MongoDB 2.5.0 (开发版本)中，包含了一个新的查询引擎实现。 查询引擎 主要的工作是通过查询条件，查询到匹配的数据。当然，这其中包括对索引的使用。现有的查询引擎在MongoDB发布的4年以来基本没有进行什么变更。它最大的问题是缺乏扩展性，也没有查询优化
MongoDB 2.5.0 (开发版本)中，包含了一个新的查询引擎实现。查询引擎主要的工作是通过查询条件，查询到匹配的数据。当然，这其中包括对索引的使用。现有的查询引擎在MongoDB发布的4年以来基本没有进行什么变更。它最大的问题是缺乏扩展性，也没有查询优化的功能（我们知道，MongoDB的查询优化实际上是通过试错的方式来做的）。
在2.5.0版本中，查询引擎被完全重写，它包括三个独立的部分：
- 一个语法树（AST：abstract syntax tree）模块，用于解析查询语句
- 一个解析模块，将BSON的查询条件解析成AST
- 一个完全重写的查询API层，其功能和现有的查询模块类似
本次改动主要是为后续的查询优化工作铺路，其中很重要的一个功能点，就是索引交集的实现。比如你的查询条件是 {a:5, b:6} ，而这时候你分别有a和b两个字段的独立索引。在现在的查询引擎下，MongoDB只能使用其中一个索引。比如找出所有a的值为5的记录，再逐条遍历，查询出b为6的值。（或者反过来查b的索引，再遍历出a为5的值）。索引交集功能则可以在现有的情况下，通过对a，b两个独立的索引求交集，直接获取到a=5,b=6的记录（目前的稳定版中，需要建一个联合索引才能实现）。除了普通索引外，这一功能还会扩展到地理位置索引，全文索引上。（比如获取所有在3.5公里以内，符合xxx条件的人）
另外利用新的查询引擎，还可以实现一些更高级的查询，比如查询出 a == b + c 的记录。以后你可以这样写：{ a : { $sum
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: [ “$b” , “$c” ] } })。这样的功能对新的查询引擎来说很容易实现。
新的查询引擎目前已经重写完成并且可以测试了。如果你有兴趣测试测试，可以下载最新的2.5.0版本试玩。【下载】
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相关代码都在github上面：
- AST Root
- Parser Root
来源：blog.mongodb.org
原文地址：MongoDB 2.5版本将提供新的查询引擎, 感谢原作者分享。
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