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Decode函数使用：

Oracle 的decode函数蛮有意思，是oracle独有的，国际标准SQL中并没有decode函数。

语法

DECODE(col|expression, search1, result1

[, search2, result2,…,]

[, default])

例子

SELECT product_id,

DECODE (warehouse_id, 1, ‘Southlake’,

2, ‘San Francisco’,

3, ‘New Jersey’,

4, ‘Seattle’,

‘Non domestic’) “Location”

FROM inventories

WHERE product_id < 1775

ORDER BY product_id, “Location”;

从上面简单的语法和例子中可以看出decode函数也可以做判断，可以实现case…when…then…else..end 和 if..then..else..end if 同样的功能。

Decode函数优点：

1、  使用DECODE函数可以避免重复扫描相同记录或重复连接相同的表，从而减少数据处理时间

例如：想要统计scott用户下emp表中部门20和部门30各有多少员工，每个部门工资成本。

 

 SELECT COUNT(*),

          SUM(SAL) FROM　EMP WHERE DEPTNO = 20;

   SELECT COUNT(*),

          SUM(SAL) FROM　EMP WHERE DEPTNO = 30;

上面的语句可以合并成一条可以达到同样的目的

SELECT COUNT(DECODE(DEPTNO,20,’X’,NULL)) D20_COUNT,

        COUNT(DECODE(DEPTNO,30,’X’,NULL)) D30_COUNT,

        SUM(DECODE(DEPTNO,20,SAL,NULL)) D20_SAL,

        SUM(DECODE(DEPTNO,30,SAL,NULL)) D30_SAL

FROM EMP；

2、  简化了代码

上面的例子如果使用case when 来实现写起来会多写一些代码

SELECT product_id,

Case warehouse_id

    When 1 then ‘Southlake’

    When 2 then ‘San Francisco’

         When 3 then ‘New Jersey’

         When 4 then ‘Seattle’

           Else  ‘Non domestic’

 End as “Location”

FROM inventories

WHERE product_id < 1775

ORDER BY product_id, “Location”;

建议：如果只是简单的判断使用decode函数简单明了

Decode函数性能对比case when性能

  If..then..end if是一种落后的判断方式，这里不做对比了。

经常在网上看到说使用某个函数或者某种写法效率怎么怎么高，执行效率如何还是得具体测试：

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。

 

总结

 

1、  case语句和decode函数执行效率方面

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。

2、  代码实现方面

使用DECODE函数可以避免重复扫描相同记录或重复连接相同的表，从而减少数据处理时间。

如果只是简单的判断使用decode函数简单明了。

 

以上文档测试并整理自互联网

官方参考文档：

Oracle® Database

SQL Language Reference

11g Release 2 (11.2)

E17118-04

5 FUNCTIONSàDECODE