Introduction to MongoDB collection.explain() Method

MongoDB is a popular NoSQL database that provides many methods for optimizing queries and operations. One very useful method is explain(), which analyzes detailed information about query execution, including the indexes used, the number of documents scanned, and more. This is very useful for debugging query performance and optimizing queries.

Syntax

The syntax of the explain() method is very simple, just pass the query to be explained as a parameter to it:

db.collection.find(query).explain()

Use Cases

The explain() method is typically used for query performance optimization. It can provide detailed information about query execution to help you decide how to better write queries and create indexes. You can use it to see which index is being used, how many documents are being scanned, and whether a covered index is being used, etc.

Examples

Suppose we have a collection named users, which contains the following documents:

{ "name": "Alice", "age": 25, "location": "New York" }
{ "name": "Bob", "age": 30, "location": "Los Angeles" }
{ "name": "Charlie", "age": 35, "location": "Chicago" }

Now we want to find users who are 30 years old, and the query statement is as follows:

db.users.find({ age: 30 })

Use the explain() method to view detailed information about query execution, the query statement is as follows:

db.users.find({ age: 30 }).explain()

The result of the query execution may look like this:

{
    "queryPlanner" : {
        "plannerVersion" : 1,
        "namespace" : "test.users",
        "indexFilterSet" : false,
        "parsedQuery" : {
            "age" : {
                "$eq" : 30
            }
        },
        "winningPlan" : {
            "stage" : "COLLSCAN",
            "filter" : {
                "age" : {
                    "$eq" : 30
                }
            },
            "direction" : "forward"
        },
        "rejectedPlans" : [ ]
    },
    "serverInfo" : {
        "host" : "hostname",
        "port" : 27017,
        "version" : "4.0.10",
        "gitVersion" : "some-git-version"
    },
    "ok" : 1
}

From the output, we can see that this query used the COLLSCAN stage, i.e., a full collection scan, without using an index. This means that the query efficiency is low and we need to create an index to optimize query performance.

Conclusion

The explain() method is a very useful query performance optimization tool in MongoDB. It can provide detailed information about query execution to help you debug and optimize queries. When query performance issues arise, using the explain() method to analyze query execution can help you quickly identify the problem and take appropriate optimization measures.