Oracle 12c partitioned table & dbms_stats

Hi,

I've a fairly big partitioned table in Oracle 12c and from time to time Oracle does analyze all partitions. The table is analyzed with:

dbms_stats.gather_schema_stats(

                                            p_schema_name,

                                            cascade         => TRUE,

                                           options         => 'GATHER AUTO'

               );

The 'GATHER AUTO' option in the context of GATHER_SCHEMA_STATS tells the database to skip tables and partitions that do not have stale statistics. 

So, I can't explain why Oracle is sometimes analyzing all partitions. And I've checked that the older partitions are not being modified.

Any ideas?

regards,

Ivan