Why is my query using the wrong index?

Question

Hi SQL Tuning experts....
I'm running Oracle 11.2.0.2 EE on Solaris 10
I've just checked my top SQL statements in OEM and drilling down on the top query of the list, I decided to run SQL Advisor on the query.
Low and behold, it came back with a profile that gives me a 99.9% improvement.
When I compared the original plan to the new plan the only difference is that the SQL Advisor is using a different index.
Now, the dumb DBA question: why? why is Oracle picking the wrong index?
original plan
SELECT STATEMENT
  COUNT STOPKEY
    VIEW
      SORT ORDER BY STOPKEY          
        TABLE ACCESS BY INDEX ROWID     ARADMIN.T2179          TABLE
          INDEX RANGE SCAN          ARADMIN.T2179_C3     INDEXnew plan...
SELECT STATEMENT
  COUNT STOPKEY
    VIEW
      SORT ORDER BY STOPKEY          
        TABLE ACCESS BY INDEX ROWID     ARADMIN.T2179               TABLE
          INDEX RANGE SCAN          ARADMIN.T2179_C536870923     INDEXThe stats:
table has approx. 3.3Mil rows
table segment is approx. 15G (5G of it is reclaimable space, but probably not related)
table has fresh stats and indexes have been recently rebuilt
indexes in this discussion are both b-tree
the C3 column is number(15) with 2836897 distinct rows
the C536870923 columns is VARCHAR2(255) with 1080533 distinct values
Here is the actual query:
     SELECT *
FROM (
SELECT T2179.C1,C536871040,C536871037,C536870944,C3,C536870918,C536870919,
C536870924,C536871048,C536871049,C536871050,C536870926,C536870925,
C536870916,C4,C536871275,C536871095,C536870913
FROM T2179
WHERE ((T2179.C536870923 = :"SYS_B_0") AND (T2179.C3 > :"SYS_B_1")) ORDER BY C1 DESC )
WHERE ROWNUM <= :"SYS_B_2"Obviously this is a good example of why we should use profiles. :-)
Edited by: ji li on Feb 14, 2013 1:32 PM
Sorry... I should have included the plan output
I'm guessing this has something to do with the cardinality of the data in the columns.

Answer 1

Dom.... I've already posted most everything. Here are the relevant init params and misc stuff you are asking for.
NAME                                 TYPE                             VALUE
object_cache_optimal_size            integer                          102400
optimizer_capture_sql_plan_baselines boolean                          FALSE
optimizer_dynamic_sampling           integer                          2
optimizer_features_enable            string                           11.2.0.2
optimizer_index_caching              integer                          0
optimizer_index_cost_adj             integer                          100
optimizer_mode                       string                           ALL_ROWS
optimizer_secure_view_merging        boolean                          TRUE
optimizer_use_invisible_indexes      boolean                          FALSE
optimizer_use_pending_statistics     boolean                          FALSE
optimizer_use_sql_plan_baselines     boolean                          TRUE
plsql_optimize_level                 integer                          2
db_file_multiblock_read_count        integer                          128
db_block_size                        integer                          8192
cursor_sharing                       string                           FORCE
SNAME                          PNAME                                    PVAL1 PVAL2
SYSSTATS_INFO                  STATUS                                         COMPLETED
SYSSTATS_INFO                  DSTART                                         12-04-2012 12:28
SYSSTATS_INFO                  DSTOP                                          12-04-2012 15:28
SYSSTATS_INFO                  FLAGS                                        0
SYSSTATS_MAIN                  CPUSPEEDNW                     2992.2746781116
SYSSTATS_MAIN                  IOSEEKTIM                                   10
SYSSTATS_MAIN                  IOTFRSPEED                                4096
SYSSTATS_MAIN                  SREADTIM                             48027.271
SYSSTATS_MAIN                  MREADTIM                             37217.102
SYSSTATS_MAIN                  CPUSPEED                                  2978
SYSSTATS_MAIN                  MBRC                                         0
SYSSTATS_MAIN                  MAXTHR                               400782336
SYSSTATS_MAIN                  SLAVETHR                               1403904
13 rows selected.
QL> l
  1  SELECT * FROM (
  2    SELECT
  3    T2179.C1,
  4    C536871040,
  5    C536871037,
  6    C536870944,
  7    C3,
  8    C536870918,
  9    C536870919,
10    C536870924,
11    C536871048,
12    C536871049,
13    C536871050,
14    C536870926,
15    C536870925,
16    C536870916,
17    C4,
18    C536871275,
19    C536871095,
20    C536870913
21    FROM aradmin.T2179
22    WHERE (
23     (C536870923 = '300885690')
24    AND
25     (C3 > '1360938353')
26          )
27    ORDER BY C1 DESC )
28* WHERE ROWNUM <= 1
no rows selected
Elapsed: 00:00:00.01
Execution Plan
Plan hash value: 2768254526
| Id  | Operation                      | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT               |          |     1 |  2315 |     6  (17)| 00:00:01 |
|*  1 |  COUNT STOPKEY                 |          |       |       |            |          |
|   2 |   VIEW                         |          |     1 |  2315 |     6  (17)| 00:00:01 |
|*  3 |    SORT ORDER BY STOPKEY       |          |     1 |   226 |     6  (17)| 00:00:01 |
|*  4 |     TABLE ACCESS BY INDEX ROWID| T2179    |     1 |   226 |     5   (0)| 00:00:01 |
|*  5 |      INDEX RANGE SCAN          | T2179_C3 |     1 |       |     3   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - filter(ROWNUM<=1)
   3 - filter(ROWNUM<=1)
   4 - filter("C536870923"='300885690')
   5 - access(SYS_OP_DESCEND("C3")<HEXTORAW('3AF1C2A1ABC9FF') )
       filter(SYS_OP_UNDESCEND(SYS_OP_DESCEND("C3"))>1360938353)
Statistics
          0  recursive calls
          0  db block gets
       2615  consistent gets
          0  physical reads
          0  redo size
       1538  bytes sent via SQL*Net to client
        513  bytes received via SQL*Net from client
          1  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
          0  rows processed