oracle数据库rowid和rownum分页算法比较

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[backcolor=rgb(223,223,223)][font=新宋体]一直以来,看到的说法大都是rowid要强于rownum分页:
在索引设计的没问题的前提下,两种方式在前几页性能上接近,越向后翻rowid的效果越明显。

注意红字部分,红字部分一定成立吗?实验说明一切。

我们先来看看[size=+0]http://www.itpub.net/thread-1318149-1-1.html][font=新宋体][backcolor=rgb(223,223,223)]11g[backcolor=rgb(223,223,223)][font=新宋体]的情况,建立实验环境:
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.

测试11g的rownum分页:
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
[backcolor=rgb(223,223,223)][font=新宋体] 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

下面测试rowid分页
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

我们看可以看到,当读900-1000行时,rowid分页(19逻辑读)确实是优于rownum分页(22逻辑读)。因为rowid方式减少了前900行的回表。

我们来看看10g的情况。
在10g建立同样的TEST表和索引,然后:
测试10g的rownum分页:
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个逻辑读,和11g是很接近的,是个正常值,也符合我们的预期。

继续测试10g的rowid分页:
swp1 >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.
[font=新宋体][backcolor=rgb(223,223,223)]-----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 999 | 55944 | 1061 (1)| 00:00:15 |
| 1 | NESTED LOOPS | | 999 | 55944 | 1061 (1)| 00:00:15 |
|* 2 | VIEW | | 999 | 24975 | 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
109 consistent gets
1 physical reads
0 redo size
5585 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

怎么逻辑读变成了109? 执行计划是和11g一样的。为什么会有这么大的差别?
rowid分页的cost一下变成了rownum方式的好几倍。

[font=新宋体][backcolor=rgb(223,223,223)]为什么?

下面的实验揭露了原因,也为我们揭露了11g一个不为人知的新特性:
[size=+0]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.
------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 16 | 1 (0)| 00:00:01 |
| 1 | INLIST ITERATOR | | | | | |
| 2 | TABLE ACCESS BY USER ROWID| TEST | 1 | 16 | 1 (0)| 00:00:01 |
------------------------------------------------------------------------------------
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
[size=+0] 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

[size=+0]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.
------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 16 | 1 (0)| 00:00:01 |
| 1 | INLIST ITERATOR | | | | | |
| 2 | TABLE ACCESS BY USER ROWID| TEST | 1 | 16 | 1 (0)| 00:00:01 |
------------------------------------------------------------------------------------
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
[size=+0] 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

我们看到,同样用rowid读取同一个block的10行,在11g中仅仅耗费了2个读,而在10g中耗费10个读。
更进一步的测试也确认了10g中,每一个rowid都会产生一个逻辑读,即使这些rowid来自一个block,也不能重用。
而11g引入了对rowid读的新特性, 那就是,如果rowid是来自一个block,那么是可以重用的。

总结:
在11g中,得益于这个11g不为人知的新特性,总体来说,rowid分页要优于rownum。
可是在10g中,相当一部分情况下rowid方式性能是远远不如rownum方式的。只有在后翻的页数非常大,并且页的大小很小,譬如10000-10010的情况下,rowid的分页才会优于rownum方式

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