How to find the objects/extents residing on a specific datafile?

This can be useful, for example, if we want to drop a datafile, and it has to be empty in order to do this.

How can we check what objects are there in a datafile?

select distinct a.owner,a.segment_name
from dba_extents a,dba_data_files b
where a.file_id=b.file_id and b.file_name=<your datafile name with path>;

Note: the query above is usually very slow, since it has to full scan a few big fixed tables.