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MongoDB Aggregate Explain ExecutionStats

Understanding MongoDB’s aggregation framework requires better familiarity with its capabilities and tools. One such tool is the explain method. Combined with the aggregate method it gives a closer look at the execution of aggregation pipelines.

The explain method acts as a window into the MongoDB’s query execution plan. It offers details like the chosen query plan, execution statistics, index usage, etc.

The explain method is useful with aggregation queries, as it throws light on the aggregation pipeline.

Executing explain(“executionStats”) on an aggregation pipeline triggers MongoDB to offer a breakdown of execution statistics for each stage within the pipeline.

Let’s take a look at this example to see the practical application in action:

db.collection.aggregate([
{ $match: { field: value } },
{ $group: { _id: "$groupField", count: { $sum: 1 } } }
]).explain("executionStats")


Here, `db.collection.aggregate([…])` outlines the aggregation pipeline.
Furthermore, `explain(“executionStats”)` requests detailed execution statistics.

When we run this aggregation query with explain(“executionStats”), MongoDB offers a document with details about the pipeline’s execution, including:

• executionStages:

  It has details on each stage, like stage type, document processing count, execution time, etc.
• executionTimeMillis:

  This is the total time taken to execute the aggregation pipeline.
• totalDocsExamined:

  This gives us the total documents examined during execution.
• totalKeysExamined:

  This is a tally of index keys inspected during execution.
• executionSuccess:

  This lets us know about the aggregation’s execution success.

Taking a closer look at the output of explain(“executionStats”) helps users understand MongoDB’s execution strategy. Additionally, we can find performance bottlenecks, optimize queries, etc.

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Conclusion

In brief, our Support Experts introduced us to MongoDB Aggregate Explain ExecutionStats.