What are covering indexes and how do they help?

Covering index is an index in a database that contains all the columns necessary to fulfill a specific query, meaning the query can be fully served solely from the index data without accessing the main table. This significantly speeds up query execution, as it reduces the number of disk read operations and decreases system load.

For example, if you have a query:

SELECT name, age FROM users WHERE city = 'Moscow';

And you create a covering index on the columns (city, name, age), the DBMS can retrieve all the needed data from the index without accessing the users table.

Advantages of covering indexes:

• Faster reads due to fewer table accesses.
• Reduced load on disk and memory.
• Improved performance of complex queries with multiple columns.

However, it is important to note that covering indexes occupy more space and may slow down insert and update operations.