Oracle aggregate functions slows query down massively

I have a table with ~12M rows of data in it. Here is the table structure:

SYSTEM_ID
BATCH_ID
MEASUREMENT_INDEX,
PARAMETER_ONE,
PARAMETER_TWO

The primary key is made up of the first three columns. I also have an index on SYSTEM_ID ASC, BATCH_ID ASC.

If I do the following query:

SELECT PARAMETER_ONE 
  FROM RESULTS_TABLE 
 WHERE SYSTEM_ID=1 
   AND BATCH_ID=100;

The results come back in about 0.002 seconds.

However, when I try to do ANY aggregate function, e.g. SUM, MAX, AVG, STDDEV, the query time raises to around 400 seconds! I have tried various permutations of this query e.g.:

SELECT AVG(PARAMETER_ONE) 
  FROM ( SELECT PARAMETER_ONE 
           FROM RESULTS_TABLE 
          WHERE SYSTEM_ID=1 
            AND BATCH_ID=100
        );

And:

SELECT DISTINCT
       AVG(PARAMETER_ONE) over (partition by system_id, batch_id)
 FROM RESULTS_TABLE
WHERE SYSTEM_ID = 123
  AND BATCH_ID = 10;

Neither of these seems to make a difference. Has anyone else had this problem??

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MORE DETAILS

The explain plan for the aggregate function query is:

Plan hash value: 2759933517

-------------------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name                         | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |                              |     1 |    32 |     2   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE              |                              |     1 |    32 |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID| RESULTS_DATA                 |     1 |    32 |     2   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | RESULTS_DATA_INDEX1          |     1 |       |     1   (0)| 00:00:01 |

Predicate Information (identified by operation id):

3 - access("

Thanks for all your input - I seem to have solved it. I recalculated table statistics using:

SQL> EXEC DBMS_STATS.GATHER_TABLE_STATS(OWNNAME=>USER,TABNAME=>'RESULTS
_DATA');

After this, the queries worked fine.

StackExchange Database Administrators Q#30090, answer score: 5