An SQL index is a data structure used to speed up searching, filtering and sorting operations in database tables. It works similarly to an index in a book, helping the database engine find data faster.

Indexes are especially useful in large tables where WHERE, JOIN, ORDER BY and GROUP BY operations are frequently used. A well-designed index can dramatically improve read performance.

However, adding indexes to every column is not a good practice. Indexes improve reads but add overhead to write operations. Every INSERT, UPDATE or DELETE may require index maintenance.

A good indexing strategy should be based on real queries, execution plans and data access patterns. Indexes are powerful, but they are only one part of database optimization.