oracle 分页优化（stopkey）
SQL优化
stopkey技术是一种预测技术,完全是为了适应大量数据的出现而出现的
从名称就可以看出,stopkey就是在关键位置上停止,终止的技术,oracle这里指的就是不再继续读取剩余的数据.
======================================================
1.创建测试表
create table vast as select * from dba_objects;
2.准备测试数据 开启执行计划
SQL> insert into vast select * from dba_objects;
72554 rows created.
3.测试，提取从500条到1000条数据
SQL>
set autotrace on explain
第一种情况
SQL> select * from (select rownum r,a.* from vast a) where r>500 and r<1000;
已用时间: 00: 00: 07.13
执行计划
----------------------------------------------------------
Plan hash value: 2564315607
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 127K| 26M| 562 (1)| 00:00:07 |
|* 1 | VIEW | | 127K| 26M| 562 (1)| 00:00:07 |
| 2 | COUNT | | | | | |
| 3 | TABLE ACCESS FULL| VAST | 127K| 25M| 562 (1)| 00:00:07 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("R"<1000 AND "R">500)
Note
-----
- dynamic sampling used for this statement (level=2)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
2102 consistent gets
0 physical reads
0 redo size
29806 bytes sent via SQL*Net to client
886 bytes received via SQL*Net from client
35 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
499 rows processed
-----------------------------------------------------------------------------------------------------------------------------------
第二种情况
select * from (select rownum r,a.* from vast a where rownum<1000) where r>500;
已用时间: 00: 00: 07.35
执行计划
----------------------------------------------------------
----------------------------------------------------------
----------------------------------------------------------
Plan hash value: 1948416614
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 999 | 214K| 7 (15)| 00:00:01 |
|* 1 | VIEW | | 999 | 214K| 7 (15)| 00:00:01 |
|* 2 | COUNT STOPKEY | | | | | |
| 3 | TABLE ACCESS FULL| VAST | 127K| 25M| 7 (15)| 00:00:01 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("R">500)
2 - filter(ROWNUM<1000)
Note
-----
- dynamic sampling used for this statement (level=2)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
50 consistent gets
0 physical reads
0 redo size
29806 bytes sent via SQL*Net to client
886 bytes received via SQL*Net from client
35 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
499 rows processed
----------------------------------------------------------
第二种情况比第一种一致性读大大减小，主要是把rownum引入到了第二层，注意执行计划中的stopkey，专门是为了提取top n 的需求优化的。
再来句简单的
select * from vast where rownum < 5;
已用时间: 00: 00: 00.04
执行计划
----------------------------------------------------------
Plan hash value: 338940916
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4 | 828 | 3 (34)| 00:00:01 |
|* 1 | COUNT STOPKEY | | | | | |
| 2 | TABLE ACCESS FULL| VAST | 127K| 25M| 3 (34)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<5)
Note
-----
- dynamic sampling used for this statement (level=2)
Statistics
----------------------------------------------------------
8 recursive calls
0 db block gets
129 consistent gets
0 physical reads
0 redo size
1896 bytes sent via SQL*Net to client
523 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
4 rows processed
条件中有rownum存在，就会有stopkey优化
，那越往后分页是否速度就会一致性读就会越大呢？
select * from (select rownum r,a.* from vast a where rownum <= 70000) where r > 69000;
已用时间: 00: 00: 04.24
执行计划
----------------------------------------------------------
Plan hash value: 1948416614
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 70000 | 14M| 310 (1)| 00:00:04 |
|* 1 | VIEW | | 70000 | 14M| 310 (1)| 00:00:04 |
|* 2 | COUNT STOPKEY | | | | | |
| 3 | TABLE ACCESS FULL| VAST | 127K| 25M| 310 (1)| 00:00:04 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("R">69000)
2 - filter(ROWNUM<=70000)
Note
-----
- dynamic sampling used for this statement (level=2)
Statistics
----------------------------------------------------------
7 recursive calls
0 db block gets
1189 consistent gets
0 physical reads
0 redo size
64623 bytes sent via SQL*Net to client
1249 bytes received via SQL*Net from client
68 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1000 rows processed
看来
stopkey 还是要注意 不能在rownum过大的时候使用。