What is an Index?
An index is a small table having only two columns. The first column has a copy of the primary or candidate key of a table and the second column having a set of pointers holding the address of the disk block where that particular key value can be found.
The advantage of using index lies in the fact is that index makes search operation perform very fast. Suppose a table has a several rows of data, each row is 20 bytes wide. If you want to search for the record number 100, the management system must thoroughly read each and each row and after reading 99x20 = 1980 bytes it will search record number 100. If we have an index, the management system starts to search for record number 100 not from the table, but from the index. The index, having only two columns, might be just 4 bytes wide in each of its rows. After reading only 99x4 = 396 bytes of data from the index the management system search an entry for record number 100, reads the address of the disk block where record number 100 is stored and directly points at the record in the physical storage device. The result is a much faster access to the record (a speed benefit of 1980:396).