本篇博文主要介绍MongoDB中一些常用的特殊索引类型，主要包括：
- 用于简单字符串搜索的全文本索引；
- 用于球体空间（2dsphere）和二维平面（2d）的地理空间索引。
一、全文索引
MongoDB有一个特殊的索引用在文档中搜索文本，之前的博客都是用精确匹配来查询字符串，这些技术有一定的限制。在搜索大块文本的速度非常慢，而且无法处理自然语言礼节的问题。全文本索引使用的是“倒排索引”的思想来做的，和当前非常开源的lucene（全文检索，Apacle基金会下的开源项目）项目是一样的思想来做的。使用全文本索引可以非常快的进行文本搜索，MongoDB支持多种语言，可惜在免费版中，并不支持世界第一的火星文语言（汉语）。查MongoDB的官网可以看到，在企业版中是支持汉语的全文索引的。
如果公司用的是免费版的MongoDB，而又需要用到中文的全文索引，建议使用lucene或者solr等开源项目来做。（没钱就得用技术来补，赤裸裸的现实。）
使用全文本检索需要专门开启这个功能才能进行使用。启动MongoDB时指定--setParameter textSearchEnabled=true选项，或者在运行时执行setParameter命令，都可以启用全文本索引。
db.adminCommand({"setParameter":1,"textSearchEnabled":true});
准备10条数据：
1 db.news.insert({"title":"SEOUL","context":"SEOUL, June 10 (Reuters) - South Korean prosecutors raided the offices of Lotte Group, the country's fifth-largest conglomerate, and several affiliates on Friday, dealing a further blow to its hotel unit's planned IPO, billed as the world's biggest this year."}); 2 db.news.insert({"title":"Many Chinese people","context":"Many Chinese people think that a job on a diplomatic team is too good to quit. So when 28-year-old Liu Xiaoxi left her embassy post as an attache late last year to start a career in photography, she quickly became a popular topic among the Chinese online community."}); 3 db.news.insert({"title":"About","context":"About 200 investigators searched 17 locations including group headquarters in central Seoul and the homes of Chairman Shin Dong-bin and other key executives, local news agency Yonhap reported, citing the Seoul Central Prosecutor's office."}); 4 db.news.insert({"title":"Three people","context":"Three people with direct knowledge of the matter told Reuters that Friday's raids were part of an investigation into a possible slush fund. They also declined to be identified."}); 5 db.news.insert({"title":"A Lotte Group spokesman","context":"A Lotte Group spokesman on Friday declined to comment on the reason for the raid, when asked whether it concerned a possible slush fund. He noted, however, that the situation was difficult given the IPO plans and Lotte Chemical's Axiall bid."}); 6 db.news.insert({"title":"According","context":"According to bourse rules, the deadline for Hotel Lotte to list is July 27, six months from the preliminary approval for the IPO. If it needed to refile its prospectus to warn investors about risks from Friday's probe, which appeared likely, it would probably not be able to meet that deadline, an exchange official told Reuters on Friday."}); 7 db.news.insert({"title":"Friday","context":"On Friday, dozens of Chinese tourists queued as usual to access elevators to the flagship Lotte Duty Free outlet in the group's headquarters complex, as TV cameras waited for investigators to emerge from office doors around the corner."}); 8 db.news.insert({"title":"Named","context":"Named after the heroine of an 18th century Goethe novel, Lotte has grown from its founding in Japan 68 years ago as a maker of chewing gum to a corporate giant with interests ranging from hotels and retail to food and chemicals. The group has annual revenue of around $60 billion in Korea."}); 9 db.news.insert({"title":"Hotel Lotte's","context":"Hotel Lotte's planned flotation of around 35 percent of its shares was intended to bring transparency and improve corporate governance at a group whose ownership structure is convoluted even by the opaque standards of South Korea's conglomerates."}); 10 db.news.insert({"title":"Shares","context":"Shares in Lotte Shopping (023530.KS) , whose units Lotte Department Store and Lotte Home Shopping were raided, fell 1.6 percent on Friday. Lotte Himart (071840.KS) , a consumer electronics retailer, dropped 2.1 percent."});