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: |
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| Returns: | A document that describes the process used to return the query results. |
Retrieve the query plan by appending explain() to a find() query, as in the following example:
db.products.find().explain()
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.
Additionally, the 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.
To determine the performance of a particular index, you can use hint() and in conjunction with explain(), as in the following example:
db.products.find().hint( { type: 1 } ).explain()
When you run explain with hint(), 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 using explain.
See also
This section explains output for queries on sharded collections. For queries on non-sharded collections, see Core Explain Output.
For queries on sharded collections, explain() returns information for each shard the query accesses.
{
"cursor" : "<Cursor Type and Index>",
"isMultiKey" : <boolean>,
"n" : <num>,
"nscannedObjects" : <num>,
"nscanned" : <num>,
"nscannedObjectsAllPlans" : <num>,
"nscannedAllPlans" : <num>,
"scanAndOrder" : <boolean>,
"indexOnly" : <boolean>,
"nYields" : <num>,
"nChunkSkips" : <num>,
"millis" : <num>,
"indexBounds" : { <index bounds> },
"allPlans" : [
{ "cursor" : "<Cursor Type and Index>",
"n" : <num>,
"nscannedObjects" : <num>,
"nscanned" : <num>,
"indexBounds" : { <index bounds> }
},
...
],
"oldPlan" : {
"cursor" : "<Cursor Type and Index>",
"indexBounds" : { <index bounds> }
}
"server" : "<host:port>",
}
Queries with $or operator execute each clause of the $or expression in parallel and can use separate indexes on the individual clauses. If the query uses indexes on any or all of the query’s clause, explain() contains output for each clause as well as the cumulative data for the entire query:
{
"clauses" : [
{
<core explain output>
},
{
<core explain output>
},
...
],
"n" : <num>,
"nscannedObjects" : <num>,
"nscanned" : <num>,
"nscannedObjectsAllPlans" : <num>,
"nscannedAllPlans" : <num>,
"millis" : <num>,
"server" : "<host:port>"
}
For queries on a sharded collection, the output contains the Core Explain Output for each accessed shard and cumulative shard information:
{
"clusteredType" : "<Shard Access Type>",
"shards" : {
"<shard1>" : [
{
<core explain output>
}
],
"<shard2>" : [
{
<core explain output>
}
],
...
},
"millisShardTotal" : <num>,
"millisShardAvg" : <num>,
"numQueries" : <num>,
"numShards" : <num>,
"cursor" : "<Cursor Type and Index>",
"n" : <num>,
"nChunkSkips" : <num>,
"nYields" : <num>,
"nscanned" : <num>,
"nscannedAllPlans" : <num>,
"nscannedObjects" : <num>,
"nscannedObjectsAllPlans" : <num>,
"millis" : <num>
}
cursor is a string that reports the type of cursor used by the query operation:
isMultiKey is a boolean. When true, the query uses a multikey index, where one of the fields in the index holds an array.
n is a number that reflects the number of documents that match the query selection criteria.
Specifies the total number of documents scanned during the query. The nscannedObjects may be lower than nscanned, such as if the index covers a query. See indexOnly. Additionally, the nscannedObjects may be lower than nscanned in the case of multikey index on an array field with duplicate documents.
Specifies the total number of documents or index entries scanned during the database operation. You want n and nscanned to be close in value as possible. The nscanned value may be higher than the nscannedObjects value, such as if the index covers a query. See indexOnly.
New in version 2.2.
nscannedObjectsAllPlans is a number that reflects the total number of documents scanned for all query plans during the database operation.
New in version 2.2.
nscannedAllPlans is a number that reflects the total number of documents or index entries scanned for all query plans during the database operation.
scanAndOrder is a boolean that is true when the query cannot use the index for returning sorted results.
When true, MongoDB must sort the documents after it retrieves them from either an index cursor or a cursor that scans the entire collection.
indexOnly is a boolean value that returns true when the query is covered by the index indicated in the cursor field. When an index covers a query, MongoDB can both match the query conditions and return the results using only the index because:
nYields is a number that reflects the number of times this query yielded the read lock to allow waiting writes execute.
nChunkSkips is a number that reflects the number of documents skipped because of active chunk migrations in a sharded system. Typically this will be zero. A number greater than zero is ok, but indicates a little bit of inefficiency.
millis is a number that reflects the time in milliseconds to complete the query.
indexBounds is a document that contains the lower and upper index key bounds. This field resembles one of the following:
"indexBounds" : {
"start" : { <index key1> : <value>, ... },
"end" : { <index key1> : <value>, ... }
},
"indexBounds" : { "<field>" : [ [ <lower bound>, <upper bound> ] ],
...
}
allPlans is an array that holds the list of plans the query optimizer runs in order to select the index for the query. Displays only when the <verbose> parameter to explain() is true or 1.
New in version 2.2.
oldPlan is a document value that contains the previous plan selected by the query optimizer for the query. Displays only when the <verbose> parameter to explain() is true or 1.
New in version 2.2.
server is a string that reports the MongoDB server.
clauses is an array that holds the Core Explain Output information for each clause of the $or expression. clauses is only included when the clauses in the $or expression use indexes.
clusteredType is a string that reports the access pattern for shards. The value is:
shards contains fields for each shard in the cluster accessed during the query. Each field holds the Core Explain Output for that shard.
millisShardTotal is a number that reports the total time in milliseconds for the query to run on the shards.
millisShardAvg is a number that reports the average time in millisecond for the query to run on each shard.
numQueries is a number that reports the total number of queries executed.
numShards is a number that reports the total number of shards queried.