Oracle分页优化

分页语句优化


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;








优化前：
select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));


PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  9w6p4hu1q1z4n, 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: 3486388599


------------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                        | Name       | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
------------------------------------------------------------------------------------------------------------------------------------
|*  1 |  VIEW                            |            |      1 |     20 |     20 |00:00:01.28 |     739 |       |       |          |
|*  2 |   COUNT STOPKEY                  |            |      1 |        |     20 |00:00:01.28 |     739 |       |       |          |
|   3 |    VIEW                          |            |      1 |  22595 |     20 |00:00:01.28 |     739 |       |       |          |
|*  4 |     SORT ORDER BY STOPKEY        |            |      1 |  22595 |     20 |00:00:01.28 |     739 |  1234K|   574K| 1096K (0)|
|   5 |      COUNT                       |            |      1 |        |  22130 |00:00:01.22 |     739 |       |       |          |
|   6 |       TABLE ACCESS BY INDEX ROWID| PAGE       |      1 |  22595 |  22130 |00:00:01.22 |     739 |       |       |          |
|*  7 |        INDEX RANGE SCAN          | IDX_PAGE_3 |      1 |  22595 |  22130 |00:00:01.20 |     150 |       |       |          |
------------------------------------------------------------------------------------------------------------------------------------


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


   1 - filter("RN">=0)
   2 - filter(ROWNUM<=20)
   4 - filter(ROWNUM<=20)
   7 - access("OBJECT_ID">1000 AND "OWNER"='SYS' AND "OBJECT_ID" IS NOT NULL)
       filter("OWNER"='SYS')
  
  
  
  
优化后：
现在加上INDEX_DESC HINT


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


PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------
SQL_ID  5tqfh5zknzyfh, 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 |
---------------------------------------------------------------------------------------------------------
|*  1 |  VIEW                            |            |      1 |     20 |     20 |00:00:00.01 |       7 |
|*  2 |   COUNT STOPKEY                  |            |      1 |        |     20 |00:00:00.01 |       7 |
|   3 |    VIEW                          |            |      1 |  22595 |     20 |00:00:00.01 |       7 |
|   4 |     COUNT                        |            |      1 |        |     20 |00:00:00.01 |       7 |
|   5 |      TABLE ACCESS BY INDEX ROWID | PAGE       |      1 |  22595 |     20 |00:00:00.01 |       7 |
|*  6 |       INDEX RANGE SCAN DESCENDING| IDX_PAGE_3 |      1 |  22595 |     20 |00:00:00.01 |       3 |
---------------------------------------------------------------------------------------------------------


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')


现在ORACLE降序扫描了索引，扫描20条记录就停止了，这次消耗的逻辑读比之前消耗的都小100倍，所以分页最好用HINT INDEX_DESC/INDEX_ASC








ORDER BY  多个表 的列 ---分页语句 无法 优化
select * from a, b where ..... order by a.xx ;
这种 分页语句 优化 就让 A 作为驱动表
走NL 去驱动B




select ... from a left join b where .... order by b
这个分页 咋搞
外连接 B 无法 作为驱动表
只能改SQL
改成 ORDER BY A 不要ORDER BY B




执行计划出现 SORT ORDER BY STOPKEY 绝对不行 分页绝对慢










总结：
1、要看 分页语句 写法 有问题没
分页语句一定要包一层(套两层),否则无法STOP，意思是只扫描一页就停止，否则会有 STOPKEY
2、看ORDER BY 的 列 
   要看 ORDER BY 的 列 出现在 1个表 还是多个表
   出现在 1个表上面 要看 是不是 有外连接
   Order BY 的表要作为驱动表，要在Order BY列上建立组合索引，
3.  ORDER BY 的列 全都 包含在 索引里面 并且  列顺序不能颠倒
   where 选择性 很低的 就他放后面
   where 列 放前面 走的是 ？
   index range scan .......
   where 列放后面 走的是 ？
   INDEX FULL SCAN ....