HowTO: Get only one entry which has the highest value in a specific column?

Question

Hi there,
ich have the following problem. I want to write logs in a table. One column is the date of the log and some others hold the information.
Now I want to get the last log and I it should be good performance too.
What would you suggest?
At the moment I think about the max() function. Is it a solution?
Would a view which orders the logs by date (DESC) improve performance? maybe the view could create a collum which numbers the logs beginning by the newest.
something like: ...ROW_NUMBER() OVER(ORDER BY logdate) AS indx ...
Thx in advance
Dominik

Answer 1

In my opinion SELECT * FROM MY_LOG WHERE LOG_DATE =
MAX(LOG_DATE) could be expensive (performance)In the first place, it issues an error saying GROUP functions are not allowed here.
MAX() solution would be better.
SQL> explain plan for select *
  2                   from (select emp.* , row_number() over (order by hiredate desc) rn
  3                         from emp)
  4                   where rn = 1;
Explained.
SQL>
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
| Id  | Operation                |  Name       | Rows  | Bytes | Cost  |
|   0 | SELECT STATEMENT         |             |    16 |  1760 |     4 |
|*  1 |  VIEW                    |             |    16 |  1760 |     4 |
|*  2 |   WINDOW SORT PUSHED RANK|             |    16 |   704 |     4 |
|   3 |    TABLE ACCESS FULL     | EMP         |    16 |   704 |     2 |
Predicate Information (identified by operation id):
   1 - filter("from$_subquery$_001"."RN"=1)
   2 - filter(ROW_NUMBER() OVER ( ORDER BY "EMP"."HIREDATE" DESC )<=1)
Note: cpu costing is off
17 rows selected.
SQL>
SQL> explain plan for select emp.* from emp
  2                   where hiredate = (select max(hiredate)
  3                                     from emp);
Explained.
SQL>
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
| Id  | Operation            |  Name       | Rows  | Bytes | Cost  |
|   0 | SELECT STATEMENT     |             |     1 |    44 |     2 |
|*  1 |  TABLE ACCESS FULL   | EMP         |     1 |    44 |     2 |
|   2 |   SORT AGGREGATE     |             |     1 |     9 |       |
|   3 |    TABLE ACCESS FULL | EMP         |    16 |   144 |     2 |
Predicate Information (identified by operation id):
   1 - filter("EMP"."HIREDATE"= (SELECT /*+ */
              MAX("EMP"."HIREDATE") FROM "EMP" "EMP"))
Note: cpu costing is off
17 rows selected.
SQL> Cheers
Sarma.