What is the purpose of a composite index in MySQL?

A MySQL composite index is an index that combines multiple columns. It is mainly used to improve the performance of queries with multiple conditions.

In specific terms, a composite index can improve the speed of multiple-column queries. When there are multiple columns used as filtering conditions in a query, a composite index can combine these columns together, allowing MySQL to locate the relevant data faster. This is because the composite index arranges the columns in the order of the index, enabling MySQL to perform range searches more efficiently.

Furthermore, a composite index can also enhance the performance of sorting operations on multiple columns. When a query requires sorting by multiple columns, a composite index can directly sort according to the order of the index without the need for additional sorting operations.

It is important to note that the creation of composite indexes should be determined based on specific query requirements and data characteristics. If only a few columns are frequently used as filtering or sorting criteria in a query, then it is sufficient to create an index that includes only those columns, to avoid creating too many composite indexes. Additionally, the column order of composite indexes should also be optimized based on the frequency of queries and filtering effects.