Order by DESC took twice as long than ASC

Is it becuase Oracle always default sorting on ascending order?

Do you have an index in ascending order on the column?

Oracle does not guarantee ordering (attribute of RDBs), however your data may already be ordered. I suggest you do some explain plans and/or select out selected rows (or the whole table if small enought) to see if the rows were loaded in order.

d.

There might also be other reasons for this.

If for example you have a GROUP BY in your query then it might not perform an aditional sorting for ORDER BY ASC at all, because ascending sorting is implicitly performed on GROUP BY. But if you have ORDER BY DESC on top of GROUP BY it mut perform an additional sort, hence longer execution time.