测试验证
现有如下SQL，每页显示10条：
select *
from test
where owner = 'SYS'
and object_id > 1000
order by object_name;
select *
from (select *
from (select a.*, rownum rn
from (select *
from test
where owner = 'SYS'
and object_id > 1000
order by object_name) a)
where rownum <= 10)
where rn >= 1;分页语句中有排序列，创建索引的时候，需要将排序列包含在索引中。所以只需要将过滤列owner、object_id以及排序列object_name组合起来创建索引。
因为owner是等值过滤，object_id列是非等值过滤，创建索引时，优先将等值过滤和排序列组合起来，然后再将非等值过滤列放在后面。
SQL> create index idx_owner_name_id on test(owner,object_name,object_id);
Index created.
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 6n5yu1h14fdrh, child number 0
-------------------------------------
select * from (select * from (select a.*, rownum rn
from (select /*+index(test idx_owner_name_id)*/
* from test
where owner = 'SYS' and object_id > 1000
order by object_name) a) where rownum
<= 10) where rn >= 1
Plan hash value: 2090516350
--------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
--------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 10 |00:00:00.01 | 15 |
|* 1 | VIEW | | 1 | 10 | 10 |00:00:00.01 | 15 |
|* 2 | COUNT STOPKEY | | 1 | | 10 |00:00:00.01 | 15 |
| 3 | VIEW | | 1 | 31163 | 10 |00:00:00.01 | 15 |
| 4 | COUNT | | 1 | | 10 |00:00:00.01 | 15 |
| 5 | VIEW | | 1 | 31163 | 10 |00:00:00.01 | 15 |
| 6 | TABLE ACCESS BY INDEX ROWID| TEST | 1 | 31163 | 10 |00:00:00.01 | 15 |
|* 7 | INDEX RANGE SCAN | IDX_OWNER_NAME_ID | 1 | 31163 | 10 |00:00:00.01 | 5 |
--------------------------------------------------------------------------------------------------------------
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)从执行计划中看到，没有sort order by。逻辑读也才15个，说明执行计划非常理想。
尝试创建以下索引：
SQL> create index idx_owner_id_name on test(owner,object_id,object_name);
Index created.
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 4ypy16hfrrfv5, child number 0
-------------------------------------
select * from (select * from (select a.*, rownum rn
from (select /*+index(test idx_owner_id_name)*/
* from test
where owner = 'SYS' and object_id > 1000
order by object_name) a) where rownum
<= 10) where rn >= 1
Plan hash value: 2498002320
---------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
---------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 10 |00:00:00.04 | 1003 | 191 | | | |
|* 1 | VIEW | | 1 | 10 | 10 |00:00:00.04 | 1003 | 191 | | | |
|* 2 | COUNT STOPKEY | | 1 | | 10 |00:00:00.04 | 1003 | 191 | | | |
| 3 | VIEW | | 1 | 31163 | 10 |00:00:00.04 | 1003 | 191 | | | |
| 4 | COUNT | | 1 | | 10 |00:00:00.04 | 1003 | 191 | | | |
| 5 | VIEW | | 1 | 31163 | 10 |00:00:00.04 | 1003 | 191 | | | |
| 6 | SORT ORDER BY | | 1 | 31163 | 10 |00:00:00.04 | 1003 | 191 | 4376K| 884K| 3889K (0)|
| 7 | TABLE ACCESS BY INDEX ROWID| TEST | 1 | 31163 | 29945 |00:00:00.03 | 1003 | 191 | | | |
|* 8 | INDEX RANGE SCAN | IDX_OWNER_ID_NAME | 1 | 31163 | 29945 |00:00:00.02 | 189 | 191 | | | |
---------------------------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("RN">=1)
2 - filter(ROWNUM<=10)
8 - access("OWNER"='SYS' AND "OBJECT_ID">1000 AND "OBJECT_ID" IS NOT NULL)执行计划中有sort order by，说明没有用到索引已经排序特性，而且逻辑读为1003个，说明执行计划是错误的。
这是因为该分页语句是根据object_name进行排序的，在创建索引的时候是按照owner，object_id，object_name顺序创建索引的。
按照索引排序的方式从表中取出5条数据观察：
SQL> select *
2 from (select rownum rn, owner, object_id, object_name
3 from test
4 where owner = 'SYS'
5 and object_id > 1000
6 order by owner, object_id, object_name)
7 where rownum <= 5;
RN OWNER OBJECT_ID OBJECT_NAME
---------- ----- ---------- ---------------
1 SYS 1001 NOEXP$
2 SYS 1002 EXPPKGOBJ$
3 SYS 1003 I_OBJTYPE
4 SYS 1004 EXPPKGACT$
5 SYS 1005 I_ACTPACKAGE在这前5条数据中，我们按照分页语句排序条件object_name进行排序，应该是第4行数据显示为第一行数据，但是它在索引中排到了第4行，所以索引中数据的顺序并不能满足分页语句中的排序要求，这就产生了sort order by，进而导致执行计划出错。
按照正确的owner，object_name，object_id顺序创建索引时，索引中前5条数据：
SQL> select *
2 from (select rownum rn, owner, object_id, object_name
3 from test
4 where owner = 'SYS'
5 and object_id > 1000
6 order by owner, object_name, object_id)
7 where rownum <= 5;
RN OWNER OBJECT_ID OBJECT_NAME
---------- ----- ---------- ----------------------------------------
1 SYS 34042 /1000323d_DelegateInvocationHa
2 SYS 44844 /1000e8d1_LinkedHashMapValueIt
3 SYS 23397 /1005bd30_LnkdConstant
4 SYS 19737 /10076b23_OraCustomDatumClosur
5 SYS 45460 /100c1606_StandardMidiFileRead索引中的数据顺序完全符合分页语句中的排序要求，这就不需要进行sort order by，所以执行计划是正确的。
