关于ROWID分页中，执行计划的错误选择与处理（一）

老实说呢,Oracle版本9以前对分页处理的真的不好，本来想来是很简单的语句，oracle偏偏就处理的有问题。。。。
具体不知道10g在这个方面是否有大的改进。我们见一个具体的例子。。。
SQL> set autot trace
SQL> select rid from
2 (select a.rowid rid,row_number() over(order by a.topic_type DESC,a.topic_last_post_id DESC) rn
3 from forum_topics a
4 WHERE a.forum_id=40
5 AND a.topic_type < 2
6 AND a.topic_status <> 3
7 ) WHERE rn < 2 and rn >= 1;

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1678 Bytes=33560)
1 0 VIEW (Cost=2 Card=1678 Bytes=33560)
2 1 WINDOW (SORT PUSHED RANK) (Cost=2 Card=1678 Bytes=31882)
3 2 INDEX (RANGE SCAN) OF 'IND_FORUM_TOP_FOR_TP_ST_ID' (NON-UNIQUE) (Cost=2 Card=1678 Bytes=31882)

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

以上的语句，只返回一行记录，查询出来了一个rowid，执行计划是索引扫描即可，可见，逻辑读是非常少的。
但是，我们把这个语句作为一个子查询，马上发现情况就变了：
SQL> SELECT t.topic_id,t.topic_type,t.topic_distillate,t.topic_vote,t.topic_status,t.topic_moved_id,
2 TO_CHAR(t.topic_time,'YYYY-MM-DD HH24:MI:SS') topic_time,
3 t.topic_last_post_id,t.topic_views,t.topic_title, t.topic_replies,
4 t.topic_poster FROM forum_topics t
5 where rowid in
6 (select rid from
7 (select a.rowid rid,row_number() over(order by a.topic_type DESC,a.topic_last_post_id DESC) rn
8 from forum_topics a
9 WHERE a.forum_id=40
10 AND a.topic_type < 2
11 AND a.topic_status <> 3
12* ) WHERE rn < 2 and rn >= 1)

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=854 Card=1678 Bytes=194648)
1 0 HASH JOIN (SEMI) (Cost=854 Card=1678 Bytes=194648)
2 1 TABLE ACCESS (FULL) OF 'FORUM_TOPICS' (Cost=444 Card=221324 Bytes=24124316)
3 1 VIEW OF 'VW_NSO_1' (Cost=2 Card=1678 Bytes=11746)
4 3 VIEW (Cost=2 Card=1678 Bytes=33560)
5 4 WINDOW (SORT PUSHED RANK) (Cost=2 Card=1678 Bytes=31882)
6 5 INDEX (RANGE SCAN) OF 'IND_FORUM_TOP_FOR_TP_ST_ID'(NON-UNIQUE) (Cost=2 Card=1678 Bytes=31882)

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
4613 consistent gets
0 physical reads
0 redo size
1167 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

这里居然走了全表扫描后与子查询的hash join（这个结果真的是可以让人吐血）。子查询才返回一条rowid，根据这个rowid的查询oracle居然不走rowid扫描后的nested loop的连接。这个时候，我们就是采用/*+ ROWID(t) */或者/*+ use_nl(t) */的提示也不能改变oracle的执行计划。但是，如果我们采用原始的rule的执行计划，会发现情况大有好转。

SQL> SELECT /*+ rule */ t.topic_id,t.topic_type,t.topic_distillate,t.topic_vote,t.topic_status,t.topic_moved_id,
2 TO_CHAR(t.topic_time,'YYYY-MM-DD HH24:MI:SS') topic_time,
3 t.topic_last_post_id,t.topic_views,t.topic_title, t.topic_replies,
4 t.topic_poster FROM forum_topics t
5 where rowid in
6 (select rid from
7 (select a.rowid rid,row_number() over(order by a.topic_type DESC,a.topic_last_post_id DESC) rn
8 from forum_topics a
9 WHERE a.forum_id=40
10 AND a.topic_type < 2
11 AND a.topic_status <> 3
12 ) WHERE rn < 2 and rn >= 1) ;

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=HINT: RULE
1 0 NESTED LOOPS
2 1 VIEW OF 'VW_NSO_1'
3 2 SORT (UNIQUE)
4 3 VIEW
5 4 WINDOW (SORT PUSHED RANK)
6 5 INDEX (RANGE SCAN) OF 'IND_FORUM_TOP_FOR_TP_ST_ID' (NON-UNIQUE)
7 1 TABLE ACCESS (BY USER ROWID) OF 'FORUM_TOPICS'

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
11 consistent gets
0 physical reads
0 redo size
1167 bytes sent via SQL*Net to client
504 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
2 sorts (memory)
0 sorts (disk)
1 rows processed

通过最后的反复测试，发现只有在如下的情况下，强行指定驱动顺序与连接方式，oracle才最终选择了正确的执行计划。
SQL> SELECT /*+ ordered use_nl(t) */ t.topic_id,t.topic_type,t.topic_distillate,t.topic_vote,
2 t.topic_status,t.topic_moved_id,TO_CHAR(t.topic_time,'YYYY-MM-DD HH24:MI:SS') topic_time,
3 t.topic_last_post_id,t.topic_views,t.topic_title, t.topic_replies,
4 t.topic_poster FROM (select rid from
5 (select a.rowid rid,row_number() over(order by a.topic_type DESC,a.topic_last_post_id DESC) rn
6 from forum_topics a
7 WHERE a.forum_id=40
8 AND a.topic_type < 2
9 AND a.topic_status <> 3
10 ) WHERE rn < 2 and rn >= 1) b,forum_topics t
11* where t.rowid=b.rid

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=1680 Card=1678 Bytes=216462)
1 0 NESTED LOOPS (Cost=1680 Card=1678 Bytes=216462)
2 1 VIEW (Cost=2 Card=1678 Bytes=33560)
3 2 WINDOW (SORT PUSHED RANK) (Cost=2 Card=1678 Bytes=31882)
4 3 INDEX (RANGE SCAN) OF 'IND_FORUM_TOP_FOR_TP_ST_ID' (NON-UNIQUE) (Cost=2 Card=1678 Bytes=31882)
5 1 TABLE ACCESS (BY USER ROWID) OF 'FORUM_TOPICS' (Cost=1 Card=1 Bytes=109)

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
11 consistent gets
0 physical reads
0 redo size
1167 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