For a file with a given record structure consisting of several fields, an index access structure is defined on a single field of file, called an indexing field.
An index on a file speeds up selection on the search key fields for the index.
An index file consists of records (called index entries) of the form:
where search key is set of attributes used to look up records in a file.
1] Dense Index:
It has an index entry for every search key value in the data file.
for example, index on ID attribute of emp relation.
Dense index are bulkier in size.
secondary index are normally dense index.
2] Sparce Index:
It has index entries for some of the search key values.
It has fewer entries than the number of records in the file.
The primary index is a sparce index.
For example employee city is an sparce index.