In some cases, indexes are not useful:
Columns that are seldom or never referenced in queries do not benefit from indexes, since the system seldom has to search for rows on the basis of values in these columns.
Columns that have many duplicates, and few unique values relative to the number of rows in the table, receive no real advantage from indexing.
If the system does have to search an unindexed column, it does so by looking at the rows one by one. The length of time it takes to perform this kind of scan is directly proportional to the number of rows in the table.
