ORA-20000

Automatic statistics gathering is enabled by default.
To check whether the statistics are collected automatically have to look at dba_scheduler_jobs.
To enable/disable statistics collection is necessary to use package dbms_scheduler.

select * 
from   dba_scheduler_jobs 
where  job_name = 'GATHER_STATS_JOB';

-- disable automatic statistics gathering
begin
  dbms_scheduler.disable('GATHER_STATS_JOB');
end;
/

-- enable automatic statistics gathering
begin
  dbms_scheduler.enable('GATHER_STATS_JOB');
end;
/

Statistics collected automatically if the property statistics_level is set to TYPICAL or ALL.

select *
from   v$parameter
where  v$parameter.name = 'statistics_level';

Job collects change all the tables, which included monitoring.

select *
from   user_tables
where  user_tables.monitoring = 'YES';

Job detects changes in the table (changing rows, adding and deleting). If the number of lines exceeds 10%, then the collection of statistics.

The information about changes of tables can be viewed in the user_tab_modifications view.

10% is not constant and can be changed using dbms_stats.set_global_prefs.

The values of all parameters of the collection of statistics can be found in the table sys.optstat_hist_control$.

begin
  dbms_stats.set_global_prefs(pname => 'STALE_PERCENT',
                              pvalue => 12);
end;
/

select *
from   sys.optstat_hist_control$;