MongoDB查询迷题(1)

本文来自 MongoDB 核心开发人员@kchodorow 的博文,是其关于 MongoDB 查询迷题的第一篇,通过几个例子介绍了在Array 中进行范围查询的一些查询规则和用法。

假如一个 Collection 中有下面一些数据:

{"x": -5} {"x": 0} {"x": 5} {"x": 10} {"x": [0, 5]} {"x": [-5, 10]} {"x": [-5, 5, 10]}

如果我们在这个 Collection 上执行下面的查询语句,会返回哪些数据呢?

db.foo.find({"x" : {"$gt" : -1, "$lt" : 6}})

结果如下:

{"x" : 0} {"x" : 5} {"x" : [0, 5]} {"x" : [-5, 10]} //这货也被查出来了!! {"x" : [-5, 5, 10]}

上面红色注释一条,其两个元素都满足大于-1小于6的条件,为什么还是被查出来了呢?让我们来看看 MongoDB 在 Array 上进行范围查询的原理吧。

MongoDB 在进行范围查询时,可以理解为是按各个条件进行对比的,对于上面有疑问的一行,其 Array 中的

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-5 是小于 6 的,而 10 是大于 -1

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的,所以系统会认为这一行同时满足这两个条件(因为系统内部不管是不是 Array 的同一个元素满足这两个条件)。

如果要在同一个元素上满足各个条件,那么我们可以使用 $elemMatch 算符,比如我们将上面的查询语句做一下修改,会得到下面的结果:

> db.foo.find({x: {$elemMatch: {$gt : -1, $lt : 6}}}) {"x" : [0, 5]} {"x" : [-5, 5, 10]}

上面红色有疑问的那一条数据已经没有了,但是等一下,还有两条数据也跟着没有了,这是什么原因呢?丢失的两条数据是下面这两条:

{"x": 0} {"x": 5}

可以看到,这两条的共同特点,是其 x 对应的 value 值并不是一个 Array。这就是为什么这两条会丢掉的原因,因为 $elemMatch 算符只会对 Array 进行筛选,如果值根本就不是一个 Array,会被直接视为不满足条件。

上面两个查询方法,条有条的原理,但是都没有得到预期的结果,那有没有什么方法可以得到预期的查询结果呢?答案是肯定的,这时候我们要结合 min() 和 max() 两个方法。

先说一下 min() 和 max() 两个方法,这两个方法作用于 MongoDB 查询的 cursor,它可以对 MongoDB 在查询中使用索引的方式进行指导,min(-1) 的意思是,MongoDB 在使用此索引的时候,只使用大于 -1 部分的索引,而 max(6) 的意思是,MongoDB 在使用此索引的时候,只使用小于 6 部分的索引。比如我们使用下面的方式,就能够成功过滤掉上面红色有疑问的一行数据了。

> db.foo.find({"x" : {"$gt" : -1, "$lt" : 6}}).min({"x" : -1}).max({"x" : 6}) {"x" : 0} {"x" : [ 0, 5 ]} {"x" : 5} {"x" : [ -5, 5, 10 ]}

Have Fun!

原文参考:www.kchodorow.com

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