Bitmap plans can estimate wrong cardinality

There is a problem query involving 3-table join in one of our prodcutin databases. The difference between the good plan and bad plan is the join order in a hash join execution plan. The reason for Oracle to pick up the bad join order is that Oracle CBO chooses the bitmap plan and this plan gives very wrong cardinality. What I did is to set "_b_tree_bitmap_plans"=false at system level, which allowing the CBO to generate the good plan. Then I created a sql plan baseline for that query from the good plan ( using sqlt/utl/coe_load_sql_baseline.sql script). After that, considering that there may be some queries that benfit from the bitmap access path, I changed back "_b_tree_bitmap_plans" to be true.

Below tests demonstrated how wrong the cardinaltiy estimeated by the bitmap plan could be.

• in the bitmap plan,  the number of rows estimated is 578K

SQL> select * from BOD_OTHR_SERIAL_ACTIVITY_I8400 I8400
  2  where
  3  (I8400.DVTQ_ACCT_NUM = 'A145779917' OR (
  4      I8400.UJTJPM_CUSTOMER_ID = '' AND I8400.UJTJPM_ACCOUNT_ID = ''
  5  ))
  6  ;

Execution Plan
----------------------------------------------------------
Plan hash value: 972526864

-------------------------------------------------------------------------------------------------------------------
| Id  | Operation                        | Name                           | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                 |                                |   578K|   170M|   161K (11)| 00:05:17 |
|   1 |  TABLE ACCESS BY INDEX ROWID     | BOD_OTHR_SERIAL_ACTIVITY_I8400 |   578K|   170M|   161K (11)| 00:05:17 |
|   2 |   BITMAP CONVERSION TO ROWIDS    |                                |       |       |            |          |
|   3 |    BITMAP OR                     |                                |       |       |            |          |
|   4 |     BITMAP CONVERSION FROM ROWIDS|                                |       |       |            |          |
|*  5 |      INDEX RANGE SCAN            | IX_I8400_04                    |       |       |     1   (0)| 00:00:01 |
|   6 |     BITMAP CONVERSION FROM ROWIDS|                                |       |       |            |          |
|*  7 |      INDEX RANGE SCAN            | IX_I8400_07                    |       |       |     3   (0)| 00:00:01 |
-------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   5 - access("I8400"."UJTJPM_CUSTOMER_ID"='' AND "I8400"."UJTJPM_ACCOUNT_ID"='')
   7 - access("I8400"."DVTQ_ACCT_NUM"='A145779917')

• -- disable the bitmap plan

SQL> alter session set "_b_tree_bitmap_plans"=false;

Session altered.

• -- Now the estimated rows is 2

SQL> select * from BOD_OTHR_SERIAL_ACTIVITY_I8400 I8400
  2  where
  3  (I8400.DVTQ_ACCT_NUM = 'A145779917' OR (
  4      I8400.UJTJPM_CUSTOMER_ID = '' AND I8400.UJTJPM_ACCOUNT_ID = ''
  5  ))
  6  ;

Execution Plan
----------------------------------------------------------
Plan hash value: 7978316

---------------------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name                           | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |                                |     2 |   618 |     9   (0)| 00:00:01 |
|   1 |  CONCATENATION               |                                |       |       |            |       |
|   2 |   TABLE ACCESS BY INDEX ROWID| BOD_OTHR_SERIAL_ACTIVITY_I8400 |     1 |   309 |     6   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | IX_I8400_07                    |     1 |       |     4  (25)| 00:00:01 |
|*  4 |   TABLE ACCESS BY INDEX ROWID| BOD_OTHR_SERIAL_ACTIVITY_I8400 |     1 |   309 |     3   (0)| 00:00:01 |
|*  5 |    INDEX RANGE SCAN          | IX_I8400_04                    |     1 |       |     2  (50)| 00:00:01 |
---------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - access("I8400"."DVTQ_ACCT_NUM"='A145779917')
   4 - filter(LNNVL("I8400"."DVTQ_ACCT_NUM"='A145779917'))
   5 - access("I8400"."UJTJPM_CUSTOMER_ID"='' AND "I8400"."UJTJPM_ACCOUNT_ID"='')

•  the acutall number of row is 1:

SQL> select count(*)  from BOD_OTHR_SERIAL_ACTIVITY_I8400 I8400
  2      where
  3      (I8400.CUST_ACCT_NUM = 'A145779917' OR (
  4          I8400.UJTJPM_CUSTOMER_ID = '' AND I8400.UJTJPM_ACCOUNT_ID = ''
  5      ))
  6      ;

  COUNT(*)
----------
         1

1 row selected.