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$explainoperator provides information on the query plan. It returns a document that describes the process and indexes used to return the query. This may provide useful insight when attempting to optimize a query. For details on the output, see cursor.explain().
You can specify the
$explainoperator in either of the following forms:
$explain runs the actual query to determine the result. Although there are some differences between running the query with
$explain and running without, generally, the performance will be similar between the two. So, if the query is slow, the
$explain operation is also slow.
$explain operation reevaluates a set of candidate query plans, which may cause the
$explain operation to perform differently than a normal query. As a result, these operations generally provide an accurate account of how MongoDB would perform the query, but do not reflect the length of these queries.