Mongodb $3.4 Write Operation Performance
Mongodb $3.4 Optimize Query Performance

Each index on a collection adds some amount of overhead to the performance of write operations.
For each insert or delete write operation on a collection, MongoDB either inserts or removes the corresponding document keys from each index in the target collection. An update operation may result in updates to a subset of indexes on the collection, depending on the keys affected by the update.

1.建立索引后的查询效率大大提升，但是索引查询的数量如果超过50%，意味着遍历更有优势。
2.索引增加会影响insert和update性能。

user有索引，扫描文档数为689，时间1ms

user无索引，全集合扫描，耗时61ms

voice_play有索引，扫描文档数1025，耗时2ms

voice_play无索引，全表扫描，耗时560ms