Oracle 11g Sys Query
Common problem in 11.2.0.3. Several workarounds are available on MOS.
Query against DBA_EXTENTS slow after upgrade to 11.2.0.3 (Doc ID 1453425.1)
The Cost Based Optimizer uses a cardinality estimate of 100,000 rows for this table, and the 11.2.0.3 execution plan is doing a full table scan. If you have a large number of extents, this query can take more than 1 hour to complete. There are a number of unpublished bugs open on slow performance for queries on DBA_EXTENTS.