【IT168 技术】在Oracle Database 12c之前，翻页查询需要试用rownum的方式进行SQL嵌套查询编写，这非常复杂，在12c中，新增了Top N查询支持特性，允许试用Offset / Limit 等限定进行Top N查询，原来的ROWNUM方式可以被替代。
其语法结构大致如下：
OFFSET <offset> [ROW | ROWS]
FETCH [FIRST | NEXT]
[<rowcount> | <percent> PERCENT] [ROW | ROWS]
[ONLY | WITH TIES]
下图是文档语法结构图：
如果仅返回TOP 5%的记录，下图中的对比可以看到新语法和原有的查询语句之间的巨大差异：
这个新的功能在内部是通过ROW_NUMBER分析函数来实现的，语法类似如下查询，返回经过排序的TOP 5数据内容：
SQL> select object_id,object_name from eygle order by object_id
2 fetch first 5 rows only;
OBJECT_ID OBJECT_NAME
---------- --------------------------------------------------
2 C_OBJ#
3 I_OBJ#
4 TAB$
5 CLU$
6 C_TS#
2 fetch first 5 rows only;
OBJECT_ID OBJECT_NAME
---------- --------------------------------------------------
2 C_OBJ#
3 I_OBJ#
4 TAB$
5 CLU$
6 C_TS#
计划，可以看到，其使用的内部函数是ROW_NUMBER() 分析函数：
SQL> set autotrace on
SQL> set linesize 150
SQL> select object_id,object_name from eygle order by object_id
2 fetch first 5 rows only;
Execution Plan
----------------------------------------------------------
Plan hash value: 3708596342
------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10000 | 1025K| | 118 (1)| 00:00:02 |
|* 1 | VIEW | | 10000 | 1025K| | 118 (1)| 00:00:02 |
|* 2 | WINDOW SORT PUSHED RANK| | 10000 | 224K| 328K| 118 (1)| 00:00:02 |
| 3 | TABLE ACCESS FULL | EYGLE | 10000 | 224K| | 46 (0)| 00:00:01 |
------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("from$_subquery$_003"."rowlimit_$$_rownumber"<=5)
2 - filter(ROW_NUMBER() OVER ( ORDER BY "OBJECT_ID")<=5)
SQL> set linesize 150
SQL> select object_id,object_name from eygle order by object_id
2 fetch first 5 rows only;
Execution Plan
----------------------------------------------------------
Plan hash value: 3708596342
------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10000 | 1025K| | 118 (1)| 00:00:02 |
|* 1 | VIEW | | 10000 | 1025K| | 118 (1)| 00:00:02 |
|* 2 | WINDOW SORT PUSHED RANK| | 10000 | 224K| 328K| 118 (1)| 00:00:02 |
| 3 | TABLE ACCESS FULL | EYGLE | 10000 | 224K| | 46 (0)| 00:00:01 |
------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("from$_subquery$_003"."rowlimit_$$_rownumber"<=5)
2 - filter(ROW_NUMBER() OVER ( ORDER BY "OBJECT_ID")<=5)
Oracle的这一新特性是被期望已久的，这将结果长久以来程序员们极为头痛的翻页算法，福音所在!
原文链接：http://www.eygle.com/archives/2012/10/12c_topn_native_query.html
