An index is a secondary database object associated with a table to improve the retrieval performance of rows from that table.

An index can be defined for a single column or multiple columns of a given table. If an index is defined on a single column of a table, the index can be viewed as ordered map between all values in that column and their row locations in the table. For examples, if you have table with a column called "company_num" and you created an index for that column. The contents of the table and the index may look like what is presented in the following picture:

Table Index Diagram

As shown in the above picture, locating the rows of company_num = 17 can be done much faster through the index.