In what cases is it recommended to use a composite index in a database to optimize queries?

A composite index in a database is created on multiple columns simultaneously and is recommended to be used when:

• Queries frequently filter or sort data by multiple fields at the same time.
• There are WHERE conditions that use several columns together.
• It is necessary to speed up the execution of complex queries with multiple criteria.

For example, if a query is often executed:

SELECT * FROM orders WHERE customer_id = ? AND order_date = ?;

then a composite index on (customer_id, order_date) can significantly speed up the search.

It is important to consider the order of columns in the index — it should match the order of usage in the query conditions. If queries often filter only by the first column of the index, the index will be effective; otherwise, it will not.

It is also advisable to avoid creating too many composite indexes to prevent slowing down insert and update operations.