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What's Oracle version and OS?
What you can do first is do a SQL trace on your session and get explain plan of the query.
My guess is on statistics.
My tip is try to run on developer environment.
Please post the trace.
and confirm that the query doesn´t have WHERE clause?
select col1,col2,col3 from table1 order by col1,col3
Though trace always provides best information
I would Check using
check v$pgastat OR AWR reports for multi passs execs happening. May need to increase PGA size
check if sort_area_size changed.
If you had a fat index (col1,col3,col2), you might had avoided sort all the way (fast full index) which might not be available now.