突然在网上看到一种说法，Oracle的WHERE条件执行顺序是自右向左的。
理由是，当ORACLE的WHERE条件中出现多个00904表示符无效错误时，错误是从右向左的顺序报的。
也有人提出解析顺序和执行顺序不是一码事，执行顺序要看执行计划。
于是，我特地作了个试验。
现做一个试验用表
SQL> select count(*) from dba_objects;
COUNT(*)
----------
72536
create table dba_objects2 as select * from dba_objects;
先来一次查询
SQL> select object_id,object_name from dba_objects2 where owner='HR' and object_id>500;
已选择41行。
执行计划
----------------------------------------------------------
Plan hash value: 4278809457
----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 426 | 40896 | 283 (1)| 00:00:04 |
|* 1 | TABLE ACCESS FULL| DBA_OBJECTS2 | 426 | 40896 | 283 (1)| 00:00:04 |
----------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OWNER"='HR' AND "OBJECT_ID">500)
Note
-----
- dynamic sampling used for this statement (level=2)
可以看出，这次Oracle是自左向右执行。那么SQL语句颠倒下会怎样？
SQL> select object_id,object_name from dba_objects2 where object_id>500 and owner='HR';
已选择41行。
执行计划
----------------------------------------------------------
Plan hash value: 4278809457
----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 426 | 40896 | 283 (1)| 00:00:04 |
|* 1 | TABLE ACCESS FULL| DBA_OBJECTS2 | 426 | 40896 | 283 (1)| 00:00:04 |
----------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OWNER"='HR' AND "OBJECT_ID">500)
Note
-----
- dynamic sampling used for this statement (level=2)
实际的执行顺序是自右向左。难道oracle的执行顺序是看开销的？把尽可能多缩小数据范围的条件放在前面执行。
为了证明自己的想法，我又增加了一个条件。
SQL> select object_id,object_name from dba_objects2 where object_id>500 and object_name like '%EM%' and owner='HR';
已选择14行。
执行计划
----------------------------------------------------------
Plan hash value: 4278809457
----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 147 | 14112 | 283 (1)| 00:00:04 |
|* 1 | TABLE ACCESS FULL| DBA_OBJECTS2 | 147 | 14112 | 283 (1)| 00:00:04 |
----------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_NAME" IS NOT NULL AND "OWNER"='HR' AND
"OBJECT_ID">500 AND "OBJECT_NAME" LIKE '%EM%')
Note
-----
- dynamic sampling used for this statement (level=2)
ORACLE 智能的增加了"OBJECT_NAME" IS NOT NULL的条件，把放在中间的 OBJECT_NAME" LIKE '%EM%'放在了最后执行。
由此可见，WHERE条件的执行顺序是按照开销的右小到大的顺序，智能调节的。
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