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 WNER = 'SYSTEM' THEN 'SYSTEM'
2 WHEN WNER = '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。