Indexing is another common method for making retrievals faster.
The time taken for this operation depends on the number of pages to be accessed. If the records are randomly stored, the page accesses depend on the volume of data. If the records are stored physically together, the number of pages depends on the size of each record also.
The number of records in the index file is same as that of the data file. Thus the access involves a Sequential access on the index file and a Direct access on the actual data file.
Retrieval Speed v/s Update Speed : Though indexes help making retrievals faster, they slow down updates on the table since updates on the base table demand update on the index field as well.
It is possible to create an index with multiple fields i.e., index on field combinations. Multiple indexes can also be created on the same table simultaneously though there may be a limit on the maximum number of indexes that can be created on a table.