OPTIONS

cursor.explain()

Definition

cursor.explain(verbose)

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. The explain() method returns a document that describes the process used to return the query results.

The explain() method has the following form:

db.collection.find().explain()

The explain() method has the following parameter:

Parameter Type Description
verbose Boolean Optional. Specifies the level of detail to include in the output. If true or 1, includes the allPlans and oldPlan fields in the output.

For an explanation of output, see Explain on Queries on Sharded Collections and Core Explain Output Fields.

Behavior

The explain() method 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.

See also

Explain Results

Explain on Queries on Unsharded Collections

For queries on unsharded collections, explain() returns the following core information.

{
  "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>",
  "filterSet" : <boolean>
}

For details on the fields, see Core Explain Output Fields.

Explain on $or Queries

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 details on the fields, see $or Query Output Fields and Core Explain Output Fields.

Explain on Queries on Sharded Collections

For queries on sharded collections, explain() returns information for each shard the query accesses. For queries on unsharded collections, see Core Explain Output Fields.

For queries on a sharded collection, the output contains the Core Explain Output Fields 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>
}

For details on these fields, see Core Explain Output Fields for each accessed shard and Sharded Collections Output Fields.

Explain Output Fields

Core Explain Output Fields

This section explains output for queries on collections that are not sharded. For queries on sharded collections, see Explain on Queries on Sharded Collections.

explain.cursor

cursor is a string that reports the type of cursor used by the query operation:

  • BasicCursor indicates a full collection scan.
  • BtreeCursor indicates that the query used an index. The cursor includes name of the index. When a query uses an index, the output of explain() includes indexBounds details.
  • GeoSearchCursor indicates that the query used a geospatial index.
  • Complex Plan indicates that MongoDB used index intersection.

For BtreeCursor cursors, MongoDB will append the name of the index to the cursor string. Additionally, depending on how the query uses an index, MongoDB may append one or both of the following strings to the cursor string:

  • reverse indicates that query transverses the index from the highest values to the lowest values (e.g. “right to left”.)
  • multi indicates that the query performed multiple look-ups. Otherwise, the query uses the index to determine a range of possible matches.
explain.isMultiKey

isMultiKey is a boolean. When true, the query uses a multikey index, where one of the fields in the index holds an array.

explain.n

n is a number that reflects the number of documents that match the query selection criteria.

explain.nscannedObjects

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.

explain.nscanned

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.

explain.nscannedObjectsAllPlans

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.

explain.nscannedAllPlans

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.

explain.scanAndOrder

scanAndOrder is a boolean that is true when the query cannot use the order of documents in the index for returning sorted results: MongoDB must sort the documents after it receives the documents from a cursor.

If scanAndOrder is false, MongoDB can use the order of the documents in an index to return sorted results.

explain.indexOnly

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:

  • all the fields in the query are part of that index, and
  • all the fields returned in the results set are in the same index.
explain.nYields

nYields is a number that reflects the number of times this query yielded the read lock to allow waiting writes execute.

explain.nChunkSkips

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.

explain.millis

millis is a number that reflects the time in milliseconds to complete the query.

explain.indexBounds

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> ] ],
                  ...
                }
explain.allPlans

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.

explain.oldPlan

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.

explain.server

New in version 2.2.

server is a string that reports the MongoDB server.

explain.filterSet

New in version 2.6.

filterSet is a boolean that indicates whether MongoDB applied an index filter for the query.

$or Query Output Fields

explain.clauses

clauses is an array that holds the Core Explain Output Fields information for each clause of the $or expression. clauses is only included when the clauses in the $or expression use indexes.

Sharded Collections Output Fields

explain.clusteredType

clusteredType is a string that reports the access pattern for shards. The value is:

  • ParallelSort, if the mongos queries shards in parallel.
  • SerialServer, if the mongos queries shards sequentially.
explain.shards

shards contains fields for each shard in the cluster accessed during the query. Each field holds the Core Explain Output Fields for that shard.

explain.millisShardTotal

millisShardTotal is a number that reports the total time in milliseconds for the query to run on the shards.

explain.millisShardAvg

millisShardAvg is a number that reports the average time in millisecond for the query to run on each shard.

explain.numQueries

numQueries is a number that reports the total number of queries executed.

explain.numShards

numShards is a number that reports the total number of shards queried.