mongdb索引查询优化

Query Selectivity

Some query operations are not selective. These operations cannot useindexes effectively or cannot use indexes at all.

The inequality operators $nin and$ne are notvery selective, as they often match a large portion of the index. As aresult, in most cases, a$nin or$ne query withan index may perform no better than a$nin or$ne query that must scan all documents in a collection.

Queries that specify regular expressions, with inline JavaScriptregular expressions or$regex operator expressions, cannotuse an index with one exception. Queries that specify regularexpressionwith anchors at the beginning of a stringcan use anindex.

$nin $ne 不会走索引

指定正则表达式和 JavaScript 内联函数  都不会走索引，只有一个例外，查询一个集合的字符串开始出使用正则表达式

在合适的情况下，建立索引可以有效地提升查询效率；但是索引对写效率有影响，所以：
1、找出最频繁的，成为瓶颈的查询语句，在列上建立索引
2、避免创建毫无意义的索引
3、在业务负载低时进行索引创建的操作