Does the order of fields in a composite index matter when optimizing queries?

Yes, the order of fields in a composite index plays an important role in query optimization.

An index is structured as a tree, where data is first sorted by the first field, then by the second, and so on. If a query filters or sorts by the first field of the index, the index will be used efficiently. If the query only uses the second field, the index may not be helpful.

For example, for an index on fields (A, B):

• A query with a condition on A or on both A and B will use the index effectively.
• A query only on B — the index will not be useful.

Therefore, when designing indexes, it is important to consider which fields are most frequently used in WHERE conditions, JOINs, or ORDER BY clauses, and to order them in the index according to their importance for queries.