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Reference : Using Indexes : Introduction

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An index is a database structure that you can use to improve the performance of database activity. A database table can have one or more indexes associated with it.
An index is defined by a field expression that you specify when you create the index. Typically, the field expression is a single field name, like emp_id. An index created on the emp_id field, for example, contains a sorted list of the employee ID values in the table. Each value in the list is accompanied by references to the rows that contain that value.
A database driver can use indexes to find rows quickly. An index on the emp_id field, for example, greatly reduces the time that the driver spends searching for a particular employee ID value. Consider the following Where clause:
WHERE EMP_id = 'E10001'
Without an index, the server must search the entire database table to find those rows having an employee ID of E10001. By using an index on the emp_id field, however, the server can quickly find those rows.
Indexes may improve the performance of SQL statements. You may not notice this improvement with small tables, but it can be significant for large tables; however, there can be disadvantages to having too many indexes. Indexes can slow down the performance of some inserts, updates, and deletes when the driver has to maintain the indexes as well as the database tables. Also, indexes take additional disk space.