Oracle分页查询格式10

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上一篇文章已经介绍了利用分析函数来进行分页的方法，总的来说，除非分页到了最后部分，利用分析函数进行分页的效率还是可以的。

综合来说，除了SQL嵌套可以少写一层外，并没有什么特别的优点来代替标准分页函数的写法。

不过上一篇测试所有的数据都是通过全表扫描得到的，如果在排序字段上存在索引，这两种不同的分页查询效率如何呢，还是继续进行测试：

SQL> ALTER TABLE T MODIFY OBJECT_NAME NOT NULL;

表已更改。

SQL> CREATE INDEX IND_T_OBJECT_NAME ON T (OBJECT_NAME);

索引已创建。

为了Oracle可以利用这个索引，将索引列置为非空，首先测试标准分页SQL语句：

SQL> SELECT OBJECT_ID, OBJECT_NAME
2 FROM
3 (
4 SELECT ROWNUM RN, OBJECT_ID, OBJECT_NAME
5 FROM
6 (
7 SELECT OBJECT_ID, OBJECT_NAME FROM T
8 ORDER BY OBJECT_NAME
9 )
10 WHERE ROWNUM <= 20
11 )
12 WHERE RN >= 11;

OBJECT_ID OBJECT_NAME
---------- ------------------------------
17869 /1005bd30_LnkdConstant
17870 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17870 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17870 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17870 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17870 /1005bd30_LnkdConstant

已选择10行。

已用时间: 00: 00: 00.05

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=826 Card=20 Bytes=1840)
1 0 VIEW (Cost=826 Card=20 Bytes=1840)
2 1 COUNT (STOPKEY)
3 2 VIEW (Cost=826 Card=4584838 Bytes=362202202)
4 3 TABLE ACCESS (BY INDEX ROWID) OF 'T' (Cost=826 Card=4584838 Bytes=132960302)
5 4 INDEX (FULL SCAN) OF 'IND_T_OBJECT_NAME' (NON-UNIQUE) (Cost=26 Card=4584838)

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

在标准SQL中为了使用索引和NESTED LOOP连接方式，一般还要加上FIRST_ROWS提示，现在还没有加上FIRST_ROWS提示，Oracle就使用了索引全扫描代替了全表扫描，而且效率相当的高，只需要0.5秒就返回了结果。

再看分析函数的表现：

SQL> SELECT OBJECT_ID, OBJECT_NAME
2 FROM
3 (
4 SELECT OBJECT_NAME, OBJECT_ID,
5 ROW_NUMBER() OVER(ORDER BY OBJECT_NAME) RN
6 FROM T
7 )
8 WHERE RN BETWEEN 11 AND 20;

已选择10行。

已用时间: 00: 01: 09.17

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=826 Card=4584838 Bytes=421805096)
1 0 VIEW (Cost=826 Card=4584838 Bytes=421805096)
2 1 WINDOW (NOSORT) (Cost=826 Card=4584838 Bytes=132960302)
3 2 TABLE ACCESS (BY INDEX ROWID) OF 'T' (Cost=826 Card=4584838 Bytes=132960302)
4 3 INDEX (FULL SCAN) OF 'IND_T_OBJECT_NAME' (NON-UNIQUE) (Cost=26 Card=4584838)

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
3197229 consistent gets
118443 physical reads
0 redo size
578 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
10 rows processed

SQL> SELECT OBJECT_ID, OBJECT_NAME
2 FROM
3 (
4 SELECT OBJECT_NAME, OBJECT_ID,
5 ROW_NUMBER() OVER(ORDER BY OBJECT_NAME) RN
6 FROM T
7 )
8 WHERE RN BETWEEN 11 AND 20;

已选择10行。

已用时间: 00: 00: 10.65

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
3197229 consistent gets
43319 physical reads
0 redo size
578 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
10 rows processed

如果说第一次执行是由于大量物理读没有缓存，导致执行时间达到了1分钟的话，那么第二次执行仍旧高得离谱的三百多万的逻辑读，就很说明问题了。执行时间居然要10秒多，比全表扫描效率还低，看执行计划就知道，这次STOP KEY没有被推到分析函数的窗口排序中，导致Oracle扫描了所有的记录。

这对于分页来说，绝对是不可接受的。不过这是在9i的环境下进行的测试：

SQL> SELECT * FROM V$VERSION;

BANNER
----------------------------------------------------------------
Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production PL/SQL Release 9.2.0.4.0 - Production
CORE 9.2.0.3.0 Production
TNS for Linux: Version 9.2.0.4.0 - Production
NLSRTL Version 9.2.0.4.0 - Production

看看10g中Oracle是否解决了这个问题：

SQL> SELECT * FROM V$VERSION;

BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - 64bi
PL/SQL Release 10.2.0.3.0 - Production
CORE 10.2.0.3.0 Production
TNS for Linux: Version 10.2.0.3.0 - Production
NLSRTL Version 10.2.0.3.0 - Production

SQL> SELECT OBJECT_ID, OBJECT_NAME
2 FROM
3 (
4 SELECT OBJECT_NAME, OBJECT_ID,
5 ROW_NUMBER() OVER(ORDER BY OBJECT_NAME) RN
6 FROM T
7 )
8 WHERE RN BETWEEN 11 AND 20;

OBJECT_ID OBJECT_NAME
---------- ------------------------------
30166 /1000e8d1_LinkedHashMapValueIt
30165 /1000e8d1_LinkedHashMapValueIt
30166 /1000e8d1_LinkedHashMapValueIt
30165 /1000e8d1_LinkedHashMapValueIt
30165 /1000e8d1_LinkedHashMapValueIt
30165 /1000e8d1_LinkedHashMapValueIt
30165 /1000e8d1_LinkedHashMapValueIt
30165 /1000e8d1_LinkedHashMapValueIt
30165 /1000e8d1_LinkedHashMapValueIt
30165 /1000e8d1_LinkedHashMapValueIt

10 rows selected.

Elapsed: 00:00:02.04

Execution Plan
----------------------------------------------------------
Plan hash value: 3047187157

-----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4969K| 436M| | 41652 (1)| 00:09:44 |
|* 1 | VIEW | | 4969K| 436M| | 41652 (1)| 00:09:44 |
|* 2 | WINDOW SORT PUSHED RANK| | 4969K| 132M| 342M| 41652 (1)| 00:09:44 |
| 3 | TABLE ACCESS FULL | T | 4969K| 132M| | 17375 (1)| 00:04:04 |
-----------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

1 - filter("RN">=11 AND "RN"<=20)
2 - filter(ROW_NUMBER() OVER ( ORDER BY "OBJECT_NAME")<=20)

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
52137 consistent gets
0 physical reads
0 redo size
725 bytes sent via SQL*Net to client
492 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
10 rows processed

SQL> SELECT /*+ FIRST_ROWS */ OBJECT_ID, OBJECT_NAME
2 FROM
3 (
4 SELECT OBJECT_NAME, OBJECT_ID,
5 ROW_NUMBER() OVER(ORDER BY OBJECT_NAME) RN
6 FROM T
7 )
8 WHERE RN BETWEEN 11 AND 20;

OBJECT_ID OBJECT_NAME
---------- ------------------------------
30165 /1000e8d1_LinkedHashMapValueIt
30166 /1000e8d1_LinkedHashMapValueIt
30165 /1000e8d1_LinkedHashMapValueIt
30166 /1000e8d1_LinkedHashMapValueIt
30165 /1000e8d1_LinkedHashMapValueIt
30166 /1000e8d1_LinkedHashMapValueIt
30165 /1000e8d1_LinkedHashMapValueIt
30166 /1000e8d1_LinkedHashMapValueIt
30165 /1000e8d1_LinkedHashMapValueIt
30166 /1000e8d1_LinkedHashMapValueIt

10 rows selected.

Elapsed: 00:00:00.00

Execution Plan
----------------------------------------------------------
Plan hash value: 3257002816

-----------------------------------------------------------------------------------------
|Id |Operation |Name |Rows |Bytes|Cost (%CPU)|Time |
-----------------------------------------------------------------------------------------
| 0|SELECT STATEMENT | | 4969K| 436M| 3679K (1)|14:18:35 |
|* 1| VIEW | | 4969K| 436M| 3679K (1)|14:18:35 |
|* 2| WINDOW NOSORT STOPKEY | | 4969K| 132M| 3679K (1)|14:18:35 |
| 3| TABLE ACCESS BY INDEX ROWID|T | 4969K| 132M| 3679K (1)|14:18:35 |
| 4| INDEX FULL SCAN |IND_T_OBJECT_NAME| 4969K| |11703 (1)|00:02:44 |
-----------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

1 - filter("RN">=11 AND "RN"<=20)
2 - filter(ROW_NUMBER() OVER ( ORDER BY "OBJECT_NAME")<=20)

Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
16 consistent gets
0 physical reads
0 redo size
755 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

10g中表的结构与数据量和9i完全一致，但是默认情况下，Oracle并没有选择使用索引扫描的方式。如果在SQL中加上FIRST_ROWS提示，那么Oracle选择索引扫描，并以接近0秒的速度将结果返回。

对比9i和10g采用分析函数分页的执行计划可以发现，92的执行计划为WINDOW (NOSORT)，而102为WINDOW NOSORT STOPKEY。显然Oracle在10g解决了9i存在的问题，这也是在上一篇文章中提到的，Oracle可能会不断完善分析函数的功能。

如果总结一下，10g中使用分析函数来进行分页，已经没有什么问题了，但是在9i中，用分析函数的方式进行分页，可能会带来严重的性能问题。