not have a distinct value for every record) is known as a clustering index.
Figure : A clustering Index on Programme of an employee file
Figure: Clustering index with separate blocks for each group of records with the same value for the clustering field
Please note the following points in the tables:
• Clustering index is one more example of a non-dense index as it has one entry for each distinct value of the clustering index field and not for all records in the file.