What purpose does an index serve in a database?

An index in a database is a data structure that improves the speed of data retrieval operations on a database table. It allows the database management system to find and access rows in a table more quickly and efficiently, similar to how an index in a book helps you locate specific information without having to read every page. When a query is executed, the database engine can use the index to find the required records more rapidly than it would by scanning the entire table. This is particularly beneficial in large databases where searching through all the data would be time-consuming.

Using an index can significantly improve performance for read operations, especially for queries that involve searching, filtering, or sorting data based on indexed columns. While indexes do require additional storage space and can slightly slow down data modification operations (such as insertions, updates, and deletions), their main purpose is to enhance the efficiency of data retrieval.

The other options do not accurately describe the primary function of an index in the context of databases. For example, increasing the size of the database or ensuring data security are not related to indexing. Additionally, while creating relationships between tables is a key aspect of relational databases, it is achieved through the use of foreign keys, not through indexing.