Did you ever try to query DBA_EXTENTS on a very large database with LMT tablespaces? I had to, in the past, in order to find which segment a corrupt block belonged to. The information about extent allocation is stored in the datafiles headers, visible though X$KTFBUE, and queries on it can be very expensive. In addition to that, the optimizer tends to start with the segments and get to this X$KTFBUE for each of them. At this time, I had quickly created a view on the internal dictionary tables, forcing to start by X$KTFBUE with materialized CTE, to replace DBA_EXTENTS.