Oracle的DECODE函式功能很強，靈活運用的話可以避免多次掃描，從而提高查詢的效能。而CASE是9i以後提供的語法，這個語法更加的靈活，提供了IF THEN ELSE的功能。

對於很多情況，DECODE和CASE都能解決問題，個人更傾向於使用DECODE，一方面是從8i保留下來的習慣，另一方面是DECODE的語法更加的簡潔，程式碼量要小一些。

不過今天在看Oracle9i的資料倉儲手冊時發現，Oracle在文件中提到CASE語句的效率會更高一些，尤其是CASE 表示式 WHEN 常量 THEN的語法，效率要比CASE WHEN 表示式 THEN的語法更高一些。對於後面這種說法倒是沒有太多的疑問，對於CASE比DECODE效率高這種說法倒是第一次看到，印象中DECODE效率很高，應該不會比CASE的效率差。

到底效率如何，還是要具體的例項來說：

SQL> CREATE TABLE T AS
2 SELECT A.*
3 FROM DBA_OBJECTS A, DBA_MVIEWS;

Table created.

SQL> SELECT COUNT(*) FROM T;

COUNT(*)
----------
6075760

下面檢查DECODE和兩種CASE語句的效率：

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

測試結果確實是CASE的簡單表示式寫法效率最高，然後是CASE的另一種寫法，DECODE效率最低。但是對於600W的記錄，最終結果只有0.01到0.02秒的查詢，實在沒有辦法得出上面的結論，因為這個差別實在是太小，以至於任何其他的一些影響都足以改變測試結果，如要一定要得出結論，那麼結論就是3種方式的效率基本相同。

不過由於CASE表示式更加靈活，使得以前DECODE必須運用的一些技巧得以簡化，這時使用CASE方式，確實可以得到一些效能上的提高，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

這裡CASE帶來效能提升的主要原因實際上是CASE避免了SIGN函式的呼叫，而並不是CASE本身的效能要高於DECODE，事實上如果這裡使用SIGN並利用CASE的所謂高效語法：

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，這時效率比DECODE還低。

根據上面的測試可以得出結論，無論是DECODE還是CASE方式的兩種寫法，執行效率沒有明顯的差別。

DECODE和CASE的效能比較

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