How does the sequence of fields in composite indexes affect query performance?

The sequence of fields in composite indexes directly affects the efficiency of query execution in the database.

An index on multiple fields works like a tree, where the first field is the root, the second is the next level, and so on. Queries that filter or sort data by the first fields of the index use the index most effectively.

If the order of fields in the index does not match the order of conditions in the query, the index may not be used or may be used partially, which reduces performance.

For example, an index on (A, B, C) will efficiently speed up queries with conditions on A, A and B, or A, B, and C, but will not be useful for a query filtering only on B or C.

Therefore, when designing indexes, it is important to consider typical queries and their filters so that the order of fields in the index corresponds to the most frequently used conditions.