Oracle性能优化3-sql优化一定要等价

做sql优化的前提瞧见是sql等价

1.MAX MIN写法的分与合

drop table t purge;

create table t as select * from dba_objects;

alter table t add constraint pk1_object_id primary key (OBJECT_ID);

set autotrace on

set linesize 1000

select min(object_id),max(object_id) from t;

和这个分开写有差别吗

select max(object_id) from t;

select min(object_id) from t;

其实这两种方式不等价,

等价的语句是

 select max, min  

    from (select max(object_id) max from t ) a,(select min(object_id) min from t) b;

2.In与 ><写法之间的异同

drop table t purge;

create table t as select * from dba_objects;

create index idx_object_id on t(object_id,object_type);

UPDATE t SET OBJECT_ID=20 WHERE ROWNUM<=26000;

UPDATE t SET OBJECT_ID=21 WHERE OBJECT_ID<>20;

COMMIT;

set linesize 266

set pagesize 1

alter session set statistics_level=all ;

select  /*+index(t,idx_object_id)*/ * from t  where object_TYPE='TABLE'  AND OBJECT_ID >= 20 AND OBJECT_ID<= 21;

select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

-------------------------------------------------------------------------------------------------------

| Id  | Operation                   | Name          | Starts | E-Rows | A-Rows |   A-Time   | Buffers |

-------------------------------------------------------------------------------------------------------

|   0 | SELECT STATEMENT            |               |      1 |        |   2925 |00:00:00.03 |    1103 |

|   1 |  TABLE ACCESS BY INDEX ROWID| T             |      1 |   2126 |   2925 |00:00:00.03 |    1103 |

|*  2 |   INDEX RANGE SCAN          | IDX_OBJECT_ID |      1 |    320 |   2925 |00:00:00.02 |     730 |

-------------------------------------------------------------------------------------------------------

select  /*+index(t,idx_object_id)*/ * from t t where object_TYPE='TABLE'  AND  OBJECT_ID IN (20,21);

select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

---------------------------------------------------------------------------------------------------------

| Id  | Operation                    | Name           | Starts | E-Rows | A-Rows |   A-Time   | Buffers |

---------------------------------------------------------------------------------------------------------

|   1 |  INLIST ITERATOR             |                |      1 |        |   2920 |00:00:00.01 |     563 |

|   2 |   TABLE ACCESS BY INDEX ROWID| t              |      2 |   2592 |   2920 |00:00:00.01 |     563 |

|*  3 |    INDEX RANGE SCAN          | IDX1_OBJECT_ID |      2 |      1 |   2920 |00:00:00.01 |     214 |

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3.count(*)与count(列)不等价

4.看似不想等，实则相等

begin

select count(*) into v_cnt from t1 ;

if v_cnt>0 

then  …A逻辑….

else 

then  …B逻辑…..

End;





我来翻译一下这段需求：

获取t1 表的记录数，判断是否大于0，如果大于0走A逻辑，否则就走B逻辑。

因此代码就如上所示来实现了。真正的需求是这样吗？

其实应该是这样的：只要T1表有记录就走A逻辑，否则走B逻辑。

两者有区别吗？其实区别还是很大的，前者可是强调获取记录数，我们是不是一定要遍历整个表得出一个记录数才知道是否大于0？

真正需求的理解可以让我们这样实现，只要从T1表中成功获取到第一条记录，就可以停止检索了，表示该表有记录了，难道事实不是这样？

因此原先的SQL1 从Select count(*) from t1; 被改造为：

Select count(*) from t1 where rownum=1; 



begin

select count(*) into v_cnt from t1 where rownum=1;

if v_cnt=1 

then  …A逻辑….

else 

then  …B逻辑…..

End;

5.从需求优化

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