在union all的视图中怎么解决分页问题

在对于union all的视图中，如果采用传统的采用rownum分页方式的时候，如

WHERE rownum < M)

WHERE linenum >=N

因为，Oracle执行到linenum >=N的时候，将不知所措，导致执行计划乱掉。如，假定bwm_users就是一个union all的视图。

代码如下：

select *

from mv_bmw_users_db1

union all

select *

from mv_bmw_users_db2

如果我们在该视图上执行如下操作，可以看到

SQL> select * from

2 (select rownum linenum,id,nick from

3 (select id,nick from bmw_users where nick ='test' order by id)

4 where rownum < 50)

5 where linenum >=1;

Execution Plan

----------------------------------------------------------

0 SELECT STATEMENT Optimizer=CHOOSE (Cost=20385 Card=49 Bytes=2401)

1 0 VIEW (Cost=20385 Card=49 Bytes=2401)

2 1 COUNT (STOPKEY)

3 2 VIEW (Cost=20385 Card=1728633 Bytes=62230788)

4 3 SORT (ORDER BY STOPKEY) (Cost=20385 Card=1728633 Bytes=62230788)

5 4 VIEW OF 'BMW_USERS' (Cost=9278 Card=1728633 Bytes=62230788)

6 5 UNION-ALL

7 6 TABLE ACCESS (FULL) OF 'MV_BMW_USERS_DB1' (Cost=4639 Card=864090 Bytes=38884050)

8 6 TABLE ACCESS (FULL) OF 'MV_BMW_USERS_DB2' (Cost=4639 Card=864543 Bytes=38904435)

Statistics

----------------------------------------------------------

0 recursive calls

0 db block gets

97298 consistent gets

20770 physical reads

0 redo size

518 bytes sent via SQL*Net to client

504 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

1 sorts (memory)

0 sorts (disk)

1 rows processed

一个非常简单的查询，在nick上是有索引的，而且表与索引都是分析过的，居然是全表扫描，耗费非常大的资源，这个时候，Oracle已经不能正确的判断使用索引了，所以错误的使用了全表，从统计信息也可以看到，该查询产生了大量的cr读与磁盘读。这个时候，就是强行指定hint也不能改变oracle的执行计划，当然，这样是行不通的，我们必须找到一个行之有效的办法。

这样的问题怎么解决呢？有两个办法，一个是仍然使用union all语句在查询中，直接查询基表而不是视图。如以上语句改造为：

SQL> select * from

2 (select rownum linenum,id,nick from

3 (select * from

4 (select id,nick from MV_BMW_USERS_DB1 where nick ='test'

5 union all

6 select id,nick from MV_BMW_USERS_DB1 where nick ='test')

7 order by id)

8 where rownum < 50)

9 where linenum >=1;

Execution Plan

----------------------------------------------------------

0 SELECT STATEMENT Optimizer=CHOOSE (Cost=17 Card=2 Bytes=98)

1 0 VIEW (Cost=17 Card=2 Bytes=98)

2 1 COUNT (STOPKEY)

3 2 VIEW (Cost=17 Card=2 Bytes=72)

4 3 SORT (ORDER BY STOPKEY) (Cost=17 Card=2 Bytes=72)

5 4 VIEW (Cost=8 Card=2 Bytes=72)

6 5 UNION-ALL

7 6 TABLE ACCESS (BY INDEX ROWID) OF 'MV_BMW_USERS_DB1' (Cost=4 Card=1 Bytes=45)

8 7 INDEX (RANGE SCAN) OF 'IND_MV_BMW_USERS_NICK1' (NON-UNIQUE) (Cost=3 Card=1)

9 6 TABLE ACCESS (BY INDEX ROWID) OF 'MV_BMW_USERS_DB1' (Cost=4 Card=1 Bytes=45)

10 9 INDEX (RANGE SCAN) OF 'IND_MV_BMW_USERS_NICK1' (NON-UNIQUE) (Cost=3 Card=1)

Statistics

----------------------------------------------------------

0 recursive calls

0 db block gets

8 consistent gets

0 physical reads

0 redo size

553 bytes sent via SQL*Net to client

504 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

1 sorts (memory)

0 sorts (disk)

2 rows processed

语句基本上是一样的，只是这次查询了基表，而不是视图，执行计划马上发生了改变，这次能使用了索引，而且成本有了很大的减少，可以看到cr读减少到只有8个块，而且磁盘读为0。

我们采用第二种方法，分析函数的办法，把语句改写为

SQL>select * from

1 (select row_number() over(order by id) rn,id,nick from bmw_users where nick ='test')

2 where rn <50 and rn >=1;

Execution Plan

----------------------------------------------------------

0 SELECT STATEMENT Optimizer=CHOOSE (Cost=13 Card=1 Bytes=49)

1 0 VIEW (Cost=13 Card=1 Bytes=49)

2 1 WINDOW (SORT PUSHED RANK) (Cost=13 Card=1 Bytes=45)

3 2 VIEW OF 'BMW_USERS' (Cost=4 Card=1 Bytes=45)

4 3 UNION-ALL (PARTITION)

5 4 TABLE ACCESS (BY INDEX ROWID) OF 'MV_BMW_USERS_DB1' (Cost=4 Card=1 Bytes=45)

6 5 INDEX (RANGE SCAN) OF 'IND_MV_BMW_USERS_NICK1' (NON-UNIQUE) (Cost=3 Card=1)

7 4 TABLE ACCESS (BY INDEX ROWID) OF 'MV_BMW_USERS_DB2' (Cost=4 Card=1 Bytes=45)

8 7 INDEX (RANGE SCAN) OF 'IND_MV_BMW_USERS_NICK2' (NON-UNIQUE) (Cost=3 Card=1)

Statistics

----------------------------------------------------------

0 recursive calls

0 db block gets

7 consistent gets

0 physical reads

0 redo size

513 bytes sent via SQL*Net to client

504 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

1 sorts (memory)

0 sorts (disk)

1 rows processed

可以看到，同样的功能，分析函数的方法是最简单的，同样也能正确的使用索引。

以上是一个简单的例子，我们再分析一个复杂的实际语句。

原始语句为：

SELECT /*+ ordered use_nl(u1,p2,u2)*/T2.*,u1.nick, u1.user_id, u1.id as userid,

u2.nick as user2, u2.user_id as id2, u2.id as userid2, p2.post_username as post_username2,

TO_CHAR(p2.post_time,'YYYY-MM-DD HH24:MI:SS') post_time

FROM

(SELECT * FROM

(SELECT T1.*, rownum as linenum

FROM

(SELECT /*+ index (t IND_FORUM_TOPICS_FOR_ID)*/t.topic_id,t.topic_type,t.topic_distillate,

t.topic_vote,t.topic_status, t.topic_moved_id,TO_CHAR(t.topic_time,'YYYY-MM-DD HH24:MI:SS') topic_time,

t.topic_last_post_id, t.topic_views,t.topic_title, t.topic_replies, t.topic_poster

FROM forum_topics t

WHERE t.forum_id = ?

AND t.topic_type < 2

AND t.topic_status <> 3

ORDER BY t.topic_type DESC, t.topic_last_post_id DESC ) T1

WHERE rownum < ?)

WHERE linenum >=?) T2,

forum_posts p2,

bmw_users u1,bmw_users u2

WHERE T2.topic_poster = u1.user_id

AND p2.post_id = T2.topic_last_post_id

AND u2.user_id = p2.poster_id

因为其中bmw_users是union all的视图，所以，该查询也使用了基表的全表扫描。如果把它改写为union all的语句，也将是异常的复杂，如，该写成union all将是这个样子

select * from (

SELECT /*+ ordered use_nl(u1,p2,u2)*/T2.*,u1.nick, u1.user_id,

u1.id as userid, u2.nick as user2, u2.user_id as id2,

u2.id as userid2, p2.post_username as post_username2,

TO_CHAR(p2.post_time,'YYYY-MM-DD HH24:MI:SS')

post_time FROM (

SELECT *FROM (

SELECT T1.*, rownum as linenum FROM(

SELECT /*+ index (t IND_FORUM_TOPICS_FOR_ID)*/t.topic_id,t.topic_type,

t.topic_distillate, t.topic_vote,t.topic_status,t.topic_moved_id,

TO_CHAR(t.topic_time,'YYYY-MM-DD HH24:MI:SS') topic_time,

t.topic_last_post_id,t.topic_views,t.topic_title, t.topic_replies,

t.topic_poster FROM forum_topics t WHERE t.forum_id = :bind0

AND t.topic_type < 2 AND t.topic_status <> 3 ORDER BY t.topic_type DESC,

t.topic_last_post_id DESC) T1

WHERE rownum < :bind1)

WHERE linenum >=:bind2

) T2,

forum_posts p2,

MV_BMW_USERS_DB1 u1,

MV_BMW_USERS_DB1 u2

WHERE T2.topic_poster = u1.user_id

AND p2.post_id = T2.topic_last_post_id

AND u2.user_id = p2.poster_id

union all

SELECT /*+ ordered use_nl(u1,p2,u2)*/T2.*,u1.nick,

u1.user_id, u1.id as userid, u2.nick as user2, u2.user_id as id2,

u2.id as userid2, p2.post_username as post_username2,

TO_CHAR(p2.post_time,'YYYY-MM-DD HH24:MI:SS') post_time

FROM (

SELECT * FROM (

SELECT T1.*, rownumas linenum FROM (

SELECT /*+ index (t IND_FORUM_TOPICS_FOR_ID)*/t.topic_id,

t.topic_type,t.topic_distillate, t.topic_vote,t.topic_status,t.topic_moved_id,

TO_CHAR(t.topic_time,'YYYY-MM-DD HH24:MI:SS') topic_time,

t.topic_last_post_id,t.topic_views,t.topic_title,

t.topic_replies, t.topic_poster FROM forum_topics t

WHERE t.forum_id = :bind3

AND t.topic_type < 2 AND t.topic_status <> 3 ORDER BY t.topic_type DESC,

t.topic_last_post_id DESC) T1

WHERE rownum < :bind4)

WHERE linenum >=:bind5

) T2,

forum_posts p2,

MV_BMW_USERS_DB1 u1,

MV_BMW_USERS_DB2 u2

WHERE T2.topic_poster = u1.user_id

AND p2.post_id = T2.topic_last_post_id

AND u2.user_id = p2.poster_id

union all

SELECT /*+ ordered use_nl(u1,p2,u2)*/T2.*,u1.nick, u1.user_id,

u1.id as userid, u2.nick as user2, u2.user_id as id2, u2.id as userid2,

p2.post_username as post_username2,

TO_CHAR(p2.post_time,'YYYY-MM-DD HH24:MI:SS') post_time

FROM (

SELECT * FROM (

SELECT T1.*, rownum as linenum FROM (

SELECT /*+ index (t IND_FORUM_TOPICS_FOR_ID)*/ t.topic_id,

t.topic_type,t.topic_distillate, t.topic_vote,t.topic_status,t.topic_moved_id,

TO_CHAR(t.topic_time,'YYYY-MM-DD HH24:MI:SS') topic_time,

t.topic_last_post_id,t.topic_views,t.topic_title, t.topic_replies,

t.topic_poster FROM forum_topics t

WHERE t.forum_id = :bind6 AND t.topic_type < 2 AND t.topic_status <> 3

ORDER BY t.topic_type DESC, t.topic_last_post_id DESC) T1

WHERE rownum < :bind7)

WHERE linenum >=:bind8

) T2,

forum_posts p2,

MV_BMW_USERS_DB2 u1,

MV_BMW_USERS_DB1 u2

WHERE T2.topic_poster = u1.user_id

AND T2.topic_last_post_id = p2.post_id

AND u2.user_id = p2.poster_id

union all

SELECT /*+ ordered use_nl(u1,p2,u2)*/T2.*,u1.nick, u1.user_id, u1.id as userid,

u2.nick as user2, u2.user_id as id2, u2.id as userid2, p2.post_username as post_username2,

TO_CHAR(p2.post_time,'YYYY-MM-DD HH24:MI:SS') post_time

FROM (

SELECT * FROM (

SELECT T1.*, rownum as linenum FROM (

SELECT /*+ index (t IND_FORUM_TOPICS_FOR_ID)*/t.topic_id,

t.topic_type,t.topic_distillate, t.topic_vote,t.topic_status,t.topic_moved_id,

TO_CHAR(t.topic_time,'YYYY-MM-DD HH24:MI:SS') topic_time,

t.topic_last_post_id,t.topic_views,t.topic_title, t.topic_replies,

t.topic_poster FROM forum_topicst WHERE t.forum_id = :bind9

AND t.topic_type < 2 AND t.topic_status <> 3

ORDER BY t.topic_type DESC, t.topic_last_post_id DESC) T1

WHERE rownum < :bind10)

WHERE linenum >=:bind11

) T2, forum_posts p2,

MV_BMW_USERS_DB2 u1,MV_BMW_USERS_DB2 u2 WHERE T2.topic_poster =

u1.user_id AND p2.post_id = T2.topic_last_post_id AND u2.user_id = p2.poster_id

)

order by topic_type DESC,topic_last_post_id desc

但是，我们利用分析函数，将显得非常简单，而且正确的使用索引

SELECT /*+ ordered use_nl(u1,p2,u2)*/ T2.*,u1.nick, u1.user_id, u1.id as userid,

u2.nick as user2, u2.user_id as id2, u2.id as userid2, p2.post_username as post_username2,

TO_CHAR(p2.post_time,'YYYY-MM-DD HH24:MI:SS') post_time

FROM (

SELECT * FROM (

SELECT /*+ index (t IND_FORUM_TOPICS_FOR_ID)*/

row_number() over(order by t.topic_type DESC, t.topic_last_post_id DESC) rn,

t.topic_id,t.topic_type,t.topic_distillate, t.topic_vote,t.topic_status,t.topic_moved_id,

TO_CHAR(t.topic_time,'YYYY-MM-DD HH24:MI:SS') topic_time,

t.topic_last_post_id,t.topic_views,t.topic_title, t.topic_replies,

t.topic_poster FROM forum_topics t

WHERE t.forum_id = ? AND t.topic_type < 2 AND t.topic_status <> 3

) T1

WHERE rn < ? and rn >= ?

) T2,

forum_posts p2,

bmw_users u1,

bmw_users u2

WHERE T2.topic_poster = u1.user_id

AND p2.post_id = T2.topic_last_post_id

AND u2.user_id = p2.poster_id