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Interviews / Databases / What is the `explain()` method and what is it used for in MongoDB?

Questions

What is the difference between SQL and NoSQL databases?
Q C
What is the `ObjectId` and what is it used for in MongoDB?
Q C
What is GridFS and when would you use it in MongoDB?
Q C
What are capped collections and when would you use them in MongoDB?
Q C
How do you do data modeling in MongoDB?
Q C
What are indexes and how do they work in MongoDB?
Q C
What is the difference between a sparse and a partial index in MongoDB?
Q C
What is the aggregation framework and how do you use it in MongoDB?
Q C
How do you do replication in MongoDB?
Q C
How do you do sharding in MongoDB?
Q C
What are transactions and how do you use them in MongoDB?
Q C
What is the `explain()` method and what is it used for in MongoDB?
Q C

What is the `explain()` method and what is it used for in MongoDB?

debugging Query Optimization Interactive Quiz Code Examples

The Scenario

You are a backend engineer at an e-commerce company. You are responsible for a service that is experiencing performance issues. The service is slow to respond to requests, and you have identified that the bottleneck is a slow database query.

The Challenge

Explain what the explain() method is in MongoDB and how you would use it to optimize the slow database query. What are the key things to look for in the output of the explain() method?

Wrong Approach

A junior engineer might not be aware of the `explain()` method. They might try to solve the problem by just adding more indexes, which might not be the most effective solution. They might not be aware of the other tools that can be used to analyze the query plan.

Right Approach

A senior engineer would know that the `explain()` method is a critical part of query optimization. They would be able to explain how to use the `explain()` method to analyze the query plan, and they would have a clear plan for how to use this information to optimize the query.

Step 1: Understand What the explain() Method Is

The explain() method is a tool that can be used to analyze the performance of a query. It returns a document that describes the query plan, which is the sequence of operations that the database will perform to execute the query.

Step 2: Use the explain() Method

Here’s how we can use the explain() method to analyze a query:

db.my_collection.find({ my_field: "my_value" }).explain("executionStats")

The executionStats argument tells the explain() method to execute the query and to include the execution statistics in the output.

Step 3: Analyze the Output

The output of the explain() method is a JSON document that contains a wealth of information about the query plan.

KeyDescription
queryPlannerInformation about the query plan that was chosen by the query optimizer.
executionStatsStatistics about the execution of the query, such as the number of documents scanned and the execution time.
winningPlan.stageThe winning query plan. Look for COLLSCAN which indicates a full collection scan.
executionStats.nReturnedThe number of documents that were returned by the query.
executionStats.totalKeysExaminedThe number of index keys that were examined.
executionStats.totalDocsExaminedThe number of documents that were examined.

Step 4: Identify and Fix the Bottleneck

By analyzing the output of the explain() method, you can identify the bottleneck in your query and take steps to fix it.

For example, if you see that the totalDocsExamined is much larger than the nReturned, it means that the database is scanning a lot of documents that are not being returned by the query. This is a sign that you need to add an index to the collection.

Practice Question

You are looking at the output of the `explain()` method and you see that the `winningPlan.stage` is `COLLSCAN`. What does this mean?