Which is the default mode in which the explain() command runs?

Question

Options

• AallPlansExecution
• BcustomExecutionStats
• CqueryPlanner
• DexecutionStats

• A
  4% (1)
• B
  8% (2)
• C
  88% (23)

Explanation

queryPlanner (C) is the default mode because it shows MongoDB's query execution plan - which index it would use - without actually running the query. This makes it fast and lightweight for inspecting optimizer decisions.

Why the distractors are wrong:

• A. allPlansExecution - A valid mode, but it must be explicitly requested; it runs all candidate plans and compares them, which is more expensive.
• B. customExecutionStats - Not a real mode in MongoDB's explain() API; this option is a distractor.
• D. executionStats - Also a valid mode, but it actually executes the query and returns runtime stats (docs examined, time taken). You must opt into it explicitly.

Memory tip: Think of the default as "plan only, no action" - queryPlanner just plans, it doesn't execute. If you want real stats, you have to ask for executionStats by name.

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