ORA-20000

Here's a simple request. The request takes into account the size of the table, including indexes, lobs and nested tables.

select '&TABLE' as table_name, sum(dba_segments.BYTES) / 1024 / 1024 as Mb
from   dba_segments
where  (dba_segments.owner, dba_segments.segment_name) in
       (
         -- table
         select '&OWNER' as owner,
                '&TABLE' as segment_name
         from   dual
         union all
         -- lob segments
         select dba_lobs.owner, dba_lobs.segment_name
         from   dba_lobs
         where  dba_lobs.owner = '&OWNER' and
                dba_lobs.table_name = '&TABLE'
         union all
         -- lob index segments
         select dba_lobs.owner, dba_lobs.index_name
         from   dba_lobs
         where  dba_lobs.owner = '&OWNER' and
                dba_lobs.table_name = '&TABLE'
         union all
         -- nested tables
         select dba_nested_tables.owner, dba_nested_tables.table_name
         from   dba_nested_tables
         start with dba_nested_tables.owner = '&OWNER' and
                    dba_nested_tables.parent_table_name = '&TABLE'
         connect by dba_nested_tables.parent_table_name = prior dba_nested_tables.table_name
         union all
         -- indexes
         select dba_indexes.owner, dba_indexes.index_name
         from   dba_indexes
         where  dba_indexes.table_owner = '&OWNER' and
                dba_indexes.table_name = '&TABLE'
       )

This small reports the tablespace name, alloc size, free space, used space, extend size, total size in all the tablespaces in Oracle database.

select a.tablespace_name ,
       round(a.bytes_alloc / 1024 / 1024, 2) m_alloc,
       round(nvl(b.bytes_free, 0) / 1024 / 1024, 2) m_free,
       round((a.bytes_alloc - nvl(b.bytes_free, 0)) / 1024 / 1024, 2) m_used,
       round((maxbytes - a.bytes_alloc)/1024/1024, 2) m_extend,
       round(maxbytes/1024/1024,2) Max
from  ( select  f.tablespace_name,
               sum(f.bytes) as bytes_alloc,
               sum(decode(f.autoextensible, 'YES',greatest(f.maxbytes, f.bytes),'NO', f.bytes)) as maxbytes
        from dba_data_files f
        group by tablespace_name
        union all
        select f.tablespace_name,
               sum(f.bytes) as bytes_alloc, 
               sum(decode(f.autoextensible, 'YES',greatest(f.maxbytes, f.bytes),'NO', f.bytes)) as maxbytes
	      from dba_temp_files f
	      group by tablespace_name         
        ) a,
      ( select  f.tablespace_name,
               sum(f.bytes)  bytes_free
        from dba_free_space f
        group by tablespace_name) b
where a.tablespace_name = b.tablespace_name (+);

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$;