MongoDB初探-细说索引
一、索引操作 索引是为了优化查询速度而生,MongoDB的索引和其他关系型数据库,比如MySQL,Oracle等的索引几乎相同,对于它们的索引优化经验同样适用于MongoDB。 1、创建索引 MongoDB中建立索引是通过ensureIndex操作完成的。下面测试了在使用索引和不使用索
一、索引操作
索引是为了优化查询速度而生,MongoDB的索引和其他关系型数据库,比如MySQL,Oracle等的索引几乎相同,对于它们的索引优化经验同样适用于MongoDB。
1、创建索引
MongoDB中建立索引是通过ensureIndex操作完成的。下面测试了在使用索引和不使用索引下的性能差别,使用explain函数进行查询性能分析。
插入测试数据:
不使用索引的查询:喎?http://www.2cto.com/kf/ware/vc/" target="_blank" class="keylink">vcD4KPHA+PGltZyBzcmM9"http://www.2cto.com/uploadfile/Collfiles/20140324/2014032409104154.jpg" alt="\">
使用索引的查询:
由以上测试可见,使用恰当的索引查找可以大大优化查询效率。
2、删除索引
索引的元信息存储在每个数据库的system.indexes集合中,该集合为保留集合,不能对其插入或删除文档。
操作只能通过ensureIndex或dropIndexes进行。
二、唯一索引
唯一索引可以确保集合的每一个文档的指定键都有唯一值。MongoDB建立唯一索引的方式如下:
db.person.ensureIndex({"name":1},{"unique":true})

如果没有对应的键,索引会将其作为null存储。所以,如果对某个键建立了唯一索引,但插入了多个缺少该索引键的文档,则由于文档包含null值而导致插入失败。
三、组合索引
通过为ensureIndex的第一个参数传入多个键值的文档来实现组合索引的建立。组合索引适用于对多条件查询的优化上,一般来说,如果索引包含N个键,则对于前几个键的查询都会有帮助。比如有个索引{"a":1,"b":1,"c":1,...,"z":1},实际上是有了{"a":1},{"a":1,"b":1},{"a":1,"b":1,"c":1}等的索引,但使用{"b":1},{"a":1,"c":1}等索引的查询则不会被优化。
四、地理空间索引
MongoDB的地理空间索引用于处理平面坐标的查询,对于查找离当前位置最近的N个场所非常适用,在LBS应用中经常使用。
1、创建空间索引
db.map.ensureIndex({"gps":"2d"})
{"gps":[0,100]} {"gps":{"x":0,"y":30}} {"gps":{"latitude":0,"longitude":100}}
db.map.ensureIndex({"gps":"2d"},{"min":-360,"max":360})
通过find函数查找:
将按照离坐标点(40,80)由近及远的方式将map集合的前5个文档返回。
db.map.find({"gps":{"$near":[40,80]}}).limit(5)
通过数据库命令查找:
db.runCommand({geoNear:"map",near:[40,80],num:5});
具体其他空间索引的查找请参考MongoDB官方文档关于索引的介绍。
五、总结
对于索引的使用,要建立合适的索引,可以使用explain查看当前索引的效果。如果索引不适合当前业务需求的话,就要修改或删除,以免不必要或不恰当的索引对数据库操作产生影响。毕竟索引需要消耗内存和文件存储。对于索引要实时维护,不要索引每一个键。

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