Question

is it possible to retrieve statistics about the minimal or maximal value of a numeric column in Oracle 10g? I have found the table USER_TAB_COL_STATISTICS having a LOW_VALUE and HIGH_VALUE column, but I am not sure whether those are the values I am looking for.

I need to find an efficient way to ask the DBS for those statistics. Using a regular MIN(a) and MAX(a) query would be too slow on large tables.

Thanks in advance.

Answer 1

Yes, LOW_VALUE and HIGH_VALUE will tell you the minimum and maximum values in the column but:

• they are stored as RAW(32) columns, so the meaning will not be immediately apparent
• they will be as of the last time statistics were gathered for the table, so may not be accurate (unless you explicitly gather stats before using them)

If you index the column then MIN(a) and MAX(a) should be very fast as in this example where T1 has 50000 rows and is indexed on OBJECT_ID:

SQL> select min(object_id) from t1;

MIN(OBJECT_ID)
--------------
           100

------------------------------------------------------------------------------------
| Id  | Operation                  | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT           |       |     1 |     5 |     2   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE            |       |     1 |     5 |            |          |
|   2 |   INDEX FULL SCAN (MIN/MAX)| T1_ID | 53191 |   259K|     2   (0)| 00:00:01 |
------------------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          2  consistent gets
          0  physical reads
          0  redo size
        419  bytes sent via SQL*Net to client
        380  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

The result is the same if you select the MAX instead of the MIN. However, if you select the MIN and MAX in a single select statement the result is different:

SQL> select min(object_id), max(object_id) from t1;

MIN(OBJECT_ID) MAX(OBJECT_ID)
-------------- --------------
           100          72809


-------------------------------------------------------------------------------
| Id  | Operation             | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |       |     1 |     5 |    34   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE       |       |     1 |     5 |            |          |
|   2 |   INDEX FAST FULL SCAN| T1_ID | 53191 |   259K|    34   (0)| 00:00:01 |
-------------------------------------------------------------------------------


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
        125  consistent gets
          0  physical reads
          0  redo size
        486  bytes sent via SQL*Net to client
        380  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

This suggests that it may be better to get them separately, though I haven’t conclusively proved that.