Mysterious plan changes

Statistics results may always be different when using sample size. Consider this 
    select * from aldo.persone sample(1)
and run it many times.

Statistics result may obviously change following data changes

col_usage$

2 instances have different plans but exactly same data and updated statistics

Each time you perform a joined query a track is keeped:

SELECT   dba_tab_columns.owner, dba_tab_columns.table_name,
         dba_tab_columns.column_name, SYS.col_usage$.*
    FROM SYS.col_usage$, dba_objects, dba_tab_columns
   WHERE obj# = dba_objects.object_id
     AND dba_tab_columns.owner = dba_objects.owner
     AND dba_tab_columns.table_name = dba_objects.object_name
     AND intcol# = dba_tab_columns.column_id
     AND dba_objects.owner = 'ALDO'
     AND object_name = 'T'
ORDER BY intcol#

See how histograms statistics are taken:

SELECT *
  FROM dba_tab_histograms
 WHERE owner = 'ALDO' AND table_name = 'T' AND column_name = 'ID'

Demonstration
This apply to method_opt=>'FOR ALL COLUMNS SIZE AUTO' when taking stats (default value in 10g)
    
create table t as
select mod(rownum, 5) id, a.*
from dba_objects a

Each time you perform a joined query a track is keeped:

SELECT   dba_tab_columns.owner, dba_tab_columns.table_name,
         dba_tab_columns.column_name, SYS.col_usage$.*
    FROM SYS.col_usage$, dba_objects, dba_tab_columns
   WHERE obj# = dba_objects.object_id
     AND dba_tab_columns.owner = dba_objects.owner
     AND dba_tab_columns.table_name = dba_objects.object_name
     AND intcol# = dba_tab_columns.column_id
     AND dba_objects.owner = 'ALDO'
     AND object_name = 'T'
ORDER BY intcol#

** No data returned *** a joined query has never been performed or 
SYS.col_usage$ has not been updated

See how histograms statistics are taken:

SELECT *
  FROM dba_tab_histograms
 WHERE owner = 'ALDO' AND table_name = 'T' AND column_name = 'ID'
 
** No data returned *** Statistics has probably never been taken

begin
dbms_stats.gather_table_stats(user, 'T', method_opt=>'FOR ALL COLUMNS SIZE AUTO');
end;

See how histograms statistics are taken:

SELECT *
  FROM dba_tab_histograms
 WHERE owner = 'ALDO' AND table_name = 'T' AND column_name = 'ID'

OWNER   TABLE_NAME  COLUMN_NAME ENDPOINT_NUMBER ENDPOINT_VALUE ENDPOINT_ACTUAL_VALUE
ALDO    T           ID          0               0	          
ALDO    T           ID          1               4	

select * from t where id = 1

begin
dbms_stats.gather_table_stats(user, 'T', method_opt=>'FOR ALL COLUMNS SIZE AUTO');
end;

Now we see the track of the joined query

SELECT   dba_tab_columns.owner, dba_tab_columns.table_name,
         dba_tab_columns.column_name, SYS.col_usage$.*
    FROM SYS.col_usage$, dba_objects, dba_tab_columns
   WHERE obj# = dba_objects.object_id
     AND dba_tab_columns.owner = dba_objects.owner
     AND dba_tab_columns.table_name = dba_objects.object_name
     AND intcol# = dba_tab_columns.column_id
     AND dba_objects.owner = 'ALDO'
     AND object_name = 'T'
ORDER BY intcol#


  
  OWNER
  TABLE_NAME
  COLUMN_NAME
  OBJ#
  INTCOL#
  EQUALITY_PREDS
  EQUIJOIN_PREDS
  NONEQUIJOIN_PREDS
  RANGE_PREDS
  LIKE_PREDS
  NULL_PREDS
  TIMESTAMP
  
      ALDO
    T
    ID
    62895
    1
    1
    0
    0
    0
    0
    0
    22/05/2006 11.44.52
  


select * from dba_tab_histograms
where owner ='ALDO' and table_name = 'T'
and column_name = 'ID'


  
  OWNER
  TABLE_NAME
  COLUMN_NAME
  ENDPOINT_NUMBER
  ENDPOINT_VALUE
  ENDPOINT_ACTUAL_VALUE
  
      ALDO
    T
    ID
    943
    0
        
      ALDO
    T
    ID
    1927
    1
        
      ALDO
    T
    ID
    2928
    2
        
      ALDO
    T
    ID
    3934
    3
        
      ALDO
    T
    ID
    4942
    4
        


As you can see when the stats are taken a different method is applied based,
in the second case more istograms are taken