Query Optimization is a process in database management systems (DBMS) that aims to improve the performance of SQL queries by selecting the most efficient way to execute them. It focuses on reducing the resources (CPU, memory, I/O operations) required to retrieve or manipulate data, which is especially important in large-scale databases or high-transaction systems.

When a query is executed, the DBMS uses a query optimizer to evaluate multiple possible execution plans. These plans are based on factors like index usage, join algorithms, and data distribution. The optimizer selects the plan that offers the lowest cost in terms of time and resource consumption. The efficiency of query optimization depends heavily on the database schema, indexing strategies, and the structure of the SQL queries.

Developers can manually optimize queries by using techniques like indexing, query rewriting, and partitioning large datasets. For example, reducing the number of unnecessary joins or filtering data early in the query can drastically reduce execution time. Query optimization is critical for improving application performance, especially for systems that handle a high volume of queries or transactions, such as e-commerce platforms, data analytics tools, and social media applications.