分页技术总结

create table page as select * from dba_objects; 

create index idx_page on page(object_id);
create index idx_page_1 on page(owner,object_id);
create index idx_page_2 on page(owner);
create index idx_page_3 on page(object_id,owner);


BEGIN
  DBMS_STATS.GATHER_TABLE_STATS(ownname          => 'SCOTT',
                                tabname          => 'PAGE',
                                estimate_percent => 100,
                                method_opt       => 'for all columns size skewonly',
                                no_invalidate    => FALSE,
                                degree           => 4,
                                cascade          => TRUE);
END;


select * from 
(
select * from 
(
select  a.*,rownum rn
  from page a 
 where object_id >1000 and owner='SYS'
 order by object_id desc
) where rownum<=20
) where rn>=0;



分页语句必须要有order by ,order by 哪个列 哪个列必须有索引

只要 分页有 sort order by  绝对是错的

肯定没走索引


SQL> select * from 
(
select * from 
(
select  a.*,rownum rn
  from page a 
 where object_id >1000 and owner='SYS'
 order by object_id desc
) where rownum<=20
) where rn>=0;  2    3    4    5    6    7    8    9   10  

20 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 824209635

-----------------------------------------------------------------------------------------
| Id  | Operation		 | Name | Rows	| Bytes |TempSpc| Cost (%CPU)| Time	|
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT	 |	|    20 |  4400 |	|   910   (1)| 00:00:11 |
|*  1 |  VIEW			 |	|    20 |  4400 |	|   910   (1)| 00:00:11 |
|*  2 |   COUNT STOPKEY 	 |	|	|	|	|	     |		|
|   3 |    VIEW 		 |	| 30424 |  6536K|	|   910   (1)| 00:00:11 |
|*  4 |     SORT ORDER BY STOPKEY|	| 30424 |  2881K|  4000K|   910   (1)| 00:00:11 |
|   5 |      COUNT		 |	|	|	|	|	     |		|
|*  6 |       TABLE ACCESS FULL  | PAGE | 30424 |  2881K|	|   234   (1)| 00:00:03 |
-----------------------------------------------------------------------------------------

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

   1 - filter("RN">=0)
   2 - filter(ROWNUM<=20)
   4 - filter(ROWNUM<=20)
   6 - filter("OWNER"='SYS' AND "OBJECT_ID">1000)


Statistics
----------------------------------------------------------
       1153  recursive calls
	  0  db block gets
       1336  consistent gets
       1036  physical reads
	  0  redo size
       3175  bytes sent via SQL*Net to client
	431  bytes received via SQL*Net from client
	  3  SQL*Net roundtrips to/from client
	 66  sorts (memory)
	  0  sorts (disk)
	 20  rows processed


分页语句:
1.必须有orderby 

2.order by 哪个列,那个列就必须要有索引,利用索引已排序的特点。

3.分页有 sort order by 重新对数据进行排序

select * from 
(
select * from 
(
select  /*+ index(a) */  a.*,rownum rn
  from page a 
 where object_id >1000 and owner='SYS'
 order by object_id desc
) where rownum<=20
) where rn>=0; 


SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));

PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------------------------------------

-------------------------------------------------------------------------------------
SQL_ID	4tub4ucyfwk79, child number 0
-------------------------------------
select * from ( select * from ( select	/*+ index(a) */  a.*,rownum rn
 from page a  where object_id >1000 and owner='SYS'  order by object_id
desc ) where rownum<=20 ) where rn>=0

Plan hash value: 4010810952

-------------------------------------------------------------------------------------------------------------------

-----------------
| Id  | Operation			 | Name       | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  

1Mem | Used-Mem |
-------------------------------------------------------------------------------------------------------------------

-----------------
|   0 | SELECT STATEMENT		 |	      |      1 |	|     20 |00:00:00.83 |     865 |	|	

|	   |
|*  1 |  VIEW				 |	      |      1 |     20 |     20 |00:00:00.83 |     865 |	|	

|	   |
|*  2 |   COUNT STOPKEY 		 |	      |      1 |	|     20 |00:00:00.83 |     865 |	|	

|	   |
|   3 |    VIEW 			 |	      |      1 |  30424 |     20 |00:00:00.83 |     865 |	|	

|	   |
|*  4 |     SORT ORDER BY STOPKEY	 |	      |      1 |  30424 |     20 |00:00:00.83 |     865 |  1824K|   

650K| 1621K (0)|
|   5 |      COUNT			 |	      |      1 |	|  29905 |00:00:00.65 |     865 |	|	

|	   |
|*  6 |       TABLE ACCESS BY INDEX ROWID| PAGE       |      1 |  30424 |  29905 |00:00:00.48 |     865 |	|	

|	   |
|*  7 |        INDEX RANGE SCAN 	 | IDX_PAGE_2 |      1 |  30835 |  30835 |00:00:00.10 |      66 |	|	

|	   |
-------------------------------------------------------------------------------------------------------------------

-----------------

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

   1 - filter("RN">=0)
   2 - filter(ROWNUM<=20)
   4 - filter(ROWNUM<=20)
   6 - filter("OBJECT_ID">1000)
   7 - access("OWNER"='SYS')


30 rows selected.


SORT ORDER BY STOPKEY 也是错的


select * from 
(
select * from 
(
select   /*+ index_desc(a idx_page_3) */    a.*,rownum rn
  from page a 
 where object_id >1000 and owner='SYS'
 order by object_id desc
) where rownum<=20
) where rn>=0; 

SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));

PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------------------------------------

-------------------------------------------------------------------------------------
SQL_ID	8vy3s6f4nh4ga, child number 0
-------------------------------------
select * from ( select * from ( select	 /*+ index_desc(a idx_page_3)
*/    a.*,rownum rn   from page a  where object_id >1000 and
owner='SYS'  order by object_id desc ) where rownum<=20 ) where rn>=0

Plan hash value: 3526010999

------------------------------------------------------------------------------------------------------------------
| Id  | Operation			 | Name       | Starts | E-Rows | A-Rows |   A-Time   | Buffers | Reads  |
------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT		 |	      |      1 |	|     20 |00:00:00.01 |       9 |      2 |
|*  1 |  VIEW				 |	      |      1 |     20 |     20 |00:00:00.01 |       9 |      2 |
|*  2 |   COUNT STOPKEY 		 |	      |      1 |	|     20 |00:00:00.01 |       9 |      2 |
|   3 |    VIEW 			 |	      |      1 |  30424 |     20 |00:00:00.01 |       9 |      2 |
|   4 |     COUNT			 |	      |      1 |	|     20 |00:00:00.01 |       9 |      2 |
|   5 |      TABLE ACCESS BY INDEX ROWID | PAGE       |      1 |  30424 |     20 |00:00:00.01 |       9 |      2 |
|*  6 |       INDEX RANGE SCAN DESCENDING| IDX_PAGE_3 |      1 |  30424 |     20 |00:00:00.01 |       4 |      2 |
------------------------------------------------------------------------------------------------------------------

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

   1 - filter("RN">=0)
   2 - filter(ROWNUM<=20)
   6 - access("OBJECT_ID" IS NOT NULL AND "OBJECT_ID">1000 AND "OWNER"='SYS')
       filter("OWNER"='SYS')


28 rows selected.


多表关联后分页:

select ... from a,b where a.id=b.id order by a.xx;

a当驱动表 nl b

分页SQL必须走NL循环 order by 谁 谁当驱动表

1. 我们把它当成只 从a出数据


2.值给a.xx建索引

3.被驱动表 b b.id建索引 




select ... from a,b where a.id=b.id order by a.xx，b.xxx;
分页的 每页 取 20条

这种SQL没法优化,order by 2个表没法优化


select ... from a,b where a.id=b.id gropu by ...order by  分页的 每页 取 20条


分页语句不能有group by ，distinct操作


select ... from a left join b where ... order by b


外链接 左链接 驱动表只能是a 那么order by a表
 


select ... from a where 过滤条件 order by 另外一个列

按where 条件列+order by 列创建索引



select ... from a where owner='SB' and xx like '%jj%'  order by money

这个时候建索引:money,owner,xx


分页语句的优化技巧：

1.分页SQL要想快最好走索引，根据order by asc/desc ，用hint index_asc/index_desc 
  强制它走索引INDEX FULL SCAN  DESCENDING/INDEX RANGE SCAN DESCENDING

2.不要让Oracle扫描整个index，确保只扫描一部分索引块，然后回表就取得数据，然后
  就count stopkey

3.一定要确保索引回表的时候不要在表上再去过滤，也就是说TABLE ACCESS BY INDEX ROWID
  上面没*,不然就可能扫描整个索引块，根据where条件可以创建组合索引
  (分区表要创建global index)

4.看执行计划的时候不要被执行计划的基数给骗了----它可能显示是有误差的