应用程序开发人员常见的数据库开发错误有哪些?


当前回答

对于基于sql的数据库:

Not taking advantage of CLUSTERED INDEXES or choosing the wrong column(s) to CLUSTER. Not using a SERIAL (autonumber) datatype as a PRIMARY KEY to join to a FOREIGN KEY (INT) in a parent/child table relationship. Not UPDATING STATISTICS on a table when many records have been INSERTED or DELETED. Not reorganizing (i.e. unloading, droping, re-creating, loading and re-indexing) tables when many rows have been inserted or deleted (some engines physically keep deleted rows in a table with a delete flag.) Not taking advantage of FRAGMENT ON EXPRESSION (if supported) on large tables which have high transaction rates. Choosing the wrong datatype for a column! Not choosing a proper column name. Not adding new columns at the end of the table. Not creating proper indexes to support frequently used queries. creating indexes on columns with few possible values and creating unnecessary indexes. ...more to be added.

其他回答

1 -在where子句中的值上不必要地使用函数,导致该索引未被使用。

例子:

where to_char(someDate,'YYYYMMDD') between :fromDate and :toDate

而不是

where someDate >= to_date(:fromDate,'YYYYMMDD') and someDate < to_date(:toDate,'YYYYMMDD')+1

在较小的程度上:不向需要函数索引的值添加函数索引……

2 -没有添加检查约束以确保数据的有效性。查询优化器可以使用约束,它们确实有助于确保您可以信任您的不变量。没有理由不使用它们。

3 -纯粹出于懒惰或时间压力而向表中添加未规范化的列。事情通常不是这样设计的,而是演变成这样的。最终的结果是,当您在未来的演进中受到丢失的数据完整性的困扰时,将会有大量的工作试图清理混乱。

想想看,重新设计一个没有数据的表是非常便宜的。一个有数百万条记录的表,没有完整性……重新设计并不便宜。因此,在创建列或表时执行正确的设计是可以分摊的。

4 -不是关于数据库本身,但确实令人讨厌。不关心SQL的代码质量。SQL是用文本表示的事实并不意味着可以将逻辑隐藏在大量的字符串操作算法中。完全有可能用文本编写SQL,使您的程序员同事能够读懂。

如果您正在使用复制(MySQL),以下函数是不安全的,除非您正在使用基于行的复制。

USER(), CURRENT_USER() (or CURRENT_USER), UUID(), VERSION(), LOAD_FILE(), and RAND()

参见:http://dev.mysql.com/doc/refman/5.1/en/replication-features-functions.html

相关子查询导致的性能差

大多数情况下,您希望避免相关子查询。如果子查询中存在对外部查询的列的引用,则子查询是相关的。当发生这种情况时,对于返回的每一行至少执行一次子查询,如果在应用包含相关子查询的条件之后应用其他条件,则可以执行更多次。

请原谅这个不自然的示例和Oracle语法,但假设您想要找到自上次商店每天销售额低于10,000美元以来在任何商店中雇用的所有员工。

select e.first_name, e.last_name
from employee e
where e.start_date > 
        (select max(ds.transaction_date)
         from daily_sales ds
         where ds.store_id = e.store_id and
               ds.total < 10000)

本例中的子查询通过store_id与外部查询相关联,并将对系统中的每个员工执行。优化此查询的一种方法是将子查询移动到内联视图。

select e.first_name, e.last_name
from employee e,
     (select ds.store_id,
             max(s.transaction_date) transaction_date
      from daily_sales ds
      where ds.total < 10000
      group by s.store_id) dsx
where e.store_id = dsx.store_id and
      e.start_date > dsx.transaction_date

In this example, the query in the from clause is now an inline-view (again some Oracle specific syntax) and is only executed once. Depending on your data model, this query will probably execute much faster. It would perform better than the first query as the number of employees grew. The first query could actually perform better if there were few employees and many stores (and perhaps many of stores had no employees) and the daily_sales table was indexed on store_id. This is not a likely scenario but shows how a correlated query could possibly perform better than an alternative.

我曾多次看到初级开发人员关联子查询,这通常会对性能产生严重影响。但是,当删除一个相关的子查询时,一定要查看之前和之后的解释计划,以确保您没有使性能变差。

这之前已经说过了,但是:索引,索引,还是索引。我见过许多性能不佳的企业web应用程序,它们通过简单地进行一些分析(查看哪些表被频繁访问),然后在这些表上添加索引来解决问题。这甚至不需要太多的SQL编写知识,而且回报是巨大的。

Avoid data duplication like the plague. Some people advocate that a little duplication won't hurt, and will improve performance. Hey, I'm not saying that you have to torture your schema into Third Normal Form, until it's so abstract that not even the DBA's know what's going on. Just understand that whenever you duplicate a set of names, or zipcodes, or shipping codes, the copies WILL fall out of synch with each other eventually. It WILL happen. And then you'll be kicking yourself as you run the weekly maintenance script.

最后:使用清晰、一致、直观的命名约定。与一段编写良好的代码应该是可读的一样,一个好的SQL模式或查询应该是可读的,并且实际上告诉您它在做什么,甚至没有注释。六个月后,当你不得不对桌子进行维护时,你会感谢自己的。“选择account_number, billing_date FROM national_accounts”比“选择ACCNTNBR, BILLDAT FROM NTNLACCTS”更容易使用。

1)不了解如何正确地在Java和数据库之间进行交互。

2)过度解析,不恰当或没有重用SQL

3)没有使用BIND变量

4)在数据库中使用SQL集逻辑时,用Java实现过程逻辑(更好)。

5)在投入生产前没有进行任何合理的性能或可伸缩性测试

6)使用水晶报表,在报表中没有正确设置模式名

7)由于不了解执行计划,使用笛卡尔式产品实现SQL(你甚至看了EXPLAIN计划吗?)