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为数据库建立索引一

Jun 13, 2016 am 10:26 AM
php one Same for dynamic exist Establish Developer I database index net

就象许多的PHP开发者一样,在刚开始建立动态网站的时候,我都是使用相对简单的数据结构。PHP在连接数据库方面的确实是十分方便(译者注:有些人认为PHP在连接不同数据库时没有一个统一的接口,不太方便,其实这可以通过一些扩展库来做到这一点),你无需看大量的设计文档就可以建立和使用数据库,这也是PHP获得成功的主要原因之一。
  前些时候,一位颇高级的程序员居然问我什么叫做索引,令我感到十分的惊奇,我想这绝不会是沧海一粟,因为有成千上万的开发者(可能大部分是使用MySQL的)都没有受过有关数据库的正规培训,尽管他们都为客户做过一些开发,但却对如何为数据库建立适当的索引所知较少,因此我起了写一篇相关文章的念头。
  最普通的情况,是为出现在where子句的字段建一个索引。为方便讲述,我们先建立一个如下的表。
  CREATE TABLE mytable (
     id serial primary key,
     category_id int not null default 0,
     user_id int not null default 0,
     adddate int not null default 0
  );
  很简单吧,不过对于要说明这个问题,已经足够了。如果你在查询时常用类似以下的语句:
   SELECT * FROM mytable WHERE category_id=1;
  最直接的应对之道,是为category_id建立一个简单的索引:
   CREATE INDEX mytable_categoryid
     ON mytable (category_id);
  OK,搞定?先别高兴,如果你有不止一个选择条件呢?例如:
   SELECT * FROM mytable WHERE category_id=1 AND user_id=2;
  你的第一反应可能是,再给user_id建立一个索引。不好,这不是一个最佳的方法。你可以建立多重的索引。


  CREATE INDEX mytable_categoryid_userid ON mytable (category_id,user_id);
  注意到我在命名时的习惯了吗?我使用"表名_字段1名_字段2名"的方式。你很快就会知道我为什么这样做了。
  现在你已经为适当的字段建立了索引,不过,还是有点不放心吧,你可能会问,数据库会真正用到这些索引吗?测试一下就OK,对于大多数的数据库来说,这是很容易的,只要使用EXPLAIN命令:
  EXPLAIN
   SELECT * FROM mytable
    WHERE category_id=1 AND user_id=2;
   This is what Postgres 7.1 returns (exactly as I expected)
   NOTICE: QUERY PLAN:
   Index Scan using mytable_categoryid_userid on
     mytable (cost=0.00..2.02 rows=1 width=16)
  EXPLAIN
以上是postgres的数据,可以看到该数据库在查询的时候使用了一个索引(一个好开始),而且它使用的是我创建的第二个索引。看到我上面命名的好处了吧,你马上知道它使用适当的索引了。
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