oracle sql调优系列1(decode)

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SQL> select * from v$version;

BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Prod
PL/SQL Release 10.2.0.1.0 - Production
CORE 10.2.0.1.0 Production
TNS for Linux: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production

SQL> set autotrace on

执行sql语句1:

SELECT COUNT(*),SUM(sal) FROM emp WHERE deptno=30
union all
SELECT COUNT(*),SUM(sal) FROM emp WHERE deptno=20;

COUNT(*)   SUM(SAL)
---------- ----------
         6       9400
         5      10875

Execution Plan
----------------------------------------------------------
Plan hash value: 2390793

----------------------------------------------------------------------------
| Id | Operation           | Name | Rows | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |      |     2 |    14 |     6 (50)| 00:00:01 |
|   1 | UNION-ALL          |      |       |       |            |          |
|   2 |   SORT AGGREGATE    |      |     1 |     7 |            |          |
|* 3 |    TABLE ACCESS FULL| EMP |     6 |    42 |     3   (0)| 00:00:01 |
|   4 |   SORT AGGREGATE    |      |     1 |     7 |            |          |
|* 5 |    TABLE ACCESS FULL| EMP |     5 |    35 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

3 - filter("DEPTNO"=30)
5 - filter("DEPTNO"=20)

Statistics
----------------------------------------------------------
          0 recursive calls
          0 db block gets
         14 consistent gets
          0 physical reads
          0 redo size
        516 bytes sent via SQL*Net to client
        385 bytes received via SQL*Net from client
          2 SQL*Net roundtrips to/from client
          0 sorts (memory)
          0 sorts (disk)
          2 rows processed

执行sql语句2(decode):

SQL> select count(decode(deptno,20,'x',null)) d20_count,
2 count(decode(deptno,30,'x',null)) d30_count,
3 sum(decode(deptno,20,sal,null)) d20_count,
4 sum(decode(deptno,30,sal,null)) d30_count
5 from emp
6 ;

D20_COUNT D30_COUNT D20_COUNT D30_COUNT
---------- ---------- ---------- ----------
5 6 10875 9400

Execution Plan
----------------------------------------------------------
Plan hash value: 2083865914

---------------------------------------------------------------------------
| Id | Operation          | Name | Rows | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |     7 |     3   (0)| 00:00:01 |
|   1 | SORT AGGREGATE    |      |     1 |     7 |            |          |
|   2 |   TABLE ACCESS FULL| EMP |    14 |    98 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          0 recursive calls
          0 db block gets
          7 consistent gets
          0 physical reads
          0 redo size
        600 bytes sent via SQL*Net to client
        385 bytes received via SQL*Net from client
          2 SQL*Net roundtrips to/from client
          0 sorts (memory)
          0 sorts (disk)
          1 rows processed

总结:sql语句1产生了14个逻辑读,sql语句2产生了7个逻辑读,很显然sql语句1的性能

远远低于sql2的性能.

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