Create Queries that Ensure Selectivity创建确保选择性的查询

Suppose you have a field called status where the possible values are new and processed. 假设您有一个名为status的字段，其中可能的值是new和processed。If you add an index on status you've created a low-selectivity index. The index will be of little help in locating records.如果添加status索引，则创建了一个低选择性索引。索引对查找记录没有什么帮助。

A better strategy, depending on your queries, would be to create a compound index that includes the low-selectivity field and another field. 根据您的查询，更好的策略是创建一个包含低选择性字段和另一个字段的复合索引。For example, you could create a compound index on status and created_at.例如，您可以创建一个关于status和created_at的复合索引。

Another option, again depending on your use case, might be to use separate collections, one for each status.另一种选择，同样取决于您的用例，可能是使用单独的集合，每个状态一个。

Consider an index { a : 1 } (i.e. an index on the key a sorted in ascending order) on a collection where a has three values evenly distributed across the collection:考虑集合上的索引{ a : 1 }（即按升序排序的键a上的索引），其中a有三个值均匀分布在集合中：

{ _id: ObjectId(), a: 1, b: "ab" }
{ _id: ObjectId(), a: 1, b: "cd" }
{ _id: ObjectId(), a: 1, b: "ef" }
{ _id: ObjectId(), a: 2, b: "jk" }
{ _id: ObjectId(), a: 2, b: "lm" }
{ _id: ObjectId(), a: 2, b: "no" }
{ _id: ObjectId(), a: 3, b: "pq" }
{ _id: ObjectId(), a: 3, b: "rs" }
{ _id: ObjectId(), a: 3, b: "tv" }

If you query for { a: 2, b: "no" } MongoDB must scan 3 documents in the collection to return the one matching result. 如果您查询{ a: 2, b: "no" }MongoDB必须扫描集合中的3个文档才能返回一个匹配的结果。Similarly, a query for { a: { $gt: 1}, b: "tv" } must scan 6 documents, also to return one result.类似地，{ a: { $gt: 1}, b: "tv" }的查询必须扫描6个文档，才能返回一个结果。

Consider the same index on a collection where a has nine values evenly distributed across the collection:考虑集合上的同一索引，其中a有九个值均匀分布在集合中：

{ _id: ObjectId(), a: 1, b: "ab" }
{ _id: ObjectId(), a: 2, b: "cd" }
{ _id: ObjectId(), a: 3, b: "ef" }
{ _id: ObjectId(), a: 4, b: "jk" }
{ _id: ObjectId(), a: 5, b: "lm" }
{ _id: ObjectId(), a: 6, b: "no" }
{ _id: ObjectId(), a: 7, b: "pq" }
{ _id: ObjectId(), a: 8, b: "rs" }
{ _id: ObjectId(), a: 9, b: "tv" }

If you query for { a: 2, b: "cd" }, MongoDB must scan only one document to fulfill the query. 如果查询{ a: 2, b: "cd" }，MongoDB必须只扫描一个文档才能完成查询。The index and query are more selective because the values of a are evenly distributed and the query can select a specific document using the index.索引和查询更具选择性，因为a的值分布均匀，查询可以使用索引选择特定文档。

However, although the index on a is more selective, a query such as { a: { $gt: 5 }, b: "tv" } would still need to scan 4 documents.然而，尽管a上的索引更具选择性，但诸如{ a: { $gt: 5 }, b: "tv" }之类的查询仍然需要扫描4个文档。