Q:

Order by two columns with unindexed second column

I am using Oracle SQL.  I'm new to SQL, i have next problem.  If i am sorting rows like that:
"ORDER BY column1 ASC, column 2 DESC", when my column1 has index, but column2 doesn't.  I know that cases when column1 value is the same in different rows will appear very rarely.  So i have a next question, how it will affect on performance?  Does it matter how many entries have the same column1 value?  As i understand, if we have table where each entry have different column1 value then sorting will only be applied to column1, without causing any performance problems, am i right? 

A:

Index is used to access data, but not directly for SORT/ORDER BY.  As database fetched set of data, it will sort it according to ORDER BY clause.  
So general answer on your question is NO: there is no impact on SORT performance
In some cases, the set can be already pre-sorted (due to fetch order for instance single INDEX RANGE SCAN will return rows sorted by key).  In this case, there is no need for sorting, and Oracle will show it as NOSORT in SQL execution plan. 