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问题:
SYS@proc> create table t as select * from v$parameter;
Table created.
SYS@proc> select value from t where name='db_block_size' and to_number(value)=8192;
VALUE
--------------------------------------------------------------------------------
8192
SYS@proc> select value from v$parameter where name='db_block_size' and to_number(value)=8192;
select value from v$parameter where name='db_block_size' and to_number(value)=8192
*
ERROR at line 1:
ORA-01722: invalid number
为什么语句“select value from t where name='db_block_size' and to_number(value)=8192;”执行成功,换成v$parameter却报错。
实验研究过程:
SYS@proc> set autotrace on
SYS@proc> analyze table t compute statistics;
Table analyzed.
SYS@proc> select value from t where name='db_block_size' and to_number(value)=8192;
VALUE
--------------------------------------------------------------------------------
8192
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 26 | 4 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T | 1 | 26 | 4 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("NAME"='db_block_size' AND TO_NUMBER("VALUE")=8192)
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
9 consistent gets
0 physical reads
0 redo size
525 bytes sent via SQL*Net to client
523 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
一开始看到这个执行计划很懵逼,完全搞不懂为什么能够执行成功,做10046,10053,改写sql加hint还是搞不懂。
最后猜想Oracle在filter("NAME"='db_block_size' AND TO_NUMBER("VALUE")=8192)这个步骤,是先对数据做name='db_block_size'的过滤,在做to_number('value')=8192的过滤。
若是能将谓词信息改变成filter(TO_NUMBER("VALUE") AND "NAME"='db_block_size'=8192)并且执行报错,那么猜想就是正确的。
尝试将sql语句的and条件调换位置"select value from t whereto_number(value)=8192 and name='db_block_size';",不过还是和原来一样,这里省略步骤。
这里构造其他测试表:
SYS@proc> create table a(id1 int,id2 int,id3 int,id4 int);
Table created.
SYS@proc> insert into a values(1,1,1,0);
1 row created.
SYS@proc> commit;
Commit complete.
SYS@proc> select * from a;
ID1 ID2 ID3 ID4
---------- ---------- ---------- ----------
1 1 1 0
这里执行以下4条sql语句:
①Select 'ok' From aaa where id1/id2=1 and id3/id4=2;
②Select 'ok' From aaa where id1/id2=2 and id3/id4=2;
③Select 'ok' From aaa where id3/id4=2 and id1/id2=1;
④Select 'ok' From aaa where id3/id4=2 and id1/id2=2;
其中①和③,②和④只是where后条件位置互换而已。
查看执行结果:
SYS@proc> Select 'ok' From aaa where id1/id2=1 and id3/id4=2;
Select 'ok' From aaa where id1/id2=1 and id3/id4=2
*
ERROR at line 1:
ORA-01476: divisor is equal to zero
SYS@proc> Select 'ok' From aaa where id1/id2=2 and id3/id4=2;
no rows selected
SYS@proc> Select 'ok' From aaa where id3/id4=2 and id1/id2=1;
Select 'ok' From aaa where id3/id4=2 and id1/id2=1
*
ERROR at line 1:
ORA-01476: divisor is equal to zero
SYS@proc> Select 'ok' From aaa where id3/id4=2 and id1/id2=2;
Select 'ok' From aaa where id3/id4=2 and id1/id2=2
*
ERROR at line 1:
ORA-01476: divisor is equal to zero
②和④只是位置不同,但是一个却正常执行,一个却报错了。
这里查看两条sql的执行计划:
SYS@proc> explain plan for Select 'ok' From aaa where id1/id2=2 and id3/id4=2;
Explained.
SYS@proc> select * from table(dbms_xplan.display());
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 864433273
-----------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 12 | 2 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| AAA | 1 | 12 | 2 (0)| 00:00:01 |
-----------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
1 - filter("ID1"/"ID2"=2 AND "ID3"/"ID4"=2)
13 rows selected.
SYS@proc> explain plan for Select 'ok' From aaa where id3/id4=2 and id1/id2=2;
Explained.
SYS@proc> select * from table(dbms_xplan.display());
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 864433273
------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 12 | 2 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| AAA | 1 | 12 | 2 (0)| 00:00:01 |
------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
1 - filter("ID3"/"ID4"=2 AND "ID1"/"ID2"=2)
13 rows selected.
这里对比谓词信息刚好是两个位置不同,导致执行结果不一样。
正好说明上边的问题的猜想:
最后猜想Oracle在filter("NAME"='db_block_size' AND TO_NUMBER("VALUE")=8192)这个步骤,是先对数据做name='db_block_size'的过滤,在做to_number('value')=8192的过滤。
若是能将谓词信息改变成filter(TO_NUMBER("VALUE") AND "NAME"='db_block_size'=8192)并且执行报错,那么猜想就是正确的。
由此问题解决。
其他:
SYS@proc> create table test (id int);
Table created.
SYS@proc> insert into test values(null);
1 row created.
SYS@proc> commit;
Commit complete.
SYS@proc> select * from test;
ID
----------
SYS@proc> set autotrace on
SYS@proc> select value from t,test a where a.id||name='db_block_size' and to_number(a.id||t.value)=8192;
VALUE
--------------------------------------------------------------------------------
8192
Execution Plan
----------------------------------------------------------
Plan hash value: 423998170
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 39 | 6 (0)| 00:00:01 |
| 1 | NESTED LOOPS | | 1 | 39 | 6 (0)| 00:00:01 |
| 2 | TABLE ACCESS FULL| TEST | 1 | 13 | 2 (0)| 00:00:01 |
|* 3 | TABLE ACCESS FULL| T | 1 | 26 | 4 (0)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter(TO_CHAR("A"."ID")||"NAME"='db_block_size' AND
TO_NUMBER(TO_CHAR("A"."ID")||"T"."VALUE")=8192)
Note
-----
- dynamic sampling used for this statement (level=2)
Statistics
----------------------------------------------------------
32 recursive calls
0 db block gets
28 consistent gets
0 physical reads
0 redo size
525 bytes sent via SQL*Net to client
523 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
4 sorts (memory)
0 sorts (disk)
1 rows processed
SYS@proc> set autotrace off
SYS@proc> select value from t,test a where to_number(a.id||t.value)=8192 and a.id||name='db_block_size';
select value from t,test a where to_number(a.id||t.value)=8192 and a.id||name='db_block_size'
*
ERROR at line 1:
ORA-01722: invalid number
SYS@proc> explain plan for select value from t,test a where to_number(a.id||t.value)=8192 and a.id||name='db_block_size';
Explained.
SYS@proc> select * from table(dbms_xplan.display());
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 423998170
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 39 | 6 (0)| 00:00:01 |
| 1 | NESTED LOOPS | | 1 | 39 | 6 (0)| 00:00:01 |
| 2 | TABLE ACCESS FULL| TEST | 1 | 13 | 2 (0)| 00:00:01 |
|* 3 | TABLE ACCESS FULL| T | 1 | 26 | 4 (0)| 00:00:01 |
---------------------------------------------------------------------------
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter(TO_NUMBER(TO_CHAR("A"."ID")||"T"."VALUE")=8192 AND
TO_CHAR("A"."ID")||"NAME"='db_block_size')
16 rows selected.
所以where后边条件的执行顺序,实际上和执行计划谓词信息的顺序有关,和where的位置无关。
网上有些在10g做实验得出结论是从右到左,在11g里边,按照相同步骤执行并得不出相同结论。
问题延伸:
filter("NAME"='db_block_size' AND TO_NUMBER("VALUE")=8192),这里是一次性扫描出全部数据在进行过滤,还是一行一行获取在判断的。
延伸链接:http://blog.itpub.net/30174570/viewspace-2149212/
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