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-- 看到很多人,写SQL代码非常随意,想法固然没错:每种方法都可以,只是结果一样就行!
-- 但是,我想:养成良好的SQL风格,会使你终生受益!
-- 因为:有时不同的查询,得到同样的结果,其效率却相差甚远......
-- 请看:
-- 查询一表中昨天生成的数据
-- 原表mobilefrends中的cdate字段上有索引,创建索引语句是:create index mobilefrends_cdate_idx on mobilefrends(cdate);
---------------------------------------------------------------------------------------------------------------------
-- 方法一:用to_char()函数
hll@SZTYORA> select count(*) from mobilefrends where to_char(cdate,'yyyy-mm-dd')=to_char(sysdate-1,'yyyy-mm-dd');
COUNT(*)
----------
82119
已用时间: 00: 00: 17.18
执行计划
----------------------------------------------------------
Plan hash value: 3731074549
------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 8 | 11773 (16)| 00:02:22 |
| 1 | SORT AGGREGATE | | 1 | 8 | | |
|* 2 | INDEX FAST FULL SCAN| MOBILEFRENDS_CDATE_IDX | 129K| 1012K| 11773 (16)| 00:02:22 |
------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter(TO_CHAR(INTERNAL_FUNCTION("CDATE"),'yyyy-mm-dd')=TO_CHAR(SYSDATE@!-1,'yyyy
-mm-dd'))
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
36896 consistent gets
0 physical reads
0 redo size
345 bytes sent via SQL*Net to client
338 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
---------------------------------------------------------------------------------------------------------------------
-- 方法二:用trunc()函数
hll@SZTYORA> select count(*) from mobilefrends where trunc(cdate)=trunc(sysdate-1);
COUNT(*)
----------
82119
已用时间: 00: 00: 16.32
执行计划
----------------------------------------------------------
Plan hash value: 3731074549
------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 8 | 12580 (22)| 00:02:31 |
| 1 | SORT AGGREGATE | | 1 | 8 | | |
|* 2 | INDEX FAST FULL SCAN| MOBILEFRENDS_CDATE_IDX | 129K| 1012K| 12580 (22)| 00:02:31 |
------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter(TRUNC(INTERNAL_FUNCTION("CDATE"))=TRUNC(SYSDATE@!-1))
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
36896 consistent gets
0 physical reads
0 redo size
345 bytes sent via SQL*Net to client
338 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
---------------------------------------------------------------------------------------------------------------------
-- 方法三:不用函数
hll@SZTYORA> select count(*) from mobilefrends where cdate>=trunc(sysdate-1) and cdate<trunc(sysdate);
COUNT(*)
----------
82119
已用时间: 00: 00: 00.43
执行计划
----------------------------------------------------------
Plan hash value: 2668176725
---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 8 | 3 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 8 | | |
|* 2 | FILTER | | | | | |
|* 3 | INDEX RANGE SCAN| MOBILEFRENDS_CDATE_IDX | 48 | 384 | 3 (0)| 00:00:01 |
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter(TRUNC(SYSDATE@!-1)<TRUNC(SYSDATE@!))
3 - access("CDATE">=TRUNC(SYSDATE@!-1) AND "CDATE"<TRUNC(SYSDATE@!))
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
221 consistent gets
0 physical reads
0 redo size
345 bytes sent via SQL*Net to client
338 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
---------------------------------------------------------------------------------------------------------------------
-- 由上三种方法可见:
-- 方法一:用to_char()函数,耗时17.18秒,逻辑读为36896次,执行的是全索引扫描;
-- 方法二:用trunc()函数,耗时16.32秒,逻辑读也是36896次,执行的也是全索引扫描;
-- 方法三:不用函数,耗时0.43秒,逻辑读为221次,执行的是索引范围扫描;
-- 请问:哪种方法好呢?
-- 在我的SQL语句中,我的原则是:
-- *(01) 尽量避免隐式类型转换
-- (例如:如果字段是日期类型的,我会在where语句等式左边用字段原型,而尽量在等式右边用函数,
绝不会在左边用函数转换成字符再去与右边比较);
-- *(02) 尽量少用函数,同样一件事情,能够用两个函数就能解决的,我绝不会嵌套三个函数;
-- (例如:求上个月的最后一天,我会用trunc(sysdate,'mm')-1,而不用last_day(add_months(sysdate,-1)) )
-- *(03) 在所有的存储过程中,我会尽量用绑定变量,以避免硬解析带来的资源消耗!
-- *(04) 在所有的存储过程中,能够用SQL语句的,我绝不会用循环去实现!
-- (例如:列出上个月的每一天,我会用connect by去递归查询一下,绝不会去用循环从上个月第一天到最后一天)
-- *(05) 当有一批处理的插入或更新时,我会用批量插入或批量更新,绝不会一条条记录的去更新!