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- cursor.explain()
cursor.explain()¶
-
cursor.
explain
()¶ The
cursor.explain()
method provides information on the query plan. The query plan is the plan the server uses to find the matches for a query. This information may be useful when optimizing a query.Parameters: - verbose (boolean) – Specifies the level of detail to include in the output. If
true
or1
, include theallPlans
andoldPlan
fields in the output document.
Returns: A document that describes the process used to return the query results.
Retrieve the query plan by appending
explain()
to afind()
query, as in the following example:For details on the output, see Explain Output.
explain
runs the actual query to determine the result. Although there are some differences between running the query withexplain
and running without, generally, the performance will be similar between the two. So, if the query is slow, theexplain
operation is also slow.Additionally, the
explain
operation reevaluates a set of candidate query plans, which may cause theexplain
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.To determine the performance of a particular index, you can use
hint()
and in conjunction withexplain()
, as in the following example:When you run
explain
withhint()
, the query optimizer does not reevaluate the query plans.Note
In some situations, the
explain()
operation may differ from the actual query plan used by MongoDB in a normal query.The
explain()
operation evaluates the set of query plans and reports on the winning plan for the query. In normal operations the query optimizer caches winning query plans and uses them for similar related queries in the future. As a result MongoDB may sometimes select query plans from the cache that are different from the plan displayed usingexplain
.See also
$explain
- Optimization Strategies for MongoDB Applications page for information regarding optimization strategies.
- Analyze Performance of Database Operations tutorial for information regarding the database profile.
- Current Operation Reporting
- verbose (boolean) – Specifies the level of detail to include in the output. If