About getting a wrong plan due to incorrect stats

Question

Hi,
can someone give an example which demonstrate how wrong stats on a table lead to a different/inefficient execution plan? I tried one but in what I tried, optimizer picked same plan when stats were incorrect. What I tried was to have a table with two columns and an index on one column and select based on this column as below:
SQL> desc tab1
Name Null? Type
COL1 NOT NULL NUMBER
COL2 NUMBER
(there is an index i1 on table col1)
SQL> select num_rows from user_tables where table_name=
2 'TAB1';
NUM_ROWS
0
SQL> select count(*) from tab1;
COUNT(*)
10000
SQL> set autot on
SQL> select * from tab1 where col1=58398;
no rows selected
Execution Plan
Plan hash value: 4092115641
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time
|
| 0 | SELECT STATEMENT | | 1 | 26 | 1 (0)| 00:00:
01 |
| 1 | TABLE ACCESS BY INDEX ROWID| TAB1 | 1 | 26 | 1 (0)| 00:00: <---oracle uses index when stats are not updated
01 |
|* 2 | INDEX RANGE SCAN | I1 | 1 | | 1 (0)| 00:00:
01 |
Predicate Information (identified by operation id):
2 - access("COL1"=58398)
Statistics
0 recursive calls
0 db block gets
2 consistent gets
0 physical reads
0 redo size
289 bytes sent via SQL*Net to client
327 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
--now update stats
SQL> exec dbms_stats.gather_table_stats(null,tabname=>'TAB1',cascade=>true);
PL/SQL procedure successfully completed.
SQL> select * from tab1 where col1=58398;
no rows selected
Execution Plan
Plan hash value: 4092115641
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time
|
| 0 | SELECT STATEMENT | | 1 | 7 | 2 (0)| 00:00:
01 |
| 1 | TABLE ACCESS BY INDEX ROWID| TAB1 | 1 | 7 | 2 (0)| 00:00: <--same plan of acess by index row id)
01 |
|* 2 | INDEX RANGE SCAN | I1 | 1 | | 1 (0)| 00:00:
01 |
Predicate Information (identified by operation id):
2 - access("COL1"=58398)
Statistics
0 recursive calls
0 db block gets
2 consistent gets
0 physical reads
0 redo size
289 bytes sent via SQL*Net to client
327 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
SQL>
In the example I tried, oracle picks the same plan whether stats are updated or not - I am trying to get one example where oracle picks wrong plan (due to incorrect stats).
thanks

Answer 1

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