Full Table Scans for small tables... in Oracle10g v.2

Question

Hi,
The Query optimizer may select to do a full table scan instead of an index scan when the table is small (it consists of blocks the number of which is less than the the value of db_file_multiblock_read_count).
So , i tried to see it using the dept table......
SQL> select blocks , extents , bytes from USER_segments where segment_name='DEPT';
    BLOCKS    EXTENTS      BYTES
         8          1      65536
SQL> SHOW PARAMETER DB_FILE_MULTIBLOCK_READ_COUNT;
NAME                                 TYPE        VALUE
db_file_multiblock_read_count        integer     16
SQL> explain plan for SELECT * FROM DEPT
  2    WHERE DEPTNO=10
  3  /
Explained
SQL>
SQL>  select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
Plan hash value: 2852011669
| Id  | Operation                   | Name    | Rows  | Bytes | Cost (%CPU)| Tim
|   0 | SELECT STATEMENT            |         |     1 |    23 |     1   (0)| 00:
|   1 |  TABLE ACCESS BY INDEX ROWID| DEPT    |     1 |    23 |     1   (0)| 00:
|*  2 |   INDEX UNIQUE SCAN         | PK_DEPT |     1 |       |     0   (0)| 00:
Predicate Information (identified by operation id):
   2 - access("DEPTNO"=10)
14 rows selectedSo , according to the above remarks
What may be the reason of not performing full table scan...????
Thanks...
Sim

Answer 1

No i have not generalized.... In the Oracle's extract
i have posted there is the word "might".....
I just want to find an example in which selecting
from a small table... query optimizer does perform a
full scan instead of a type of index scan...Sorry for that... I don't mean to be rude :)
See following...
create table index_test(id int, name varchar2(10));
create index index_test_idx on index_test(id);
insert into index_test values(1, 'name');
commit;
-- No statistics
select * from index_test where id = 1;
SELECT STATEMENT ALL_ROWS-Cost : 3
  TABLE ACCESS FULL MAXGAUGE.INDEX_TEST(1) ("ID"=1)
-- If first rows mode?
alter session set optimizer_mode = first_rows;
select * from index_test where id = 1;
SELECT STATEMENT FIRST_ROWS-Cost : 802
  TABLE ACCESS BY INDEX ROWID MAXGAUGE.INDEX_TEST(1)
   INDEX RANGE SCAN MAXGAUGE.INDEX_TEST_IDX (ID) ("ID"=1)
-- If statistics is gathered
exec dbms_stats.gather_table_stats(user, 'INDEX_TEST', cascade=>true);
alter session set optimizer_mode = first_rows;
select * from index_test where id = 1;
SELECT STATEMENT ALL_ROWS-Cost : 2
  TABLE ACCESS BY INDEX ROWID MAXGAUGE.INDEX_TEST(1)
   INDEX RANGE SCAN MAXGAUGE.INDEX_TEST_IDX (ID) ("ID"=1)
alter session set optimizer_mode = all_rows;
select * from index_test where id = 1;
SELECT STATEMENT ALL_ROWS-Cost : 2
  TABLE ACCESS BY INDEX ROWID MAXGAUGE.INDEX_TEST(1)
   INDEX RANGE SCAN MAXGAUGE.INDEX_TEST_IDX (ID) ("ID"=1) See some dramatic changes by the difference of parameters and statistics?
Jonathan Lewis has written a great book on cost mechanism of Oracle optimizer.
It will tell almost everything about your questions...
http://www.amazon.com/Cost-Based-Oracle-Fundamentals-Jonathan-Lewis/dp/1590596366/ref=sr_1_1?ie=UTF8&s=books&qid=1195209336&sr=1-1