Most recently created row

Question

We have an error_log table which will log errors after the execution of a stored proc.
We have a create_time field which stores the time of creation of the new record.
How can i view the most recently created row? Is the below mentioned query(courtesy of OTN) Ok? Or are there any other better options?
select err_msg from (select err_msg from err_log order by create_time desc)
where rownum=1 ;

Answer 1

That seems to be quite expensive
SQL> get t
  1  with t as (select 'test' err_msg, sysdate create_time from dual
  2             union all select 'test2' err_msg, sysdate -1 create_time from dual
  3             union all select 'test3' err_msg, sysdate -2 create_time from dual
  4             union all select 'test4' err_msg, sysdate -3 create_time from dual)
  5  select *
  6    from t, (select max(t.create_time) over(order by create_time desc) maxtime
  7               from t) tab
  8   where t.create_time = tab.maxtime
  9*  and rownum = ceil(abs(sin(42)))
SQL> /
ERR_M CREATE_TI MAXTIME
test  17-JAN-08 17-JAN-08
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=14 Card=1 Bytes=22)
   1    0   TEMP TABLE TRANSFORMATION
   2    1     LOAD AS SELECT
   3    2       UNION-ALL
   4    3         FAST DUAL (Cost=2 Card=1)
   5    3         FAST DUAL (Cost=2 Card=1)
   6    3         FAST DUAL (Cost=2 Card=1)
   7    3         FAST DUAL (Cost=2 Card=1)
   8    1     COUNT (STOPKEY)
   9    8       HASH JOIN (Cost=6 Card=1 Bytes=22)
  10    9         VIEW (Cost=2 Card=4 Bytes=52)
  11   10           TABLE ACCESS (FULL) OF 'SYS_TEMP_0FD9D7BD6_7502F1D5' (TABLE (TEMP)) (Cost=2 Card=4 Bytes=52)
  12    9         VIEW (Cost=3 Card=4 Bytes=36)
  13   12           WINDOW (SORT) (Cost=3 Card=4 Bytes=36)
  14   13             VIEW (Cost=2 Card=4 Bytes=36)
  15   14               TABLE ACCESS (FULL) OF 'SYS_TEMP_0FD9D7BD6_7502F1D5' (TABLE (TEMP)) (Cost=2 Card=4 Bytes=52)
Statistics
          2  recursive calls
          8  db block gets
          8  consistent gets
          1  physical reads
        648  redo size
        683  bytes sent via SQL*Net to client
        660  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL>But if you code it a little bit trickier, it suddenly costs "nothing"
SQL> get t
  1  with t as (select 'test' err_msg, sysdate create_time from dual
  2             union all select 'test2' err_msg, sysdate -1 create_time from dual
  3             union all select 'test3' err_msg, sysdate -2 create_time from dual
  4             union all select 'test4' err_msg, sysdate -3 create_time from dual)
  5  select *
  6    from  (select err_msg, create_time, row_number() over(order by create_time desc) rn
  7               from t) tab
  8*  where rn = ceil(abs(sin(42)))
SQL> /
ERR_M CREATE_TI         RN
test  17-JAN-08          1
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=9 Card=4 Bytes=104)
   1    0   VIEW (Cost=9 Card=4 Bytes=104)
   2    1     WINDOW (SORT PUSHED RANK) (Cost=9 Card=4 Bytes=52)
   3    2       VIEW (Cost=8 Card=4 Bytes=52)
   4    3         UNION-ALL
   5    4           FAST DUAL (Cost=2 Card=1)
   6    4           FAST DUAL (Cost=2 Card=1)
   7    4           FAST DUAL (Cost=2 Card=1)
   8    4           FAST DUAL (Cost=2 Card=1)
Statistics
          1  recursive calls
          0  db block gets
          0  consistent gets
          0  physical reads
          0  redo size
        673  bytes sent via SQL*Net to client
        660  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL>