what is a purpose of index with order if the query has order 1 column, do we need to use order in index

When a query sorts by a single column, an index is not required to explicitly define the sorting direction. Modern relational databases such as PostgreSQL, MySQL, and SQL Server utilize B-Tree indexes, which are inherently bidirectional. This means that whether the query sorts in ascending or descending order, the database can efficiently scan the index in either direction, starting from either the beginning or the end.

Creating an index with an explicit ascending or descending order (e.g., `CREATE INDEX idx_name ON table(col DESC)`) for a single-column query does not provide any performance benefit over the default ascending index. The database can read the single-column index both forward and backward with equal speed.

The need for specifying the sort direction in an index becomes relevant only in situations involving composite indexes, which consist of multiple columns. In such cases, the query might involve sorting by different directions for different columns. For example, if a query is structured as `SELECT * FROM users ORDER BY score DESC, created_at ASC;`, a default composite index on `(score, created_at)` would be defined as `(score ASC, created_at ASC)`.

In this scenario, the database can scan the index straightforwardly for queries that follow the same sort directions as the index. However, the database cannot efficiently handle queries that require mixed sorting directions, such as `ORDER BY score DESC, created_at ASC`. To optimize performance for these specific cases, it is necessary to create an index that matches the query's ORDER BY directions, like `CREATE INDEX idx_score_date ON users (score DESC, created_at ASC);`.

In summary, for single-column sorts, simply creating a regular index (e.g., `CREATE INDEX idx ON table(column)`) will ensure the database efficiently handles both ascending and descending queries. For composite indexes with queries that sort some columns in ascending and others in descending order, the index definition must align with the exact sorting requirements specified in the query to optimize performance.