MongoDB字符串查询中的比较

最近因为想了解一下MongoDB，就找了《MongoDB权威指南》来看，后来又想多了解一下别人的一些思路，遂报了官网的一个线上培训，以下是从他们上星期开始的一期在线培训的第二课中Inequalities On Strings中摘下来的一段，觉得有意思就手打出来了，算是给自己的一个备忘。

简言之，MongoDB将字符串按UTF-8进行字典排序，并可将$gt（$ greater than，大于）和$lt（$ less than，小于）用于字符串查询，原文如下：

Right now MongoDB has exactly zero knowledge of locales. In effect, the comparisons that we perform for $ less than, $ greater than an so forth are going to sort according to the total ordering of the UTF-8 code units. That is to say, according to a lexicographic sorting of the bytes in the UTF-8 representation of the string. This happens to be correct only in the POSIX  or C locales. That is to say, MongoDB compares and sorts things in a Ascii-betically correct fashion. If it so happens that there's a local for which sorting things in UTF-8 byte order happens to be correct, then MongoDB happens to agree with that as well.  A future release of MongoDB is likely to have better support locale-aware sorting collation, and so forth.

以下是培训课程中举的一个例子，列出所有name字段首字母在D前的文档：

>db.people.find( { name: { $lt:"D" } } );