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oracle sql调优系列1(decode)

原创 Linux操作系统 作者:alsrt 时间:2012-06-05 22:18:18 0 删除 编辑
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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