For queries sorting by a single column, explicit direction in index creation is unnecessary because databases can read single-column B-tree indexes equally efficiently forward and backward. This capability eliminates the costly filesort operation, as the index already stores data in order. When a query requests ascending order, the database scans the index from the start. For descending order, it scans the same index in reverse. Therefore, creating an index with a descending direction for a single column offers no performance benefit over the default ascending index.However, specifying ASC or DESC becomes critical for composite indexes when a query involves mixed sort directions across multiple columns. A default composite index might not efficiently support queries with mixed sorting requirements. To optimize such queries, the index definition must precisely match the query's specified sort directions. For instance, a query ordering by score DESC and created_at ASC requires an index explicitly defined as (score DESC, created_at ASC).In summary, create regular indexes for single-column sorts, and they will efficiently handle both ascending and descending queries. For multi-column sorts with the same direction, a composite index is suitable. When dealing with mixed directions in multi-column sorting, ensure the index definition perfectly mirrors the query's ORDER BY clause. This approach guarantees optimal performance by avoiding expensive sorting operations.