What is an index in MySQL

An index in MySQL is a data structure that improves the speed of data retrieval operations on a database table at the cost of additional space and decreased performance on data modification operations. Indexes are used to find rows with specific column values quickly, which helps significantly enhance the performance of queries that filter data or perform searches.

MySQL supports various types of indexes, including primary keys, unique indexes, and full-text indexes. A primary key is a unique identifier for a record in a table, while a unique index ensures that no two rows have the same value in a specified column. Full-text indexes are used for full-text searches, allowing for more complex search capabilities.

By creating an index, MySQL can avoid scanning the entire table and can instead quickly access the data it needs, thus improving query performance.

For example, to create an index on a column named `username` in a `users` table, you could use the following SQL command: