Estatísticas

Atualiza a estatistica de um schema :

execute dbms_stats.gather_schema_stats('SCOTT');

Atualize a estatistica da sua intancia :

execute dbms_stats.gather_database_stats;

Atualize a estatistica de uma tabela do seu schema

exec dbms_stats.gather_table_stats('', '');

Analise com anda a estatistica dos objetos 

set pages 999 lines 100
select a.owner
, a.total_tables tables
, nvl(b.analyzed_tables,0) analyzed
from (select owner
 , count(*) total_tables
 from dba_tables
 group by owner) a
, (select owner
 , count(last_analyzed) analyzed_tables
 from dba_tables
 where last_analyzed is not null
 group by owner) b
where a.owner = b.owner (+)
and a.owner not in ('SYS', 'SYSTEM')
order by a.total_tables - nvl(b.analyzed_tables,0) desc
/