Several years ago I wrote a note about reporting dba_free_space and dba_extents to produce a map of the space usage in a tablespace in anticipation of messing about with moving or rebuilding objects to try and reduce the size of the files in the tablespace. In the related page where I published the script I pointed out that a query against dba_extents would be expensive because it makes use of structure x$ktfbue which generates the information dynamically by reading segment header blocks.
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