Anomaly with rownum (COUNT (STOPKEY)) and FTS testcase . Please explain.

Question

Hi,
I've done simply test on 9.2.0.8 EE AIX DB which results I cant explain .
create table t (id number , filler varchar2(1024));
insert into t select rownum r , rpad('*', 1024 ,'*') from dual connect by level <= 1000000;
exec dbms_stats.gather_table_stats(user, 'T');
and here starts strange part for me:
I did noprint for column filler (think its irrelevant)
SQL> select /*+FULL(t) */ * from t where id < 10 and rownum < 11;
        ID
         1
         2
         3
         4
         5
         6
         7
         8
         9
9 rows selected.
Execution Plan
   0      SELECT STATEMENT Optimizer=RULE (Cost=8708 Card=9 Bytes=9261
   1    0   COUNT (STOPKEY)
   2    1     TABLE ACCESS (FULL) OF 'T' (Cost=8708 Card=9 Bytes=9261)
Statistics
          0  recursive calls
          0  db block gets
     142977  consistent gets
     113244  physical reads
          0  redo size
       1339  bytes sent via SQL*Net to client
        237  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          9  rows processed
SQL> select /*+FULL(t) */ * from t where id < 10 and rownum < 10;
        ID
         1
         2
         3
         4
         5
         6
         7
         8
         9
9 rows selected.
Execution Plan
   0      SELECT STATEMENT Optimizer=RULE (Cost=8708 Card=9 Bytes=9261
   1    0   COUNT (STOPKEY)
   2    1     TABLE ACCESS (FULL) OF 'T' (Cost=8708 Card=9 Bytes=9261)
Statistics
          0  recursive calls
          0  db block gets
          6  consistent gets
          0  physical reads
          0  redo size
       1339  bytes sent via SQL*Net to client
        237  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          9  rows processed
here with changed optimizer_mode
SQL> alter session set optimizer_mode=choose;
Session altered.
SQL> select /*+FULL(t) */ * from t where id < 10 and rownum < 10;
        ID
         1
         2
         3
         4
         5
         6
         7
         8
         9
9 rows selected.
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=8708 Card=9 Bytes=92
          61)
   1    0   COUNT (STOPKEY)
   2    1     TABLE ACCESS (FULL) OF 'T' (Cost=8708 Card=9 Bytes=9261)
Statistics
          0  recursive calls
          0  db block gets
          6  consistent gets
          0  physical reads
          0  redo size
       1339  bytes sent via SQL*Net to client
        237  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          9  rows processed
SQL> select /*+FULL(t) */ * from t where id < 10 and rownum < 11;
        ID
         1
         2
         3
         4
         5
         6
         7
         8
         9
9 rows selected.
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=8708 Card=9 Bytes=92
          61)
   1    0   COUNT (STOPKEY)
   2    1     TABLE ACCESS (FULL) OF 'T' (Cost=8708 Card=9 Bytes=9261)
Statistics
          0  recursive calls
          0  db block gets
     142977  consistent gets
     113247  physical reads
          0  redo size
       1339  bytes sent via SQL*Net to client
        237  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          9  rows processedPlease check consistent gets value . Same number of rows and plan but resource utilization much differs.
Regards.
Greg
Edited by: user10388717 on Feb 5, 2010 5:44 AM

Answer 1

Hi,
I do not see any anomaly or problem.
You have two different query
select /*+FULL(t) */ * from t where id < 10 and rownum < 10;
select /*+FULL(t) */ * from t where id < 10 and rownum < 11;According to the inserted value, all the numbers are unique and are from 1 to 1000000.
Now when you say id<10, it would get only 9 records in your case and Oracle has stored the id < 10 at the same place.
So when you do id<10 and rownum<10, it get 9 values in 6 consistent gets and since both the condition is fullfilled it stops the scanning and return you the value. It does not scan to the HMW of the table.
But when you do id < 10 and rownum < 11, it would get 9 values in 6 consistent gets but still the query is not completed and hence it scan to hmw of the table and hence you are getting high consistent gets.
Regards
Anurag