索引与聚合管道进行排序

Question

I'm developing an application using MongoDB as its database, and for sorting data, I encountered an interesting argument from a colleague that index can be used instead of aggregation pipeline for getting sorted data.

I tried this and it actually works; using an index with specified field and direction does return sorted data when queried. When using aggregation pipeline, I also obtained the same result.

I have created an index with the following specification:

index name: batch_deleted_a_desc

num: asc
marked: asc
value: desc

Using aggregation pipeline:

> db.test.aggregate([{$match: {num:"3",marked:false}}, {$sort:{"value":-1}}])
{ "_id" : ObjectId("5d70b40ba7bebd3d7c135615"), "value" : 4, "marked" : false, "num" : "3" }
{ "_id" : ObjectId("5d70b414a7bebd3d7c135616"), "value" : 2, "marked" : false, "num" : "3" }
{ "_id" : ObjectId("5d70b3fea7bebd3d7c135614"), "value" : 1, "marked" : false, "num" : "3" }

Using index:

> db.test.find({num:"3",marked:false})
{ "_id" : ObjectId("5d70b40ba7bebd3d7c135615"), "value" : 4, "marked" : false, "num" : "3" }
{ "_id" : ObjectId("5d70b414a7bebd3d7c135616"), "value" : 2, "marked" : false, "num" : "3" }
{ "_id" : ObjectId("5d70b3fea7bebd3d7c135614"), "value" : 1, "marked" : false, "num" : "3" }

As you can see, the results are the same. But I am unsure that using index for getting sorted data is a good practice, and yet using aggregation pipeline is (sometimes) taking more effort than just creating index.

So, which would be the best option?

Answer 1

In the context of the question, the better option would be the aggregation because it explicitly specifies the sort .

In the query example, results are being returned in order specified by the index because the query is using the index { num: 1, marked: 1, value: 1} . However, nothing specified in the query will guarantee that ordering, meaning results may change at some point in the future. For example, consider the case where the index { num: 1, marked: 1, updated_at: 1 } were to be created. The query planner may decide to use this index instead, which may result in results in a different order.

In the absence of a sort, a query would return results in the order of the index being used, but you should not rely upon that ordering without explicitly specifying it. Quoting the docs :

Unless you specify the sort() method or use the $near operator, MongoDB does not guarantee the order of query results.