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HomeDatabaseMysql TutorialOracle数据库设计字段类型选择错误的隐患

虽然这两条SQL消耗的资源是一样的,但SQL1的评估结果不对,在多表关联的时候,这个绝对是个隐患,非常容易导致执行计划走错。除了

Oracle数据类型不准确的一个隐患,下面来构造一张表存日期字段,一个存varchar2,一个存date,,做一个测试。之前也写过:

字段类型设计与实际业务不符引发的问题 

SQL> drop table test purge;
SQL> create table test as select
      to_char(to_date('2014-01-01','yyyy-MM-dd')+rownum,'yyyymmdd') s_date,
      to_date('2014-01-01','yyyy-MM-dd')+rownum d_date
    from all_objects;

 SQL> create index ind_t_sdate on test(s_date) nologging;
 SQL> create index ind_t_ddate on test(d_date) nologging;
 SQL> exec dbms_stats.gather_table_stats(user,'test',cascade => true);

 SQL> set timing on
 SQL> set autotrace traceonly
 SQL> select * from test where s_date between '20140201' and '20140222';
已选择22行。
 已用时间:  00: 00: 00.00
执行计划
----------------------------------------------------------
Plan hash value: 953148778
 -------------------------------------------------------------------------------------------
 | Id  | Operation                  | Name        | Rows  | Bytes | Cost (%CPU)| Time    |
 -------------------------------------------------------------------------------------------
 |  0 | SELECT STATEMENT            |            |    3 |    51 |    3  (0)| 00:00:01 |
 |  1 |  TABLE ACCESS BY INDEX ROWID| TEST        |    3 |    51 |    3  (0)| 00:00:01 |
 |*  2 |  INDEX RANGE SCAN          | IND_T_SDATE |    3 |      |    2  (0)| 00:00:01 |
 
-------------------------------------------------------------------------------------------

--可以看到CBO评估出来的行数是3,明明返回的是22
Predicate Information (identified by operation id):
 ---------------------------------------------------
    2 - access("S_DATE">='20140201' AND "S_DATE"统计信息
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          7  consistent gets
          0  physical reads
          0  redo size
        944  bytes sent via SQL*Net to client
        349  bytes received via SQL*Net from client
          3  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          22  rows processed

 SQL> select * from test
      where d_date between to_date('20140201', 'yyyymmdd') and
            to_date('20140222', 'yyyymmdd');
已选择22行。
 已用时间:  00: 00: 00.00
执行计划
----------------------------------------------------------
Plan hash value: 112387541
 -------------------------------------------------------------------------------------------
 | Id  | Operation                  | Name        | Rows  | Bytes | Cost (%CPU)| Time    |
 -------------------------------------------------------------------------------------------
 |  0 | SELECT STATEMENT            |            |    23 |  391 |    3  (0)| 00:00:01 |
 |  1 |  TABLE ACCESS BY INDEX ROWID| TEST        |    23 |  391 |    3  (0)| 00:00:01 |
 |*  2 |  INDEX RANGE SCAN          | IND_T_DDATE |    23 |      |    2  (0)| 00:00:01 |
 
-------------------------------------------------------------------------------------------

--可以看到CBO评估出来基本是准确的。

Predicate Information (identified by operation id):
 ---------------------------------------------------
    2 - access("D_DATE">=TO_DATE(' 2014-02-01 00:00:00', 'syyyy-mm-dd hh24:mi:ss')
              AND "D_DATE"统计信息
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          7  consistent gets
          0  physical reads
          0  redo size
        944  bytes sent via SQL*Net to client
        349  bytes received via SQL*Net from client
          3  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
 
        22  rows processed

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