Indexing:

• 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.