Explain reveals access paths and work such as examined keys and documents. Compare those counts with returned results to find inefficient queries.
Before you start
You should understand documents, collections and indexes. Sketch representative documents and the reads and writes they must support. MongoDB-specific examples assume a collection with the shown fields; deployment topology, permissions and existing indexes can affect the operational behavior being discussed.
The practical goal is to reason through this situation: A query returning ten records after examining thousands suggests a poor match between index and filter. Read the walkthrough first, then try the interview exercise before opening its answer. The important part is explaining the decision and its consequences, rather than remembering a definition alone.
Step-by-step walkthrough
Step 1: Record returned versus examined
A small result can require a large scan.
Step 2: Use realistic data
Tiny collections conceal selectivity and sorting costs.
Step 3: Compare total tradeoffs
Include index write and storage overhead alongside read improvement.
Worked scenario
A query returning ten records after examining thousands suggests a poor match between index and filter.
A query returns ten documents but examines 50,000 because its filter poorly matches the available index. A candidate compound index may reduce examination, but test the actual sort and skewed data distribution. An index name in the plan alone does not prove the query became efficient.
Common mistake
A tiny development dataset can hide the cost of a collection scan.
Verify the behavior
Compare examined keys, documents, returned count and timing under the same workload.
Interview exercise
Verify a query optimization.
Answer and reasoning
Test realistic distributions and sort patterns, compare explain statistics and include index maintenance costs.
Continue learning
Compare the scenario with the MongoDB interview questions and test your understanding with the MongoDB MCQs. For terminology and implementation details, consult the reference material.