select * from ( select rownum as rn ,a.* from ( select
* from t100 a
order by object_id desc )a where rownum<=40 ) a where rn>=1
SQL>
SQL> select count(*) from t100;
COUNT(*)
----------
3020896
BEGIN
DBMS_STATS.GATHER_TABLE_STATS(ownname => 'SYSTEM',
tabname => 'T100',
estimate_percent => 100,
method_opt => 'for all columns size repeat',
no_invalidate => FALSE,
degree => 8,
cascade => TRUE);
END;
---高级执行计划:
11G：
alter session set statistics_level=all; ---再运行SQL
select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));
select *
from (select rownum as rn, a.*
from (select * from t100 a order by object_id desc) a
where rownum <= 40) a
where rn >= 1
SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));
PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-----------------------
SQL_ID 1h6g4u7aa6sqd, child number 1
-------------------------------------
select * from (select rownum as rn, a.* from (select * from
t100 a order by object_id desc) a where rownum <= 40) a where
rn >= 1
Plan hash value: 3078193190
-------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
-------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 40 |00:00:01.44 | 42846 | 111 | | | |
|* 1 | VIEW | | 1 | 40 | 40 |00:00:01.44 | 42846 | 111 | | | |
|* 2 | COUNT STOPKEY | | 1 | | 40 |00:00:01.44 | 42846 | 111 | | | |
| 3 | VIEW | | 1 | 3020K| 40 |00:00:01.44 | 42846 | 111 | | | |
|* 4 | SORT ORDER BY STOPKEY| | 1 | 3020K| 40 |00:00:01.44 | 42846 | 111 | 1186K| 567K| 1054K (0)|
| 5 | TABLE ACCESS FULL | T100 | 1 | 3020K| 3020K|00:00:00.46 | 42846 | 111 | | | |
-------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("RN">=1)
2 - filter(ROWNUM<=40)
4 - filter(ROWNUM<=40)
已选择26行。
分页SQL返回40条记录,实际确需要访问 3020K 记录,显然不合理
创建索引:
create index t100_idx1 on t100(object_id);
select * from ( select rownum as rn ,a.* from ( select
* from t100 a
order by object_id desc )a where rownum<=40 ) a where rn>=1
SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));
PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-----------------------
SQL_ID 2umgv7yv9y4vu, child number 0
-------------------------------------
select * from (select rownum as rn, a.* from (select * from
t100 a order by object_id desc) a where rownum <= 40) a where
rn >= 1
Plan hash value: 3078193190
----------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 40 |00:00:01.54 | 42846 | | | |
|* 1 | VIEW | | 1 | 40 | 40 |00:00:01.54 | 42846 | | | |
|* 2 | COUNT STOPKEY | | 1 | | 40 |00:00:01.54 | 42846 | | | |
| 3 | VIEW | | 1 | 3020K| 40 |00:00:01.54 | 42846 | | | |
|* 4 | SORT ORDER BY STOPKEY| | 1 | 3020K| 40 |00:00:01.54 | 42846 | 1186K| 567K| 1054K (0)|
| 5 | TABLE ACCESS FULL | T100 | 1 | 3020K| 3020K|00:00:00.51 | 42846 | | | |
----------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("RN">=1)
2 - filter(ROWNUM<=40)
4 - filter(ROWNUM<=40)
已选择26行。
没有走索引
SQL> select *
from (select rownum as rn, a.*
from (select /*+ index(a t100_idx1)*/ * from t100 a order by object_id desc) a
where rownum <= 40) a
where rn >= 1
2 3 4 5 6 ;
已选择40行。
执行计划
----------------------------------------------------------
Plan hash value: 3078193190
-----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 40 | 8800 | | 78467 (1)| 00:15:42 |
|* 1 | VIEW | | 40 | 8800 | | 78467 (1)| 00:15:42 |
|* 2 | COUNT STOPKEY | | | | | | |
| 3 | VIEW | | 3020K| 596M| | 78467 (1)| 00:15:42 |
|* 4 | SORT ORDER BY STOPKEY| | 3020K| 279M| 386M| 78467 (1)| 00:15:42 |
| 5 | TABLE ACCESS FULL | T100 | 3020K| 279M| | 11645 (1)| 00:02:20 |
-----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("RN">=1)
2 - filter(ROWNUM<=40)
4 - filter(ROWNUM<=40)
SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));
PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-----------------------
SQL_ID 1jx0gdw60vc3n, child number 1
-------------------------------------
select * from (select rownum as rn, a.* from (select /*+
index(a t100_idx1)*/ * from t100 a where a.object_id is not null order
by object_id desc) a where rownum <= 40) a where rn >= 1
Plan hash value: 2711774789
----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 40 |00:00:06.25 | 3027K| | | |
|* 1 | VIEW | | 1 | 40 | 40 |00:00:06.25 | 3027K| | | |
|* 2 | COUNT STOPKEY | | 1 | | 40 |00:00:06.25 | 3027K| | | |
| 3 | VIEW | | 1 | 3020K| 40 |00:00:06.25 | 3027K| | | |
|* 4 | SORT ORDER BY STOPKEY | | 1 | 3020K| 40 |00:00:06.25 | 3027K| 149M| 4122K| 132M (0)|
| 5 | TABLE ACCESS BY INDEX ROWID| T100 | 1 | 3020K| 3020K|00:00:04.33 | 3027K| | | |
|* 6 | INDEX FULL SCAN | T100_IDX1 | 1 | 3020K| 3020K|00:00:00.78 | 6697 | | | |
----------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("RN">=1)
2 - filter(ROWNUM<=40)
4 - filter(ROWNUM<=40)
6 - filter("A"."OBJECT_ID" IS NOT NULL)
已选择28行。
此时走索引了,但是仍旧访问了3020K ,显然不够优化
SQL> select *
from (select rownum as rn, a.*
from (select /*+ index_desc(a t100_idx1)*/ * from t100 a where a.object_id is not null order by object_id desc) a
where rownum <= 40) a
where rn >= 1 2 3 4 5
6 ;
SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));
PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-----------------------
SQL_ID 9pv0wyn2mk187, child number 0
-------------------------------------
select * from (select rownum as rn, a.* from (select /*+
index_desc(a t100_idx1)*/ * from t100 a where a.object_id is not null
order by object_id desc) a where rownum <= 40) a where rn >= 1
Plan hash value: 4052825412
---------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads |
---------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 40 |00:00:00.01 | 46 | 1 |
|* 1 | VIEW | | 1 | 40 | 40 |00:00:00.01 | 46 | 1 |
|* 2 | COUNT STOPKEY | | 1 | | 40 |00:00:00.01 | 46 | 1 |
| 3 | VIEW | | 1 | 40 | 40 |00:00:00.01 | 46 | 1 |
| 4 | TABLE ACCESS BY INDEX ROWID| T100 | 1 | 40 | 40 |00:00:00.01 | 46 | 1 |
|* 5 | INDEX FULL SCAN DESCENDING| T100_IDX1 | 1 | 3020K| 40 |00:00:00.01 | 6 | 1 |
---------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("RN">=1)
2 - filter(ROWNUM<=40)
5 - filter("A"."OBJECT_ID" IS NOT NULL)
已选择26行。
此时访问了40条记录,是最优化的
SORT ORDER BY STOPKEY
Sorts a subset of the result according to the order by clause. Used for top-N queries if pipelined execution is not possible.
排序结果的子集根据order by子句
SORT ORDER BY STOPKEY 是指:
排序过后取数据
走索引后返回 3020K|条
是要对3020K|排序,所以效率低