Gather_schema_stats and gather_table_stats, the result different?

Question

Hi All
Oracle : 10.2.0.4
OS : AIX
Environment : Test but restore from production database
My scenario has 4 as below and set method_opt = 'FOR ALL COLUMNS SIZE REPEAT' and degree = 2 and I deleted stats before run each step and base on same data and no work on test database.
1. gather_schema_stats : estimate_percent = 15 and options = 'GATHER AUTO'
2. gather_schema_stats : estimate_percent = AUTO_SAMPLE_SIZE and options = 'GATHER AUTO'
3. gather_table_stats : estimate_percent = 15
4. gather_table_stats : estimate_percent = AUTO_SAMPLE_SIZE
The result seem like as below on table A
scenario 1 : num_rows = 181928599 and sample_size = 569047 and gather stats time = 27 minutes
scenario 2 : num_rows = 185865585 and sample_size = 185865585 and gather stats time = 30 minutes
scenario 3 : num_rows = 185869720 and sample_size = 27880458 and gather stats time = 13 minutes
scenario 4 : num_rows = 185839451 and sample_size = 660578 and gather stats time = 17 minutes
The question is Why sampling size between gather_schema_stats and gather_table_stats on same estimate_percent and data that different? Did Oracle not gather statistics as estimate_percent parameter? and sometime explain plan had been changed. Weekly task is gather_schema_stats with estimate_percent = 15 on production.
Notes: I tried to search in forum but I cannot found that match as my question.
Thanks,
Hiko

Answer 1

An estimate_percent lower than 100 produces an estimation, nothing more, nothing less. An estimation by definition is not always 100% accurate, it can be more, it can be less than the actual number of rows in the tables.
It can even varry between two execution of gather stats within the same extimate_percent value:
15:39:11 SQL> create table mytable as
16:48:22   2  select rownum rn
16:48:33   3  from dual
16:48:35   4  connect by rownum<1000000;
Table created.
Elapsed: 00:00:05.09
16:48:52 SQL> exec dbms_stats.gather_table_stats(user,'MYTABLE',estimate_percent=>1)
PL/SQL procedure successfully completed.
Elapsed: 00:00:00.40
16:49:24 SQL> select num_rows from user_tables where table_name='MYTABLE';
  NUM_ROWS
   1002000
Elapsed: 00:00:00.09
16:49:47 SQL> exec dbms_stats.gather_table_stats(user,'MYTABLE',estimate_percent=>1)
PL/SQL procedure successfully completed.
Elapsed: 00:00:00.04
16:50:03 SQL> select num_rows from user_tables where table_name='MYTABLE';
  NUM_ROWS
   1014200
Elapsed: 00:00:00.06
16:50:09 SQL> exec dbms_stats.gather_table_stats(user,'MYTABLE',estimate_percent=>100)
PL/SQL procedure successfully completed.
Elapsed: 00:00:01.71
16:51:52 SQL> select num_rows from user_tables where table_name='MYTABLE';
  NUM_ROWS
    999999
Elapsed: 00:00:00.04
16:51:55 SQL> exec dbms_stats.gather_table_stats(user,'MYTABLE',estimate_percent=>100)
PL/SQL procedure successfully completed.
Elapsed: 00:00:01.71
16:52:02 SQL> select num_rows from user_tables where table_name='MYTABLE';
  NUM_ROWS
    999999
Elapsed: 00:00:00.04
16:52:04 SQL> select count(*) from mytable;
  COUNT(*)
    999999
Elapsed: 00:00:00.14
16:52:18 SQL>Nicolas.