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AES加密解密
大家在做SQL优化的过程中,可能都知道一个事实:某些情况下使用UNION替换OR可以提高SQL的运行效率。
您知道这个“某些情况”指的是什么么?
解释一下,“某些情况”指的是:使用的表字段要有索引。
这个实验,给大家展示一下这个结论
1.创建测试表
sec@ora10g> drop table t;
Table dropped.
sec@ora10g> create table t as select * from all_objects;
Table created.
sec@ora10g> alter table t rename column owner to x;
Table altered.
sec@ora10g> alter table t rename column object_name to y;
Table altered.
sec@ora10g> update t set x = 'secooler';
4785 rows updated.
OK,通过上面的初始化,我们得到了这个测试表T,我们关心它的第一个和第二个字段,修改名字后是x字段和y字段
x字段内容统一修改为“secooler”,以便模拟使用这个字段得到大量返回结果
y字段指定特定值后,模拟返回一条记录
2.开启autotrace,跟踪不同的SQL执行(为使执行计划稳定,请多次执行,得到稳定输出结果)
用到的测试SQL语句是以下三条
1)返回记录多的条件放在where子句的前面
select * from t where x = 'secooler' or y = 'T';
2)返回记录多的条件放在where子句的后面
select * from t where y = 'T' or x = 'secooler';
3)使用UNION改写上面的OR语句
select * from t where x = 'secooler'
union
select * from t where y = 'T'
/
3.先看一下,在没有创建索引情况下的实验效果
sec@ora10g> set autotrace traceonly
sec@ora10g> select * from t where x = 'secooler' or y = 'T';
4782 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4583 | 572K| 16 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T | 4583 | 572K| 16 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("X"='secooler' OR "Y"='T')
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
111 consistent gets
0 physical reads
0 redo size
206142 bytes sent via SQL*Net to client
1009 bytes received via SQL*Net from client
49 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
4782 rows processed
sec@ora10g> select * from t where y = 'T' or x = 'secooler';
4782 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4583 | 572K| 16 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T | 4583 | 572K| 16 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("Y"='T' OR "X"='secooler')
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
111 consistent gets
0 physical reads
0 redo size
206142 bytes sent via SQL*Net to client
1009 bytes received via SQL*Net from client
49 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
4782 rows processed
sec@ora10g> select * from t where x = 'secooler'
union
select * from t where y = 'T'
/
2 3 4
4782 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2618920678
------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4584 | 573K| | 168 (12)| 00:00:03 |
| 1 | SORT UNIQUE | | 4584 | 573K| 1448K| 168 (12)| 00:00:03 |
| 2 | UNION-ALL | | | | | | |
|* 3 | TABLE ACCESS FULL| T | 4583 | 572K| | 16 (0)| 00:00:01 |
|* 4 | TABLE ACCESS FULL| T | 1 | 128 | | 16 (0)| 00:00:01 |
------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter("X"='secooler')
4 - filter("Y"='T')
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
126 consistent gets
0 physical reads
0 redo size
253890 bytes sent via SQL*Net to client
1009 bytes received via SQL*Net from client
49 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
4782 rows processed
4.在没有创建索引情况下的实验结论
1)无论是返回记录多的条件放在where子句的前面还是后面,从执行计划上看,效率是一样的。
2)没有创建索引的情况下,使用UNION改写后效率没有提高,反而下降了
5.在看一下,创建所需的索引情况后的实验效果
sec@ora10g> create index idx1_t on t(x);
Index created.
sec@ora10g> create index idx2_t on t(y);
Index created.
sec@ora10g> select * from t where x = 'secooler' or y = 'T';
4782 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4583 | 572K| 16 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T | 4583 | 572K| 16 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("X"='secooler' OR "Y"='T')
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
111 consistent gets
0 physical reads
0 redo size
206142 bytes sent via SQL*Net to client
1009 bytes received via SQL*Net from client
49 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
4782 rows processed
sec@ora10g> select * from t where y = 'T' or x = 'secooler';
4782 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4583 | 572K| 16 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T | 4583 | 572K| 16 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("Y"='T' OR "X"='secooler')
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
111 consistent gets
0 physical reads
0 redo size
206142 bytes sent via SQL*Net to client
1009 bytes received via SQL*Net from client
49 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
4782 rows processed
sec@ora10g> select * from t where x = 'secooler'
union
select * from t where y = 'T'
/
2 3 4
4782 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 4276936497
------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4584 | 573K| | 153 (3)| 00:00:02 |
| 1 | SORT UNIQUE | | 4584 | 573K| 1448K| 153 (3)| 00:00:02 |
| 2 | UNION-ALL | | | | | | |
|* 3 | TABLE ACCESS FULL | T | 4583 | 572K| | 16 (0)| 00:00:01 |
| 4 | TABLE ACCESS BY INDEX ROWID| T | 1 | 128 | | 2 (0)| 00:00:01 |
|* 5 | INDEX RANGE SCAN | IDX2_T | 1 | | | 1 (0)| 00:00:01 |
------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter("X"='secooler')
5 - access("Y"='T')
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
66 consistent gets
0 physical reads
0 redo size
253890 bytes sent via SQL*Net to client
1009 bytes received via SQL*Net from client
49 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
4782 rows processed
6.在创建所需的索引后的实验结果
1)无论是返回记录多的条件放在where子句的前面还是后面,从执行计划上看,效率还是一样的。
2)从“consistent gets”参数上看,使用UNION改写OR后,效率得到有效的提升。
7.小结
通过上面的实验,可以得到在CBO模式下,无论是返回记录多的条件放在where子句的前面还是后面,从执行计划上看,效率是一样的。
另外一个重要结论就是:在考虑使用UNION改写OR的时候,一定要注意查看使用的字段是否已经创建了索引。
您知道这个“某些情况”指的是什么么?
解释一下,“某些情况”指的是:使用的表字段要有索引。
这个实验,给大家展示一下这个结论
1.创建测试表
sec@ora10g> drop table t;
Table dropped.
sec@ora10g> create table t as select * from all_objects;
Table created.
sec@ora10g> alter table t rename column owner to x;
Table altered.
sec@ora10g> alter table t rename column object_name to y;
Table altered.
sec@ora10g> update t set x = 'secooler';
4785 rows updated.
OK,通过上面的初始化,我们得到了这个测试表T,我们关心它的第一个和第二个字段,修改名字后是x字段和y字段
x字段内容统一修改为“secooler”,以便模拟使用这个字段得到大量返回结果
y字段指定特定值后,模拟返回一条记录
2.开启autotrace,跟踪不同的SQL执行(为使执行计划稳定,请多次执行,得到稳定输出结果)
用到的测试SQL语句是以下三条
1)返回记录多的条件放在where子句的前面
select * from t where x = 'secooler' or y = 'T';
2)返回记录多的条件放在where子句的后面
select * from t where y = 'T' or x = 'secooler';
3)使用UNION改写上面的OR语句
select * from t where x = 'secooler'
union
select * from t where y = 'T'
/
3.先看一下,在没有创建索引情况下的实验效果
sec@ora10g> set autotrace traceonly
sec@ora10g> select * from t where x = 'secooler' or y = 'T';
4782 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4583 | 572K| 16 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T | 4583 | 572K| 16 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("X"='secooler' OR "Y"='T')
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
111 consistent gets
0 physical reads
0 redo size
206142 bytes sent via SQL*Net to client
1009 bytes received via SQL*Net from client
49 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
4782 rows processed
sec@ora10g> select * from t where y = 'T' or x = 'secooler';
4782 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4583 | 572K| 16 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T | 4583 | 572K| 16 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("Y"='T' OR "X"='secooler')
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
111 consistent gets
0 physical reads
0 redo size
206142 bytes sent via SQL*Net to client
1009 bytes received via SQL*Net from client
49 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
4782 rows processed
sec@ora10g> select * from t where x = 'secooler'
union
select * from t where y = 'T'
/
2 3 4
4782 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2618920678
------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4584 | 573K| | 168 (12)| 00:00:03 |
| 1 | SORT UNIQUE | | 4584 | 573K| 1448K| 168 (12)| 00:00:03 |
| 2 | UNION-ALL | | | | | | |
|* 3 | TABLE ACCESS FULL| T | 4583 | 572K| | 16 (0)| 00:00:01 |
|* 4 | TABLE ACCESS FULL| T | 1 | 128 | | 16 (0)| 00:00:01 |
------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter("X"='secooler')
4 - filter("Y"='T')
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
126 consistent gets
0 physical reads
0 redo size
253890 bytes sent via SQL*Net to client
1009 bytes received via SQL*Net from client
49 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
4782 rows processed
4.在没有创建索引情况下的实验结论
1)无论是返回记录多的条件放在where子句的前面还是后面,从执行计划上看,效率是一样的。
2)没有创建索引的情况下,使用UNION改写后效率没有提高,反而下降了
5.在看一下,创建所需的索引情况后的实验效果
sec@ora10g> create index idx1_t on t(x);
Index created.
sec@ora10g> create index idx2_t on t(y);
Index created.
sec@ora10g> select * from t where x = 'secooler' or y = 'T';
4782 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4583 | 572K| 16 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T | 4583 | 572K| 16 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("X"='secooler' OR "Y"='T')
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
111 consistent gets
0 physical reads
0 redo size
206142 bytes sent via SQL*Net to client
1009 bytes received via SQL*Net from client
49 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
4782 rows processed
sec@ora10g> select * from t where y = 'T' or x = 'secooler';
4782 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4583 | 572K| 16 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T | 4583 | 572K| 16 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("Y"='T' OR "X"='secooler')
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
111 consistent gets
0 physical reads
0 redo size
206142 bytes sent via SQL*Net to client
1009 bytes received via SQL*Net from client
49 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
4782 rows processed
sec@ora10g> select * from t where x = 'secooler'
union
select * from t where y = 'T'
/
2 3 4
4782 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 4276936497
------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4584 | 573K| | 153 (3)| 00:00:02 |
| 1 | SORT UNIQUE | | 4584 | 573K| 1448K| 153 (3)| 00:00:02 |
| 2 | UNION-ALL | | | | | | |
|* 3 | TABLE ACCESS FULL | T | 4583 | 572K| | 16 (0)| 00:00:01 |
| 4 | TABLE ACCESS BY INDEX ROWID| T | 1 | 128 | | 2 (0)| 00:00:01 |
|* 5 | INDEX RANGE SCAN | IDX2_T | 1 | | | 1 (0)| 00:00:01 |
------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter("X"='secooler')
5 - access("Y"='T')
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
66 consistent gets
0 physical reads
0 redo size
253890 bytes sent via SQL*Net to client
1009 bytes received via SQL*Net from client
49 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
4782 rows processed
6.在创建所需的索引后的实验结果
1)无论是返回记录多的条件放在where子句的前面还是后面,从执行计划上看,效率还是一样的。
2)从“consistent gets”参数上看,使用UNION改写OR后,效率得到有效的提升。
7.小结
通过上面的实验,可以得到在CBO模式下,无论是返回记录多的条件放在where子句的前面还是后面,从执行计划上看,效率是一样的。
另外一个重要结论就是:在考虑使用UNION改写OR的时候,一定要注意查看使用的字段是否已经创建了索引。
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- **解决方案**:使用 `UNION ALL` 替换 `OR`,可以更高效地处理多个独立的查询。 **5. 避免使用 `LIKE` 开头的通配符** - **问题分析**:使用 `%` 开头的 `LIKE` 语句(例如 `LIKE '%abc%'`)通常会导致全表...
它不仅支持多种编程语言的语法高亮显示,还具备代码折叠、自动完成等功能,极大地提高了程序员的工作效率。然而,默认情况下,EditPlus并不直接支持SQL(Structured Query Language)这种数据库查询语言。 #### 二...
8. **使用`UNION ALL`**:在某些情况下,将`DISTINCT`替换为多个`UNION ALL`子查询可能更有效,因为`UNION ALL`不进行去重操作,只合并结果集。 在实际应用中,优化SQL查询需要根据具体的数据库结构、数据分布和...
一种常见的方法是使用编辑距离(Levenshtein Distance),它计算将一个字符串转换为另一个字符串所需的最少单字符编辑(插入、删除或替换)次数。 接下来,我们可以利用Python的内置数据结构,如列表和字典,来实现...
面向对象的软件工程是面向对象方法在软件工程领域的全面应用,它包括面向对象的分析(OOA)、面向对象的设计(OOD)、面向对象的编程(OOP)、面向对象的测试(OOT)和面向对象的软件维护(OOSM)等主要内容。...