oracle函数之case和decode的用法区别及性能比较

    在oracle世界，你可以使用：

    1）case表达式      或者

    2）decode函数

    来实现逻辑判断。Oracle的DECODE函数功能很强，灵活运用的话可以避免多次扫描，从而提高查询的性能。而CASE是9i以后提供的语法，这个语法更加的灵活，提供了IF
THEN ELSE的功能。   

    case表达式

    case表达式，可分两种，简单和搜索，简单case后接表达式，如：



    对于简单的case需要几点注意：

    1）寻找when的优先级：从上到下

    2）再多的when，也只有一个出口，即其中有一个满足了expr就马上退出case

    3）不能把return_expr和else_expr指定为null，而且，expr、comparison_expr和return_expr的数据类型必须相同。



    搜索case：

    CASE WHEN condition THEN return_expr

              [WHEN condition THEN return_expr]

              ...

    ELSE else_expr

    END

    例子：

SELECT (CASE WHEN cust_credit_limit BETWEEN  0 AND 3999 THEN  ' 0 - 3999'
WHEN cust_credit_limit BETWEEN  4000 AND 7999 THEN ' 4000 - 7999'
WHEN cust_credit_limit BETWEEN  8000 AND 11999 THEN  ' 8000 - 11999'
WHEN cust_credit_limit BETWEEN  12000 AND 16000 THEN '12000 - 16000' END)
AS BUCKET, COUNT(*) AS Count_in_Group
FROM customers WHERE cust_city = 'Marshal' GROUP BY
(CASE WHEN cust_credit_limit BETWEEN  0 AND 3999 THEN ' 0 - 3999'
WHEN cust_credit_limit BETWEEN  4000 AND 7999 THEN ' 4000 - 7999'
WHEN cust_credit_limit BETWEEN  8000 AND 11999 THEN  ' 8000 - 11999'
WHEN cust_credit_limit BETWEEN  12000 AND 16000 THEN '12000 - 16000' END);

BUCKET        COUNT_IN_GROUP
------------- --------------
0 - 3999                  8
4000 - 7999               7
8000 - 11999              7
12000 - 16000              1

    用decode可以违反第3NF（行不可再分，列不可再分，列不可重复）：列重复

hr@ORCL> select * from a;

ID NAME
---------- ----------
1 a
2 b
3 c
1 a

hr@ORCL> select sum(decode(id,1,1,0)) think,
2             sum(decode(id,2,2,0)) water,
3             sum(decode(id,3,3,0)) linshuibin
4        from a;

THINK      WATER LINSHUIBIN
---------- ---------- ----------
2          2          3

 

    一个字段，decode函数可以完全改写简单case；

    多个字段，需要复杂的case，方可。

    语法：

    DECODE(value,if1,then1,if2,then2,if3,then3,...,else)，表示如果value等于if1时，DECODE函数的结果返then1,...,如果不等于任何一个if值，则返回else。可以用函数或表达式来替代value，if，then，else从而作出一些更有用的比较。

    来看看具体的运用：

    1 假设我们想给百度职员加工资，其标准是：工资在8000元以下的将加20％；工资在8000元以上的加15％

    则：

    select decode(sign(salary - 8000),1,salary*1.15,-1,salary*1.2,salary) "revised_salary" from employee

    2 表table_subject，有subject_name列。要求按照：语、数、外的顺序进行排序

    则：

    select * from table_subject order by decode(subject_name, '语文', 1, '数学', 2, , '外语',3)

    decode和简单case的性能比较

 

   

Oracle的DECODE函数功能很强，灵活运用的话可以避免多次扫描，从而提高查询的性能。而CASE是9i以后提供的语法，这个语法更加的灵活，提供了IF
THEN ELSE的功能。

对于很多情况，DECODE和CASE都能解决问题，个人更倾向于使用DECODE，一方面是从8i保留下来的习惯，另一方面是DECODE的语法更加的简洁，代码量要小一些。
不过今天在看Oracle9i的数据仓库手册时发现，Oracle在文档中提到CASE语句的效率会更高一些，尤其是CASE表达式 WHEN
常量 THEN的语法，效率要比CASE WHEN表达式 THEN的语法更高一些。对于后面这种说法倒是没有太多的疑问，对于CASE比DECODE效率高这种说法倒是第一次看到，印象中DECODE效率很高，应该不会比CASE的效率差。
到底效率如何，还是要具体的实例来说：
SQL> CREATE TABLE T AS

2 SELECT A.*

3 FROM DBA_OBJECTS A, DBA_MVIEWS;

Table created.

SQL> SELECT COUNT(*) FROM T;

COUNT(*)

----------

6075760

下面检查DECODE和两种CASE语句的效率：
SQL> SET ARRAY 1000

SQL> SET TIMING ON

SQL> SET AUTOT TRACE

SQL> SELECT DECODE(OWNER, 'SYSTEM', 'SYSTEM', 'SYS', 'SYSTEM', 'USER')

2 FROM T;

6075760 rows selected.

Elapsed: 00:00:07.24

Execution Plan

----------------------------------------------------------

Plan hash value: 1601196873

--------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

--------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 4245K| 68M| 13828 (1)| 00:03:14 |

| 1 | TABLE ACCESS FULL| T | 4245K| 68M| 13828 (1)| 00:03:14 |

--------------------------------------------------------------------------

Note

-----

- dynamic sampling used for this statement

Statistics

----------------------------------------------------------

0 recursive calls

0 db block gets

47551 consistent gets

0 physical reads

0 redo size

46288564 bytes sent via SQL*Net to client

67317 bytes received via SQL*Net from client

6077 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

6075760 rows processed

SQL> SELECT CASE OWNER WHEN 'SYSTEM' THEN 'SYSTEM'

2 WHEN 'SYS' THEN 'SYSTEM'

3 ELSE 'USER' END

4 FROM T;

6075760 rows selected.

Elapsed: 00:00:07.22

Execution Plan

----------------------------------------------------------

Plan hash value: 1601196873

--------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

--------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 4245K| 68M| 13828 (1)| 00:03:14 |

| 1 | TABLE ACCESS FULL| T | 4245K| 68M| 13828 (1)| 00:03:14 |

--------------------------------------------------------------------------

Note

-----

- dynamic sampling used for this statement

Statistics

----------------------------------------------------------

0 recursive calls

0 db block gets

47551 consistent gets

0 physical reads

0 redo size

46288578 bytes sent via SQL*Net to client

67317 bytes received via SQL*Net from client

6077 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

6075760 rows processed

SQL> SELECT CASE WHEN OWNER = 'SYSTEM' THEN 'SYSTEM'

2 WHEN OWNER = 'SYS' THEN 'SYSTEM'

3 ELSE 'USER' END

4 FROM T;

6075760 rows selected.

Elapsed: 00:00:07.23

Execution Plan

----------------------------------------------------------

Plan hash value: 1601196873

--------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

--------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 4245K| 68M| 13828 (1)| 00:03:14 |

| 1 | TABLE ACCESS FULL| T | 4245K| 68M| 13828 (1)| 00:03:14 |

--------------------------------------------------------------------------

Note

-----

- dynamic sampling used for this statement

Statistics

----------------------------------------------------------

0 recursive calls

0 db block gets

47551 consistent gets

0 physical reads

0 redo size

46288585 bytes sent via SQL*Net to client

67317 bytes received via SQL*Net from client

6077 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

6075760 rows processed

测试结果确实是CASE的简单表达式写法效率最高，然后是CASE的另一种写法，DECODE效率最低。但是对于600W的记录，最终结果只有0.01到0.02秒的查询，实在没有办法得出上面的结论，因为这个差别实在是太小，以至于任何其他的一些影响都足以改变测试结果，如要一定要得出结论，那么结论就是3种方式的效率基本相同。
不过由于CASE表达式更加灵活，使得以前DECODE必须运用的一些技巧得以简化，这时使用CASE方式，确实可以得到一些性能上的提高，比如：
SQL> SELECT DECODE(SIGN(OBJECT_ID), 1, '+', -1, '-', '0')

2 FROM T;

6075760 rows selected.

Elapsed: 00:00:04.94

Execution Plan

----------------------------------------------------------

Plan hash value: 1601196873

--------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

--------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 4245K| 52M| 13840 (1)| 00:03:14 |

| 1 | TABLE ACCESS FULL| T | 4245K| 52M| 13840 (1)| 00:03:14 |

--------------------------------------------------------------------------

Note

-----

- dynamic sampling used for this statement

Statistics

----------------------------------------------------------

0 recursive calls

0 db block gets

47551 consistent gets

0 physical reads

0 redo size

31491431 bytes sent via SQL*Net to client

67317 bytes received via SQL*Net from client

6077 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

6075760 rows processed

SQL> SELECT CASE WHEN OBJECT_ID > 0 THEN '+'

2 WHEN OBJECT_ID < 0 THEN '-'

3 ELSE '0' END

4 FROM T;

6075760 rows selected.

Elapsed: 00:00:04.60

Execution Plan

----------------------------------------------------------

Plan hash value: 1601196873

--------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

--------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 4245K| 52M| 13840 (1)| 00:03:14 |

| 1 | TABLE ACCESS FULL| T | 4245K| 52M| 13840 (1)| 00:03:14 |

--------------------------------------------------------------------------

Note

-----

- dynamic sampling used for this statement

Statistics

----------------------------------------------------------

0 recursive calls

0 db block gets

47551 consistent gets

0 physical reads

0 redo size

31491449 bytes sent via SQL*Net to client

67317 bytes received via SQL*Net from client

6077 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

6075760 rows processed

这里CASE带来性能提升的主要原因实际上是CASE避免了SIGN函数的调用，而并不是CASE本身的性能要高于DECODE，事实上如果这里使用SIGN并利用CASE的所谓高效语法：
SQL> SELECT CASE SIGN(OBJECT_ID) WHEN 1 THEN '+'

2 WHEN -1 THEN '-'

3 ELSE '0' END

4 FROM T;

6075760 rows selected.

Elapsed: 00:00:04.97

Execution Plan

----------------------------------------------------------

Plan hash value: 1601196873

--------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

--------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 4245K| 52M| 13840 (1)| 00:03:14 |

| 1 | TABLE ACCESS FULL| T | 4245K| 52M| 13840 (1)| 00:03:14 |

--------------------------------------------------------------------------

Note

-----

- dynamic sampling used for this statement

Statistics

----------------------------------------------------------

0 recursive calls

0 db block gets

47551 consistent gets

0 physical reads

0 redo size

31491445 bytes sent via SQL*Net to client

67317 bytes received via SQL*Net from client

6077 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

6075760 rows processed

可以看到，这时效率比DECODE还低。
根据上面的测试可以得出结论，无论是DECODE还是CASE方式的两种写法，执行效率没有明显的差别。