How does the use of indexes affect the speed of query execution in a database?

Indexes in a database create an additional data structure that allows for quick retrieval of records without scanning all rows of the table. It's similar to a pointer in a book: instead of reading the entire book, you go directly to the needed page.

When a query contains search conditions (for example, WHERE), an index helps the database quickly determine where the matching records are, significantly reducing the query execution time.

Example: if you have a users table with millions of records and you are searching for a user by email, an index on the email field will allow you to find the user almost instantly, instead of scanning the entire table.