不要制造不必要的混乱把优化器给弄糊涂了

一.10.2.0.1下(其它版本没有测试过)
SQL> explain plan for SELECT PRODUCT_ID,COUNT(PRODUCT_ID) FROM RES_ARTICLE_INFO WHERE NVL(PRODUCT_ID,0)>0 GROUP BY PRODUCT_ID;
SQL> select * from table(dbms_xplan.display());
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 48838 | 143K| 39247 (2)| 00:07:51 |
| 1 | HASH GROUP BY | | 48838 | 143K| 39247 (2)| 00:07:51 |
|* 2 | TABLE ACCESS FULL| RES_ARTICLE_INFO | 117K| 343K| 39236 (2)| 00:07:51 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter(NVL("PRODUCT_ID",0)>0)

SQL> explain plan for SELECT /*+ index_ffs(a IND_ARTINFO_PROD_ID) */PRODUCT_ID,COUNT(PRODUCT_ID)
FROM RES_ARTICLE_INFO a
WHERE NVL(PRODUCT_ID,0)>0
GROUP BY PRODUCT_ID;
SQL> select * from table(dbms_xplan.display());
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 48838 | 143K| 39247 (2)| 00:07:51 |
| 1 | HASH GROUP BY | | 48838 | 143K| 39247 (2)| 00:07:51 |
|* 2 | TABLE ACCESS FULL| RES_ARTICLE_INFO | 117K| 343K| 39236 (2)| 00:07:51 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter(NVL("PRODUCT_ID",0)>0)
我这里强制加上提示也不走这个执行路径,说明优化器认为这个执行路径不在考虑范围内.

SQL> explain plan for SELECT PRODUCT_ID,COUNT(PRODUCT_ID) FROM RES_ARTICLE_INFO WHERE PRODUCT_ID>0 GROUP BY PRODUCT_ID;
SQL> select * from table(dbms_xplan.display());
---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 17938 | 53814 | 495 (20)| 00:00:06 |
| 1 | HASH GROUP BY | | 17938 | 53814 | 495 (20)| 00:00:06 |
|* 2 | INDEX FAST FULL SCAN| IND_ARTINFO_PROD_ID | 764K| 2240K| 415 (5)| 00:00:05 |
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("PRODUCT_ID">0)

这里PRODUCT_ID确实是可以为空的,存在索引index IND_ARTINFO_PROD_ID on RES_ARTICLE_INFO(product_id)
其实NVL(PRODUCT_ID,0)>0 和PRODUCT_ID>0 是完全等价的,但这里显然优化器没有意识到这一点,对于NVL(PRODUCT_ID,0)>0 这样的写法,它认为你会访问product为null的数据行,所以它选择了全表扫描,而没有像后者那样走了索引的快速全扫描.

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二.10.2.0.4下
create table zsj_test
as select owner,object_id,object_name,object_type,to_date('2000-01-01','yyyy-mm-dd')+rownum last_ddl_time
from all_objects
where rownum<=50000;

alter table zsj_test modify(last_ddl_time not null); --last_ddl_time上是有not null约束的,并且这个列上有索引
create index ind1_zsj_test on zsj_test(last_ddl_time);
exec dbms_stats.gather_table_stats(user,'ZSJ_TEST',cascade => TRUE,estimate_percent => 100,method_opt => 'FOR ALL INDEXED COLUMNS SIZE 1');

SELECT * FROM
(
SELECT OWNER,OBJECT_ID,OBJECT_NAME,OBJECT_TYPE,LAST_DDL_TIME FROM ZSJ_TEST ORDER BY LAST_DDL_TIME DESC
)
WHERE ROWNUM<=10;

-----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 670 | 3 (0)| 00:00:01 |
|* 1 | COUNT STOPKEY | | | | | |
| 2 | VIEW | | 10 | 670 | 3 (0)| 00:00:01 |
| 3 | TABLE ACCESS BY INDEX ROWID| ZSJ_TEST | 50000 | 2490K| 3 (0)| 00:00:01 |
| 4 | INDEX FULL SCAN DESCENDING| IND1_ZSJ_TEST | 10 | | 2 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<=10)
Statistics
----------------------------------------------------------
6 consistent gets
10 rows processed
可以通过索引的降序全扫描避免排序的.

SELECT * FROM
(
SELECT OWNER,OBJECT_ID,OBJECT_NAME,OBJECT_TYPE,LAST_DDL_TIME FROM ZSJ_TEST T ORDER BY LAST_DDL_TIME DESC NULLS LAST
)
WHERE ROWNUM<=10; --这里画蛇添足添加了一个毫无必要的nulls last

--------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 670 | | 730 (1)| 00:00:09 |
|* 1 | COUNT STOPKEY | | | | | | |
| 2 | VIEW | | 50000 | 3271K| | 730 (1)| 00:00:09 |
|* 3 | SORT ORDER BY STOPKEY| | 50000 | 2490K| 6680K| 730 (1)| 00:00:09 |
| 4 | TABLE ACCESS FULL | ZSJ_TEST | 50000 | 2490K| | 91 (2)| 00:00:02 |
--------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<=10)
3 - filter(ROWNUM<=10)
Statistics
----------------------------------------------------------
395 consistent gets
10 rows processed
你可以看到它变成全表扫描后排序的操作了

SELECT * FROM
(
SELECT /*+ index_desc(t)*/OWNER,OBJECT_ID,OBJECT_NAME,OBJECT_TYPE,LAST_DDL_TIME FROM ZSJ_TEST T ORDER BY LAST_DDL_TIME DESC NULLS LAST
)
WHERE ROWNUM<=10;

--------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 670 | | 1167 (1)| 00:00:14 |
|* 1 | COUNT STOPKEY | | | | | | |
| 2 | VIEW | | 50000 | 3271K| | 1167 (1)| 00:00:14 |
|* 3 | SORT ORDER BY STOPKEY | | 50000 | 2490K| 6680K| 1167 (1)| 00:00:14 |
| 4 | TABLE ACCESS BY INDEX ROWID| ZSJ_TEST | 50000 | 2490K| | 528 (1)| 00:00:07 |
| 5 | INDEX FULL SCAN DESCENDING| IND1_ZSJ_TEST | 50000 | | | 135 (1)| 00:00:02 |
--------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<=10)
3 - filter(ROWNUM<=10)
Statistics
----------------------------------------------------------
526 consistent gets
10 rows processed
强制走索引降序扫描,可也避免不了排序操作,逻辑读更大了.

SELECT * FROM
(
SELECT /*+ index(t)*/OWNER,OBJECT_ID,OBJECT_NAME,OBJECT_TYPE,LAST_DDL_TIME FROM ZSJ_TEST T ORDER BY LAST_DDL_TIME DESC NULLS LAST
)
WHERE ROWNUM<=10;

--------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 670 | | 1167 (1)| 00:00:14 |
|* 1 | COUNT STOPKEY | | | | | | |
| 2 | VIEW | | 50000 | 3271K| | 1167 (1)| 00:00:14 |
|* 3 | SORT ORDER BY STOPKEY | | 50000 | 2490K| 6680K| 1167 (1)| 00:00:14 |
| 4 | TABLE ACCESS BY INDEX ROWID| ZSJ_TEST | 50000 | 2490K| | 528 (1)| 00:00:07 |
| 5 | INDEX FULL SCAN | IND1_ZSJ_TEST | 50000 | | | 135 (1)| 00:00:02 |
--------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<=10)
3 - filter(ROWNUM<=10)
Statistics
----------------------------------------------------------
526 consistent gets
10 rows processed

这里因为一个画蛇添足的nulls last(字段上有not null约束的),优化器放弃了最优的执行计划.

这些都是工作中实际碰到的问题,你当然可以说是因为优化器不足够智能.可问题是我们写出这样的语句的时候,我们是否足够清醒,知道自己在干什么呢?优化器不足够智能,这个我们必须承认,我们也改变不了,我们能做的就是意识到优化器还不足够智能,所以我们在写语句的时候,要保持足够的清醒,知道自己在干什么,不要制造不必要的混乱把优化器给弄糊涂了

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其实http://blog.csdn.net/zhaosj1726/archive/2010/12/05/6056347.aspx 这里提到的sql2无法通过索引避免排序也是同样的问题,没有意识到排序字段中使用的是列的别名.

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以后工作中碰到了类似需要注意的问题,也会补充到这里的.