April 24, 2012

rowid paging benefit from 11g new feature

Consider below request:
We need to get the 900 to 1000 records after ordered by time.

We call this kind of request as "paging".  Because in most case we encounter this kind of request in web page's redirect buttons:  page 1, page 2, page 3..... 
We can jump to any page via clicking the conrrespoding buttion.

There are two kinds of paging method in oracle: Rownum paging and Rowid paging.
Almost all the documents which i read all announce that rowid is a better method then rownum for paging.
They said:
For the first a few pages, two methods have similiar performance.But as long as the page number become bigger and bigger, rowid paging perform better and better then rownum paging.



Pay attention to the red part. Is that really true?  Let's make a experiment.

First test in 11g. Create test table as below:
11gR2 >create table test(id number,status VARCHAR2(7),type VARCHAR2(19),created date);
Table created.

11gR2 >insert into test select OBJECT_ID,STATUS,OBJECT_TYPE,CREATED from dba_objects;
12926 rows created.

11gR2 >alter table test modify created not null;
Table altered.

11gR2 >create index test_ind1 on test(CREATED);
Index created.

11gR2 >ANALYZE TABLE TEST compute statistics;
Table analyzed.


We are going to read row 900-1000 from table test order by created.

Test 11g paging with rownum method:
11gR2 >select *
       from (
             select rownum rn,t.*
             from
             (select  id,status,type,created from test order by created) t
             where rownum<1000)
       where rn >900; 
99 rows selected.
--------------------------------------------------------------------------------------------
| Id  | Operation                      | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT               |           |   999 | 50949 |     9   (0)| 00:00:01 |
|*  1 |  VIEW                          |           |   999 | 50949 |     9   (0)| 00:00:01 |
|*  2 |   COUNT STOPKEY                |           |       |       |            |          |
|   3 |    VIEW                        |           |   999 | 37962 |     9   (0)| 00:00:01 |
|   4 |     TABLE ACCESS BY INDEX ROWID| TEST      | 12926 |   277K|     9   (0)| 00:00:01 |
|   5 |      INDEX FULL SCAN           | TEST_IND1 |   999 |       |     4   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
         22  consistent gets
          0  physical reads
          0  redo size
       3703  bytes sent via SQL*Net to client
        590  bytes received via SQL*Net from client
          8  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
         99  rows processed


Test 11g paging with rowid method:
11gR2 >select /*+ ordered use_nl(p s) */ *
      from (
            select rownum rn,rd
            from (select  rowid rd from test order by created)
            t where rownum<1000) p,
            test s
      where rn>900 and p.rd=s.rowid;  2    3    4    5    6    7
99 rows selected.
-----------------------------------------------------------------------------------------
| Id  | Operation                   | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |           |   999 | 52947 |  1003   (0)| 00:00:13 |
|   1 |  NESTED LOOPS               |           |   999 | 52947 |  1003   (0)| 00:00:13 |
|*  2 |   VIEW                      |           |   999 | 24975 |     4   (0)| 00:00:01 |
|*  3 |    COUNT STOPKEY            |           |       |       |            |          |
|   4 |     VIEW                    |           |   999 | 11988 |     4   (0)| 00:00:01 |
|   5 |      INDEX FULL SCAN        | TEST_IND1 | 12926 |   239K|     4   (0)| 00:00:01 |
|   6 |   TABLE ACCESS BY USER ROWID| TEST      |     1 |    28 |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
         19  consistent gets
          0  physical reads
          0  redo size
       5450  bytes sent via SQL*Net to client
        590  bytes received via SQL*Net from client
          8  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
         99  rows processed


From the red part we can see rowid paging(19 logic read)  do  better then  rownum paging(22 logic read).
The reason is rowid method reduce the "TABLE ACCESS BY ROWID" for first 900 rows.


Let's check in 10g:
Build the Test table as excatly the same with 11g, created the index and gathered stats.

Test 11g paging with rownum method:
10gR2 >select *
       from (
             select rownum rn,t.*
             from
             (select  id,status,type,created from test order by created) t
             where rownum<1000)
       where rn >900; 
99 rows selected.
--------------------------------------------------------------------------------------------
| Id  | Operation                      | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT               |           |   999 | 50949 |    17   (0)| 00:00:01 |
|*  1 |  VIEW                          |           |   999 | 50949 |    17   (0)| 00:00:01 |
|*  2 |   COUNT STOPKEY                |           |       |       |            |          |
|   3 |    VIEW                        |           |   999 | 37962 |    17   (0)| 00:00:01 |
|   4 |     TABLE ACCESS BY INDEX ROWID| TEST      | 45620 |  1113K|    17   (0)| 00:00:01 |
|   5 |      INDEX FULL SCAN           | TEST_IND1 |   999 |       |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
         19  consistent gets
          0  physical reads
          0  redo size
       3842  bytes sent via SQL*Net to client
        558  bytes received via SQL*Net from client
          8  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
         99  rows processed


19 logic read, very close to 11g. It is a normal value and close to our expected.

Test 11g paging with rowid method:
10gR2 >select /*+ ordered use_nl(p s) */ *
      from (
            select rownum rn,rd
            from (select  rowid rd from test order by created)
            t where rownum<200) p,
            test s
      where rn>100 and p.rd=s.rowid;
99 rows selected.
-----------------------------------------------------------------------------------------
| Id  | Operation                   | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |           |   199 | 11144 |   261   (1)| 00:00:04 |
|   1 |  NESTED LOOPS               |           |   199 | 11144 |   261   (1)| 00:00:04 |
|*  2 |   VIEW                      |           |   199 |  4975 |    62   (2)| 00:00:01 |
|*  3 |    COUNT STOPKEY            |           |       |       |            |          |
|   4 |     VIEW                    |           | 45620 |   534K|    62   (2)| 00:00:01 |
|   5 |      INDEX FULL SCAN        | TEST_IND1 | 45620 |   846K|    62   (2)| 00:00:01 |
|   6 |   TABLE ACCESS BY USER ROWID| TEST      |     1 |    31 |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
        108  consistent gets
          0  physical reads
          0  redo size
       5252  bytes sent via SQL*Net to client
        558  bytes received via SQL*Net from client
          8  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
         99  rows processed


wow, why logic turn to 108? The execution plan is same with 11g, but why so big difference in logic read?
      rownum    rowid
11g:      22       19
10g:      19      108


Why?
Below is the Reason:
11gR2 >select rowid,id from test where rowid in ('AAADSHAABAAAH3hAAA','AAADSHAABAAAH3hAAB','AAADSHAABAAAH3hAAC','AAADSHAABAAAH3hAAD','AAADSHAABAAAH3hAAE','AAADSHAABAAAH3hAAF','AAADSHAABAAAH3hAAG','AAADSHAABAAAH3hAAH','AAADSHAABAAAH3hAAI','AAADSHAABAAAH3hAAJ');
ROWID                      ID
------------------ ----------
AAADSHAABAAAH3hAAA         20
AAADSHAABAAAH3hAAB         46
AAADSHAABAAAH3hAAC         28
AAADSHAABAAAH3hAAD         15
AAADSHAABAAAH3hAAE         29
AAADSHAABAAAH3hAAF          3
AAADSHAABAAAH3hAAG         25
AAADSHAABAAAH3hAAH         41
AAADSHAABAAAH3hAAI         54
AAADSHAABAAAH3hAAJ         40
10 rows selected.
Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          2  consistent gets
          0  physical reads
          0  redo size
        875  bytes sent via SQL*Net to client
        524  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
         10  rows processed


10gR2 >select rowid,id from test where rowid in ('AAAzo5AABAAAKnWAAA','AAAzo5AABAAAKnWAAB','AAAzo5AABAAAKnWAAC','AAAzo5AABAAAKnWAAD','AAAzo5AABAAAKnWAAE','AAAzo5AABAAAKnWAAF','AAAzo5AABAAAKnWAAG','AAAzo5AABAAAKnWAAH','AAAzo5AABAAAKnWAAI','AAAzo5AABAAAKnWAAJ');
ROWID                      ID
------------------ ----------
AAAzo5AABAAAKnWAAA         30
AAAzo5AABAAAKnWAAB          8
AAAzo5AABAAAKnWAAC         14
AAAzo5AABAAAKnWAAD         34
AAAzo5AABAAAKnWAAE         45
AAAzo5AABAAAKnWAAF         39
AAAzo5AABAAAKnWAAG         47
AAAzo5AABAAAKnWAAH         51
AAAzo5AABAAAKnWAAI         11
AAAzo5AABAAAKnWAAJ         48
10 rows selected.

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


A Rowid is made by: OOOOOOFFFBBBBBBRRR
O is object id,F is file id,B is block id,R is row number.

From above we can see, read 10 rows from same block in 11g cost 2 block read while in 10g cost 10 block read.

Further research also confirmed in 10g, every rowid will gernerate an logic read, even all the rowid are come from same block.
But in 11g some new machanism are introduced, if next rowid is in the same block with current  row, then current block can be reuse.

2 Comments:

jeff1978 said...

Thanks a lot Kevin....this one really helps.

Is there a way to speed up the count query (to know the total records) for such pagination query ?

Path Infotech said...

Path Infotech is in the field of oracle training program from past several years.

For more info : Oracle Certification Courses

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