How to correctly apply composite indexes in a database and what should be considered when using them?

Composite (composite) indexes in a database are indexes that include multiple table columns. They help speed up queries that filter or sort data based on several fields simultaneously.

Proper use of composite indexes:

• Order of columns matters. The index is effective for queries that use the first columns of the index in the same order. For example, an index on (A, B) is useful for queries filtering by A or by A and B, but not by B alone.
• Use for frequently used filter and sort combinations. Analyze real queries and create indexes tailored to them.
• Do not create overly long composite indexes unnecessarily. This increases the index size and slows down insert/update operations.

Example of creating a composite index in SQL:

CREATE INDEX idx_user_status_date ON orders (user_id, status, order_date);

When used in Go with a database, it is important to ensure that ORM or drivers correctly generate queries to effectively utilize composite indexes.