Oracle优化——单表分页优化

单表分页优化思路:


--创建测试表:

SQL> create table t_test as select * from dba_objects; 


Table created.


如,下面的sql (没有过滤条件,只有排序),要将查询结果分页显示,每页显示10条,如:

select * from t_test order by object_id;


例子:

1、分页查询sql语句,如下(通常会采用下面的方法,但是这是错误的分页框架)

语法:select * from (select t.*,rownum rn from (需要分页的sql)t ) where rn >=1 and rn <=10;


SQL> select * from (select t.*,rownum rn from (select * from t_test order by object_id) t) where rn >=1 and rn <=10;
Execution Plan
----------------------------------------------------------
Plan hash value: 3603170480
----------------------------------------------------------------------------------------
| Id  | Operation             | Name   | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |        | 80700 |    16M|       |  3918   (1)| 00:00:48 |
|*  1 |  VIEW                 |        | 80700 |    16M|       |  3918   (1)| 00:00:48 |
|   2 |   COUNT               |        |       |       |       |            |          |
|   3 |    VIEW               |        | 80700 |    15M|       |  3918   (1)| 00:00:48 |
|   4 |     SORT ORDER BY     |        | 80700 |    15M|    19M|  3918   (1)| 00:00:48 |
|   5 |      TABLE ACCESS FULL| t_test | 80700 |    15M|       |   287   (1)| 00:00:04 |
----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("RN"<=10 AND "RN">=1)
Note
-----
   - dynamic sampling used for this statement (level=2)
Statistics
----------------------------------------------------------
        331  recursive calls
          0  db block gets
       1211  consistent gets
       1024  physical reads
          0  redo size
       2075  bytes sent via SQL*Net to client
        523  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
         10  rows processed


   从上面的执行计划可以看出,该sql走了全表扫描,如果该表很大,则先要将该表排序,再取出其中10行数据,这样会出现严重的性能问题,所以该sql不能走全表扫描,必须走索引扫描。(因为索引已经排序了,可使用索引来消除排序)一般分页语句中都有排序。

如下,在object_id列建立索引,并在索引中添加一个常量0,如下:(在索引中添加一个常量0,是因为object_id列允许位null,如果不添加常量(不一定是0,可以是1,2,3,也可以是英文字母),索引中就不能存储null值,;因为sql中并没有剔除null值,所以我们必须要添加一个常量,让索引存储null,这样才能使sql走索引。)

SQL> create index idx_test on t_test(object_id,0);


SQL> select * from (select t.*,rownum rn from (select /*+ index)t_test idx_test) */ * from t_test order by object_id) t) where rn >=1 and rn <=10;


SQL> alter session set statistics_level=all;

SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
-----------------------------------------
SQL_ID  gvq2rvmutr85w, child number 0
-------------------------------------
 select * from (select t.*,rownum rn from (select /*+ index)t_test
idx_test) */ * from t_test order by object_id) t) where rn >=1 and rn
<=10
Plan hash value: 3119682446
-----------------------------------------------------------------------------------------------------
| Id  | Operation                      | Name     | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-----------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT               |          |      1 |        |     10 |00:00:00.08 |    1245 |
|*  1 |  VIEW                          |          |      1 |  80700 |     10 |00:00:00.08 |    1245 |
|   2 |   COUNT                        |          |      1 |        |  71903 |00:00:00.07 |    1245 |
|   3 |    VIEW                        |          |      1 |  80700 |  71903 |00:00:00.06 |    1245 |
|   4 |     TABLE ACCESS BY INDEX ROWID| t_test   |      1 |  80700 |  71903 |00:00:00.04 |    1245 |
|   5 |      INDEX FULL SCAN           | IDX_test |      1 |  80700 |  71903 |00:00:00.01 |     181 |
-----------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter(("RN"<=10 AND "RN">=1))
Note
-----
   - dynamic sampling used for this statement (level=2)
28 rows selected.

(因为sql语句没有where过滤条件,强制走索引只能走index full scan,无法走索引范围扫描(index range scan)。E-Rows的显示 说明了扫描了索引中所有叶子块,一共消耗了1245个逻辑读;理想的执行计划是:index full scan只扫描一个最多几个索引叶子块,最根本的原因还在于这个分页框架错了!)


2、正确的分页框架:

语法:select * from (select * from (select a.*,rownum rn from (需要分页的sql) a ) where rownum <=10) where rn >=1;


SQL> select * from (select * from (select a.*,rownum rn from (select /*+ index(t_test idx_test) */ * from t_test order by object_id) a ) where rownum <=10) where rn >=1;
SQL> alter session set statistics_level=all;
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
-----------------------------------------
SQL_ID  cgjp65zfj1yqa, child number 0
-------------------------------------
select * from (select * from (select a.*,rownum rn from (select /*+
index(t_test idx_test) */ * from t_test order by object_id) a ) where
rownum <=10) where rn >=1
Plan hash value: 1201925926
-------------------------------------------------------------------------------------------------------
| Id  | Operation                        | Name     | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                 |          |      1 |        |     10 |00:00:00.01 |       5 |
|*  1 |  VIEW                            |          |      1 |     10 |     10 |00:00:00.01 |       5 |
|*  2 |   COUNT STOPKEY                  |          |      1 |        |     10 |00:00:00.01 |       5 |
|   3 |    VIEW                          |          |      1 |  80700 |     10 |00:00:00.01 |       5 |
|   4 |     COUNT                        |          |      1 |        |     10 |00:00:00.01 |       5 |
|   5 |      VIEW                        |          |      1 |  80700 |     10 |00:00:00.01 |       5 |
|   6 |       TABLE ACCESS BY INDEX ROWID| t_test   |      1 |  80700 |     10 |00:00:00.01 |       5 |
|   7 |        INDEX FULL SCAN           | IDX_test |      1 |  80700 |     10 |00:00:00.01 |       3 |
-------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("RN">=1)
   2 - filter(ROWNUM<=10)
Note
-----
   - dynamic sampling used for this statement (level=2)
31 rows selected.


(从上面的执行计划可以看到,sql走 index full scan,只扫描了10条数据(id=7 A-Rows=10)就停止了(id=2 COUNT STOPKEY)一共消耗了5个逻辑读;该执行计划利用索引已经排序的特性只扫描索引获取了10条数据,然后再利用count stopkey特性,获取到分页需要的数据,sql立即停止运行,这才是最佳的执行计划。)


注意:

  如果分页语句中有排序(order by),要利用索引已经排序特性,将order by的列包含在索引中,同时也要利用rownum的count stopkey特性来优化分页sql。如果分页中没有排序,可以直接利用rownum的count stopkey特性来优化分页sql。


例子:

如下两条sq(注意,过滤条件是等值过滤,当然也有order by)l,改成分页语句,并查看执行计划,如下:

select * from t_test where owner='SCOTT' order by object_id;

select * from t_test where owner='SYS' order by object_id;

(其中第一条sql语句的过滤条件是where owner='SCOTT';该过滤条件能过滤掉表中大部分数据。第二条sql语句的过滤条件where owner='SYS',能过滤表中一半数据)


---执行第一条sql语句:

SQL> select * from (select * from (select a.*,rownum rn from (select /*+ index(t_test idx_test) */ * from t_test where owner='SCOTT' order by object_id) a ) where rownum <=10) where rn >=1;
SQL> alter session set statistics_level=all;
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
--------------------------------------------
SQL_ID  0w9tbrwkn9tn6, child number 0
-------------------------------------
 select * from (select * from (select a.*,rownum rn from (select /*+
index(t_test idx_test) */ * from t_test where owner='SCOTT' order by
object_id) a ) where rownum <=10) where rn >=1
Plan hash value: 1201925926
-------------------------------------------------------------------------------------------------------
| Id  | Operation                        | Name     | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                 |          |      1 |        |     10 |00:00:00.04 |    1245 |
|*  1 |  VIEW                            |          |      1 |     10 |     10 |00:00:00.04 |    1245 |
|*  2 |   COUNT STOPKEY                  |          |      1 |        |     10 |00:00:00.04 |    1245 |
|   3 |    VIEW                          |          |      1 |     12 |     10 |00:00:00.04 |    1245 |
|   4 |     COUNT                        |          |      1 |        |     10 |00:00:00.04 |    1245 |
|   5 |      VIEW                        |          |      1 |     12 |     10 |00:00:00.04 |    1245 |
|*  6 |       TABLE ACCESS BY INDEX ROWID| t_test   |      1 |     12 |     10 |00:00:00.04 |    1245 |
|   7 |        INDEX FULL SCAN           | IDX_test |      1 |  80700 |  71901 |00:00:00.01 |     181 |
-------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("RN">=1)
   2 - filter(ROWNUM<=10)
   6 - filter("OWNER"='SCOTT')
Note
-----
   - dynamic sampling used for this statement (level=2)
32 rows selected.

---执行第2条语句,如下:

SQL> select * from (select * from (select a.*,rownum rn from (select /*+ index(t_test idx_test) */ * from t_test where owner='SYS' order by object_id) a ) where rownum <=10) where rn >=1;
SQL> alter session set statistics_level=all;
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
-------------------------------------------------------
SQL_ID  dfwkpppbtc8h7, child number 0
-------------------------------------
select * from (select * from (select a.*,rownum rn from (select /*+
index(t_test idx_test) */ * from t_test where owner='SYS' order by
object_id) a ) where rownum <=10) where rn >=1
Plan hash value: 1201925926
-------------------------------------------------------------------------------------------------------
| Id  | Operation                        | Name     | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                 |          |      1 |        |     10 |00:00:00.01 |       5 |
|*  1 |  VIEW                            |          |      1 |     10 |     10 |00:00:00.01 |       5 |
|*  2 |   COUNT STOPKEY                  |          |      1 |        |     10 |00:00:00.01 |       5 |
|   3 |    VIEW                          |          |      1 |  28483 |     10 |00:00:00.01 |       5 |
|   4 |     COUNT                        |          |      1 |        |     10 |00:00:00.01 |       5 |
|   5 |      VIEW                        |          |      1 |  28483 |     10 |00:00:00.01 |       5 |
|*  6 |       TABLE ACCESS BY INDEX ROWID| t_test   |      1 |  28483 |     10 |00:00:00.01 |       5 |
|   7 |        INDEX FULL SCAN           | IDX_test |      1 |  80700 |     10 |00:00:00.01 |       3 |
-------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("RN">=1)
   2 - filter(ROWNUM<=10)
   6 - filter("OWNER"='SYS')
Note
-----
   - dynamic sampling used for this statement (level=2)
32 rows selected.


   从上面的执行计划可以看出,两条sql都走了 index full scan,第1条sql从索引中扫描了71901条数据(id=7 A-Rows=71901),在回表的时候对数据进行了大量过滤(id=6),最后得到10条数据,耗费了1245个逻辑读。

   第2条sql从索引中扫描了10条数据,耗费了5个逻辑读。可以看出,第二条sql的执行计划是正确的,而第一条sql的执行计划是错误的,应该尽量在索引扫描的时候就取得10行数据。

(为什么上面的两条sql只有过滤条件不一样,而第一条sql的执行计划就错了呢?这是因为第一条sql的过滤条件where owner='SCOTT',在表中只有很少数据,通过扫描object_id列的索引,然后在回表去匹配owner='SCOTT',因为owner='SCOTT'数据量很少,要搜索大量数据才能匹配上。而第二条sql的过滤条件owner='SYS',因为数据量多,只需要搜索少量的数据就能匹配上。)


---优化第一条sql:(就必须让过滤条件的列出现在索引中,如下:)


SQL> create index idx_test_all on t_test(owner,object_id);  ---创建一个组合索引,包含owner字段
SQL>  select * from (select * from (select a.*,rownum rn from (select /*+ index(t_test idx_test_all) */ * from t_test where owner='SCOTT' order by object_id) a ) where rownum <=10) where rn >=1;
SQL>  select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
---------------------------------------------、
SQL_ID  9mm61b7j943sf, child number 0
-------------------------------------
 select * from (select * from (select a.*,rownum rn from (select /*+
index(t_test idx_test_all) */ * from t_test where owner='SCOTT' order
by object_id) a ) where rownum <=10) where rn >=1
Plan hash value: 3696904346
-----------------------------------------------------------------------------------------------------------
| Id  | Operation                        | Name         | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-----------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                 |              |      1 |        |     10 |00:00:00.01 |       6 |
|*  1 |  VIEW                            |              |      1 |     10 |     10 |00:00:00.01 |       6 |
|*  2 |   COUNT STOPKEY                  |              |      1 |        |     10 |00:00:00.01 |       6 |
|   3 |    VIEW                          |              |      1 |     12 |     10 |00:00:00.01 |       6 |
|   4 |     COUNT                        |              |      1 |        |     10 |00:00:00.01 |       6 |
|   5 |      VIEW                        |              |      1 |     12 |     10 |00:00:00.01 |       6 |
|   6 |       TABLE ACCESS BY INDEX ROWID| t_test       |      1 |     12 |     10 |00:00:00.01 |       6 |
|*  7 |        INDEX RANGE SCAN          | IDX_test_ALL |      1 |     12 |     10 |00:00:00.01 |       3 |
-----------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("RN">=1)
   2 - filter(ROWNUM<=10)
   7 - access("OWNER"='SCOTT')
Note
-----
   - dynamic sampling used for this statement (level=2)
32 rows selected.

(这时候会发现,sql走了索引范围扫描,从索引中扫描了10条数据,一共耗费了6个逻辑读。这说明现在的执行计划是正确的。)


注意:

  在实际的生成环境中,过滤条件一般都是绑定变量,我们无法控制传参究竟传入哪个值,这就不能确定返回数据究竟是多还是少了,所以,建议最好将排序的列包含在索引中。


但是要注意:如果排序列有多个列,创建索引的时候,我们要将所有的排序列包含在索引中,并且要注意排序列先后顺序,而且还要注意列时升序还是降序。如果分页语句中排序列只有一个列,但是是降序显示的,创建索引的时候就没必要降序创建索引了,我们可以使用hint:index_desc 让索引降序扫描就可以了。


例子:

(创建索引,只能是object_id在前,object_name在后,另外object_name是降序显示的,那么在创建索引的时候,还要指定object_name列降序排序。下面的sql也没有过滤条件,在创建索引的时候还要加个常量,如下所示:)

SQL> create index idx_test_2 on t_test(object_id,object_name desc,0);  ---创建一个组合索引,包含owner字段
SQL>  select * from (select * from (select a.*,rownum rn from (select /*+ index(t_test idx_test_2) */ * from t_test  order by object_id,object_name desc) a ) where rownum <=10) where rn >=1;
SQL>  select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
----------------------------------------
SQL_ID  g8zgbvt1u1qjq, child number 0
-------------------------------------
select * from (select * from (select a.*,rownum rn from (select /*+
index(t_test idx_test_2) */ * from t_test  order by
object_id,object_name desc) a ) where rownum <=10) where rn >=1
Plan hash value: 2251915778
------------------------------------------------------------------------------------------------------------------
| Id  | Operation                        | Name       | Starts | E-Rows | A-Rows |   A-Time   | Buffers | Reads  |
------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                 |            |      1 |        |     10 |00:00:00.01 |       5 |      1 |
|*  1 |  VIEW                            |            |      1 |     10 |     10 |00:00:00.01 |       5 |      1 |
|*  2 |   COUNT STOPKEY                  |            |      1 |        |     10 |00:00:00.01 |       5 |      1 |
|   3 |    VIEW                          |            |      1 |  80700 |     10 |00:00:00.01 |       5 |      1 |
|   4 |     COUNT                        |            |      1 |        |     10 |00:00:00.01 |       5 |      1 |
|   5 |      VIEW                        |            |      1 |  80700 |     10 |00:00:00.01 |       5 |      1 |
|   6 |       TABLE ACCESS BY INDEX ROWID| t_test     |      1 |  80700 |     10 |00:00:00.01 |       5 |      1 |
|   7 |        INDEX FULL SCAN           | IDX_test_2 |      1 |  80700 |     10 |00:00:00.01 |       3 |      1 |
------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("RN">=1)
   2 - filter(ROWNUM<=10)
Note
-----
   - dynamic sampling used for this statement (level=2)
31 rows selected.


如果在创建索引的时候没有指定 object_name列降序排序,那么执行计划中会出现sort order by。因为索引中排序和分页语句中排序不一致,如:


SQL> create index idx_test_3 on t_test(object_id,object_name,0);
SQL>  select * from (select * from (select a.*,rownum rn from (select /*+ index(t_test idx_test_3) */ * from t_test  order by object_id,object_name desc) a ) where rownum <=10) where rn >=1;
SQL>  select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
---------------------------------------------
SQL_ID  bapu5jn5vn22r, child number 0
-------------------------------------
select * from (select * from (select a.*,rownum rn from (select /*+
index(t_test idx_test_3) */ * from t_test  order by
object_id,object_name desc) a ) where rownum <=10) where rn >=1
Plan hash value: 3547841113
-----------------------------------------
| Id  | Operation                         | Name       | Starts | E-Rows | A-Rows |   A-Time   | Buffers | Reads  |  OMem |  1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                  |            |      1 |        |     10 |00:00:00.10 |    1490 |    426 |       |       |          |
|*  1 |  VIEW                             |            |      1 |     10 |     10 |00:00:00.10 |    1490 |    426 |       |       |          |
|*  2 |   COUNT STOPKEY                   |            |      1 |        |     10 |00:00:00.10 |    1490 |    426 |       |       |          |
|   3 |    VIEW                           |            |      1 |  80700 |     10 |00:00:00.10 |    1490 |    426 |       |       |          |
|   4 |     COUNT                         |            |      1 |        |     10 |00:00:00.10 |    1490 |    426 |       |       |          |
|   5 |      VIEW                         |            |      1 |  80700 |     10 |00:00:00.10 |    1490 |    426 |       |       |          |
|   6 |       SORT ORDER BY               |            |      1 |  80700 |     10 |00:00:00.10 |    1490 |    426 |    10M|  1272K| 9811K (0)|
|   7 |        TABLE ACCESS BY INDEX ROWID| t_test     |      1 |  80700 |  71903 |00:00:00.06 |    1490 |    426 |       |       |          |
|   8 |         INDEX FULL SCAN           | IDX_test_3 |      1 |  80700 |  71903 |00:00:00.03 |     427 |    426 |       |       |          |
----------------------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("RN">=1)
   2 - filter(ROWNUM<=10)
Note
-----
   - dynamic sampling used for this statement (level=2)
32 rows selected.


(从上面可以看出,执行计划中出现了 sort order by,这就意味着分页语句没有利用到索引已经排序的特性,执行计划是错误的,这时候就需要创建正确的索引。)


例子3:

一条sql(,过滤条件有等值条件,也有非等值条件,当然也有order by),如下,将下面的sql分页查询:

select * from t_test where owner='SYS' and object_id > 1000 order by object_name;


--:创建索引(因为owner是等值过滤,object_Id是非等值过滤,创建索引的时候要优先将等值过滤列和排序列组合在一起,然后再将非等值过滤列放到后面)如下:

create index idx_test_4 on t_test(owner,object_name,object_id);
select * from (select * from (select a.*,rownum rn from (select /*+ index(t_test idx_test_4) */ * from t_test where owner='SYS' and object_id > 1000 order by object_name) a ) where rownum <=10) where rn >=1;
SQL>  select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
------------------------------------------:
SQL_ID  4z6tjgrdjm5a1, child number 0
-------------------------------------
select * from (select * from (select a.*,rownum rn from (select /*+
index(t_test idx_test_4) */ * from t_test where owner='SYS' and
object_id > 1000 order by object_name) a ) where rownum <=10) where rn
>=1
Plan hash value: 1432357471
------------------------------------------------------------------------------------------------------------------
| Id  | Operation                        | Name       | Starts | E-Rows | A-Rows |   A-Time   | Buffers | Reads  |
------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                 |            |      1 |        |     10 |00:00:00.01 |      14 |      2 |
|*  1 |  VIEW                            |            |      1 |     10 |     10 |00:00:00.01 |      14 |      2 |
|*  2 |   COUNT STOPKEY                  |            |      1 |        |     10 |00:00:00.01 |      14 |      2 |
|   3 |    VIEW                          |            |      1 |  25683 |     10 |00:00:00.01 |      14 |      2 |
|   4 |     COUNT                        |            |      1 |        |     10 |00:00:00.01 |      14 |      2 |
|   5 |      VIEW                        |            |      1 |  25683 |     10 |00:00:00.01 |      14 |      2 |
|   6 |       TABLE ACCESS BY INDEX ROWID| t_test     |      1 |  25683 |     10 |00:00:00.01 |      14 |      2 |
|*  7 |        INDEX RANGE SCAN          | IDX_test_4 |      1 |    256 |     10 |00:00:00.01 |       4 |      2 |
------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("RN">=1)
   2 - filter(ROWNUM<=10)
   7 - access("OWNER"='SYS' AND "OBJECT_ID">1000)
       filter("OBJECT_ID">1000)
Note
-----
   - dynamic sampling used for this statement (level=2)
34 rows selected.

(可以看出,执行计划中没有出现 sort order by,逻辑读也只有14个,说明执行计划是正确的。)



注意:

  ①:如果分页语句中有排序(order by),要利用索引已经排序的特性,将order by的列按照排序的先后顺序包含在索引中,同时要注意排序是升序还是降序。如果分页语句中有过滤条件,我们要注意过滤条件是否有等值过滤条件,如果有等值过滤条件,要将等值过滤条件优先组合在一起,然后将排序列放在等值过滤条件后面,最后将非等值过滤列放排序列后面。如果分页语句中没有等值过滤条件,我们应该先将排序列放在索引前面,将非等值过滤列放后面,最后利用rownum的count stopkey特性来优化分页sql。如果分页中没有排序,可以直接利用rownum的count stopkey特性来优化分页sql。


 ②:要想一眼看出执行计划是否正确,就要先看分页语句有没有order by,再看执行计划有没有sort order by,如果执行计划中有sort order by,说明执行计划一般是错误的。


 ③:如果分页语句中排序的表是分区表,这时我们要看分页语句中是否有跨区分区扫描,如果有跨分区扫描,创建的索引一般都是global索引,如果不创建global索引,就无法保证分页的顺序与所有的顺序是一致的。如果就只扫描一个分区,这时可以创建local索引:


例子:创建分区表,并导入数据

 CREATE TABLE p_test

   (    "OWNER" VARCHAR2(30),

        "OBJECT_NAME" VARCHAR2(128),

        "SUBOBJECT_NAME" VARCHAR2(30),

        "OBJECT_ID" NUMBER,

        "DATA_OBJECT_ID" NUMBER,

        "OBJECT_TYPE" VARCHAR2(19),

        "CREATED" DATE,

        "LAST_DDL_TIME" DATE,

        "TIMESTAMP" VARCHAR2(19),

        "STATUS" VARCHAR2(7),

        "TEMPORARY" VARCHAR2(1),

        "GENERATED" VARCHAR2(1),

        "SECONDARY" VARCHAR2(1),

        "NAMESPACE" NUMBER,

        "EDITION_NAME" VARCHAR2(30)

   ) partition by range (object_id)

   (

   partition p1 values less than (10000),

   partition p2 values less than (20000),

   partition p3 values less than (30000),

   partition p4 values less than (40000),

   partition p5 values less than (50000),

   partition p6 values less than (60000),

   partition p7 values less than (70000),

   partition px values less than (maxvalue));


 SQL> insert into p_test select * from dba_objects;


71917 rows created.  


---例子1:分页语句:

select * from (selct * from (select a.*,rownum rn from (select * from p_test order by object_id) a ) where rownum <=10) where rn >=1;


  (这个分页语句没有过滤条件,因此会扫描表中的所有分区。因为排序列恰好是范围分区列,范围分区每个分区的数据也是递增的,这时我们创建索引可以创建为local索引。但是如果将范围分区改为list分区或者hash分区,这时我们就必须创建global索引,因为list分区和hash分区是无序的。)


SQL> create index idx_test_id on p_test(object_id,0) local;  ---创建local索引


select * from (select * from (select a.*,rownum rn from (select /*+ index(p_test idx_test_id ) */ * from p_test order by object_id) a ) where rownum <=10) where rn >=1;
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
SQL_ID  bxw1059jmgxvx, child number 0
-------------------------------------
select * from (select * from (select a.*,rownum rn from (select /*+
index(p_test idx_test_id ) */ * from p_test order by object_id) a )
where rownum <=10) where rn >=1
Plan hash value: 1291390031
--------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                               | Name        | Starts | E-Rows | A-Rows |   A-Time   | Buffers | Reads  |
--------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                        |             |      1 |        |     10 |00:00:00.01 |       5 |      1 |
|*  1 |  VIEW                                   |             |      1 |     10 |     10 |00:00:00.01 |       5 |      1 |
|*  2 |   COUNT STOPKEY                         |             |      1 |        |     10 |00:00:00.01 |       5 |      1 |
|   3 |    VIEW                                 |             |      1 |  63696 |     10 |00:00:00.01 |       5 |      1 |
|   4 |     COUNT                               |             |      1 |        |     10 |00:00:00.01 |       5 |      1 |
|   5 |      VIEW                               |             |      1 |  63696 |     10 |00:00:00.01 |       5 |      1 |
|   6 |       PARTITION RANGE ALL               |             |      1 |  63696 |     10 |00:00:00.01 |       5 |      1 |
|   7 |        TABLE ACCESS BY LOCAL INDEX ROWID| P_TEST      |      1 |  63696 |     10 |00:00:00.01 |       5 |      1 |
|   8 |         INDEX FULL SCAN                 | IDX_TEST_ID |      1 |  63696 |     10 |00:00:00.01 |       3 |      1 |
--------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("RN">=1)
   2 - filter(ROWNUM<=10)
Note
-----
   - dynamic sampling used for this statement (level=2)
32 rows selected.


例子2:分页语句(根据object_name排序)

select * from (selct * from (select a.*,rownum rn from (select * from p_test order by object_name) a ) where rownum <=10) where rn >=1;


这时候我们需要创建global索引,因为如果是本地索引就会产生 sort order by

SQL> drop index idx_test_id ; 
SQL> create index idx_test_id on p_test(object_name,0); 
select * from (select * from (select a.*,rownum rn from (select /*+ index(p_test idx_test_id ) */ * from p_test order by object_name) a ) where rownum <=10) where rn >=1;
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
SQL_ID  25pm0f6b6m49x, child number 0
-------------------------------------
select * from (select * from (select a.*,rownum rn from (select /*+
index(p_test idx_test_id ) */ * from p_test order by object_name) a )
where rownum <=10) where rn >=1
Plan hash value: 246970912
--------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                               | Name        | Starts | E-Rows | A-Rows |   A-Time   | Buffers | Reads  |
--------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                        |             |      1 |        |     10 |00:00:00.01 |      10 |      2 |
|*  1 |  VIEW                                   |             |      1 |     10 |     10 |00:00:00.01 |      10 |      2 |
|*  2 |   COUNT STOPKEY                         |             |      1 |        |     10 |00:00:00.01 |      10 |      2 |
|   3 |    VIEW                                 |             |      1 |  63696 |     10 |00:00:00.01 |      10 |      2 |
|   4 |     COUNT                               |             |      1 |        |     10 |00:00:00.01 |      10 |      2 |
|   5 |      VIEW                               |             |      1 |  63696 |     10 |00:00:00.01 |      10 |      2 |
|   6 |       TABLE ACCESS BY GLOBAL INDEX ROWID| P_TEST      |      1 |  63696 |     10 |00:00:00.01 |      10 |      2 |
|   7 |        INDEX FULL SCAN                  | IDX_TEST_ID |      1 |  63696 |     10 |00:00:00.01 |       4 |      2 |
--------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("RN">=1)
   2 - filter(ROWNUM<=10)
Note
-----
   - dynamic sampling used for this statement (level=2)
31 rows selected.
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