谓词推入导致的数据不一致案例
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2022-05-29 10:34:55
现象 下面的语句， 能查到deviceid 为DEV005M0的记录， 但是加上deviceid = 'DEV005M0'这个条件后， 查询语句查不出结果了。 语句如下： 当注释掉deviceid = 'DEV005M0'， 查询结果如下： 当增加deviceid = 'DEV005M0'，查询结果 ......
现象
下面的语句， 能查到deviceid 为dev005m0的记录， 但是加上deviceid = 'dev005m0'这个条件后， 查询语句查不出结果了。
语句如下：
select * from
( select deviceid deviceid, port, cvlan, status, funcswitch,
decode(funcswitch, 3, pvlan, 1, pvlan) svlan,
decode(funcswitch, 3, pvlan, 1,
lead(pvlan)over(partition by deviceid, port, cvlan, status order by pvlan asc)) evlan
from vlanstatus_pre2bak
)
where funcswitch <> 2 and evlan is null -- and deviceid = 'dev005m0';
当注释掉deviceid = 'dev005m0'， 查询结果如下：
当增加deviceid = 'dev005m0'，查询结果没有记录：
deviceid的数据类型为char(8), vlanstatus_pre2bak的deviceid数据也没有空格等特殊字符， 非常诡异。下面来具体分析。
分析如下
1. 查询语句， 查询没有记录
select * from
( select deviceid deviceid, port, cvlan, status, funcswitch,
decode(funcswitch, 3, pvlan, 1, pvlan) svlan,
decode(funcswitch, 3, pvlan, 1,
lead(pvlan)over(partition by deviceid, port, cvlan, status order by pvlan asc)) evlan
from vlanstatus_pre2bak
)
where funcswitch <> 2 and evlan is null and deviceid = 'dev005m0' ;
或者：
with tmptab as
(
select deviceid deviceid, port, cvlan, status, funcswitch,
decode(funcswitch, 3, pvlan, 1, pvlan) svlan,
decode(funcswitch, 3, pvlan, 1,
lead(pvlan)over(partition by deviceid, port, cvlan, status order by pvlan asc)) evlan
from vlanstatus_pre2bak
)
select * from tmptab
where funcswitch <> 2 and evlan is null and deviceid = 'dev005m0' ;
查看执行计划：
sql> set lines 200
sql> set pages 200
sql> explain plan for
2 select * from
3 ( select deviceid deviceid, port, cvlan, status, funcswitch,
4 decode(funcswitch, 3, pvlan, 1, pvlan) svlan,
5 decode(funcswitch, 3, pvlan, 1,
6 lead(pvlan)over(partition by deviceid, port, cvlan, status order by pvlan asc)) evlan
7 from vlanstatus_pre2bak
8 )
9 where funcswitch <> 2 and evlan is null and deviceid = 'dev005m0' ;
已解释。
sql> select * from table(dbms_xplan.display);
plan_table_output
----------------------------------------------------------------------------------------------------
plan hash value: 2175325539
------------------------------------------------------------------------------------------
| id | operation | name | rows | bytes | cost (%cpu)| time |
------------------------------------------------------------------------------------------
| 0 | select statement | | 70 | 49350 | 692 (1)| 00:00:09 |
|* 1 | view | | 70 | 49350 | 692 (1)| 00:00:09 |
| 2 | window sort | | 70 | 3430 | 692 (1)| 00:00:09 |
|* 3 | table access full| vlanstatus_pre2bak | 70 | 3430 | 691 (1)| 00:00:09 |
------------------------------------------------------------------------------------------
predicate information (identified by operation id):
---------------------------------------------------
1 - filter("funcswitch"<>2 and "evlan" is null)
3 - filter("deviceid"='dev005m0')
已选择16行。
sql>
查看id为3的谓词过滤（3 - filter("deviceid"='dev005m0')）， 说明先在表vlanstatus_pre2bak执行了deviceid = 'dev005m0'操作。id为1的谓词过滤只有两个（1 - filter("funcswitch"<>2 and "evlan" is null)），说明这个查询语句先在子查询里面过滤了deviceid = 'dev005m0'的记录，然后在整个查询视图执行过滤条件funcswitch<>2 and evlan is null。这个现象就是谓词推入。
2. 使用materialize的hint避免谓词推入
with tmptab as
(
select /*+materialize*/ deviceid deviceid, port, cvlan, status, funcswitch,
decode(funcswitch, 3, pvlan, 1, pvlan) svlan,
decode(funcswitch, 3, pvlan, 1,
lead(pvlan)over(partition by deviceid, port, cvlan, status order by pvlan asc)) evlan
from vlanstatus_pre2bak
)
select * from tmptab
where funcswitch <> 2 and evlan is null and deviceid = 'dev005m0' ;
sql> set lines 200
sql> set pages 200
sql> explain plan for
2 with tmptab as
3 (
4 select /*+materialize*/ deviceid deviceid, port, cvlan, status, funcswitch,
5 decode(funcswitch, 3, pvlan, 1, pvlan) svlan,
6 decode(funcswitch, 3, pvlan, 1,
7 lead(pvlan)over(partition by deviceid, port, cvlan, status order by pvlan asc)) evlan
8 from vlanstatus_pre2bak
9 )
10 select * from tmptab
11 where funcswitch <> 2 and evlan is null and deviceid = 'dev005m0' ;
已解释。
sql> select * from table(dbms_xplan.display);
plan_table_output
----------------------------------------------------------------------------------------------------
plan hash value: 1444871666
----------------------------------------------------------------------------------------------------
| id | operation | name | rows | bytes |tempspc| cost (%cpu)| time |
----------------------------------------------------------------------------------------------------
| 0 | select statement | | 313k| 210m| | 5062 (1)| 00:01:01 |
| 1 | temp table transformation | | | | | | |
| 2 | load as select | sys_temp_0fd9d66ce_df9dbbfb | | | | | |
| 3 | window sort | | 313k| 14m| 20m| 4492 (1)| 00:00:54 |
| 4 | table access full | vlanstatus_pre2bak | 313k| 14m| | 690 (1)| 00:00:09 |
|* 5 | view | | 313k| 210m| | 570 (1)| 00:00:07 |
| 6 | table access full | sys_temp_0fd9d66ce_df9dbbfb | 313k| 14m| | 570 (1)| 00:0
----------------------------------------------------------------------------------------------------
predicate information (identified by operation id):
---------------------------------------------------
5 - filter("funcswitch"<>2 and "evlan" is null and "deviceid"='dev005m0')
已选择18行。
sql>
查看id为5的谓词过滤条件， 三个条件都是发生在视图view上面。
3. 使用trim或者拼接一个空的字符
select * from
( select trim(deviceid) deviceid, port, cvlan, status, funcswitch,
decode(funcswitch, 3, pvlan, 1, pvlan) svlan,
decode(funcswitch, 3, pvlan, 1,
lead(pvlan)over(partition by deviceid, port, cvlan, status order by pvlan asc)) evlan
from vlanstatus_pre2bak
)
where funcswitch <> 2 and evlan is null and deviceid = 'dev005m0' ;
查看执行计划：
sql> set lines 200
sql> set lines 300
sql> set pages 200
sql> explain plan for
2 select * from
3 ( select trim(deviceid) deviceid, port, cvlan, status, funcswitch,
4 decode(funcswitch, 3, pvlan, 1, pvlan) svlan,
5 decode(funcswitch, 3, pvlan, 1,
6 lead(pvlan)over(partition by deviceid, port, cvlan, status order by pvlan asc)) evlan
7 from vlanstatus_pre2bak
8 )
9 where funcswitch <> 2 and evlan is null and deviceid = 'dev005m0';
已解释。
sql> select * from table(dbms_xplan.display);
plan_table_output
----------------------------------------------------------------------------------------------------
plan hash value: 2175325539
--------------------------------------------------------------------------------------------------
| id | operation | name | rows | bytes |tempspc| cost (%cpu)| time |
--------------------------------------------------------------------------------------------------
| 0 | select statement | | 313k| 209m| | 4492 (1)| 00:00:54 |
|* 1 | view | | 313k| 209m| | 4492 (1)| 00:00:54 |
| 2 | window sort | | 313k| 14m| 20m| 4492 (1)| 00:00:54 |
| 3 | table access full| vlanstatus_pre2bak | 313k| 14m| | 690 (1)| 00:00:09 |
--------------------------------------------------------------------------------------------------
predicate information (identified by operation id):
---------------------------------------------------
1 - filter("funcswitch"<>2 and "evlan" is null and "deviceid"='dev005m0')
已选择15行。
sql>
查看谓词过滤， 三个过滤条件都发生在id为1的view上面。
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