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Explain Results

On this page

  • Explain Output Structure
  • Explain Output for MongoDB 5.1 and Later
  • queryPlanner
  • executionStats
  • serverInfo
  • Execution Plan Statistics for Query with $lookup Pipeline Stage
  • Collection Scan
  • Covered Queries
  • Index Intersection
  • $or Expression
  • $sort and $group Stages
  • Sort Stage

To return information on query plans and execution statistics of the query plans, MongoDB provides:

Important

  • Only the most important output fields are shown on this page.

  • The output is subject to change.

  • Some fields are for internal use and are not documented.

The explain results present the query plans as a tree of stages. The output structure can differ based on which query engine the operation uses. Operations can use the classic query engine or the slot-based execution query engine.

To see how the output structure can differ between the two execution engines, see the following examples:

Each stage passes its resulting documents or index keys to the parent node. The leaf nodes access the collection or the indices. The internal nodes use the documents or the index keys that result from the child nodes. The root node indicates the stage that MongoDB ultimately derives the result set from.

Stages are descriptive of the operation. For example:

  • COLLSCAN for a collection scan

  • IXSCAN for scanning index keys

  • FETCH for retrieving documents

  • GROUP for grouping documents

  • SHARD_MERGE for merging results from shards

  • SHARDING_FILTER for filtering out orphan documents from shards

This section shows the explain output for MongoDB 5.1 and later. To see the explain output for older versions of MongoDB, refer to the documentation for that version.

explain.explainVersion

Integer field with the explain planner version.

explainVersion is:

  • 1 when slot based query execution is not used.

  • 2 when slot based query execution is used.

New in version 5.1.

explain.queryPlanner information details the plan selected by the query optimizer.

These examples may combine the output structures of MongoDB's classic and slot-based execution engines. They are not meant to be representative. Your output may differ significantly.

explain.queryPlanner

Contains information on the selection of the query plan by the query optimizer.

explain.queryPlanner.namespace

A string that specifies the namespace with the names of the database and the collection accessed by the query. The namespace has the format <database>.<collection>.

explain.queryPlanner.indexFilterSet

A boolean that specifies whether MongoDB applied an index filter for the query shape.

explain.queryPlanner.queryHash

A hexadecimal string that represents the hash of the query shape and is dependent only on the query shapes. queryHash can help identify slow queries (including the query filter of write operations) with the same query shape.

Note

As with any hash function, two different query shapes may result in the same hash value. However, the occurrence of hash collisions between different query shapes is unlikely.

For more information on queryHash and planCacheKey, see queryHash and planCacheKey.

explain.queryPlanner.planCacheKey

A hash of the key for the plan cache entry associated with the query.

Unlike the explain.queryPlanner.queryHash, the explain.queryPlanner.planCacheKey is a function of both the query shape and the currently available indexes for that shape. That is, if indexes that can support the query shape are added/dropped, the planCacheKey value may change whereas the queryHash value would not change.

For more information on queryHash and planCacheKey, see queryHash and planCacheKey.

explain.queryPlanner.optimizedPipeline

A boolean that indicates that the entire aggregation pipeline operation was optimized away, and instead, fulfilled by a tree of query plan execution stages.

For example, starting in MongodB 4.2, the following aggregation operation can be fulfilled by the tree of query plan execution rather than using the aggregation pipeline.

db.example.aggregate([ { $match: { someFlag: true } } ] )

The field is only present if the value is true and only applies to explain on aggregation pipeline operations. When true, because the pipeline was optimized away, no aggregation stage information appears in the output.

explain.queryPlanner.winningPlan

A document that details the plan selected by the query optimizer.

explain.queryPlanner.winningPlan.stage

A string that denotes the name of the stage.

Each stage consists of information specific to the stage. For example, an IXSCAN stage will include the index bounds along with other data specific to the index scan. If a stage has a child stage or multiple child stages, the stage will have an inputStage or inputStages.

This field appears if the operation used the classic query execution engine.

explain.queryPlanner.winningPlan.inputStage

A document that describes the child stage, which provides the documents or index keys to its parent. The field is present if the parent stage has only one child.

explain.queryPlanner.winningPlan.inputStages

An array of documents describing the child stages. Child stages provide the documents or index keys to the parent stage. The field is present if the parent stage has multiple child nodes. For example, stages for $or expressions or index intersection consume input from multiple sources.

This field appears if the operation used the classic query execution engine.

explain.queryPlanner.winningPlan.queryPlan

A document that details the plan selected by the query optimizer. MongoDB presents the plan as a tree of stages.

This document appears if the query used the slot-based execution query engine.

New in version 5.1.

explain.queryPlanner.winningPlan.queryPlan.stage

A string that denotes the name of the stage.

Each stage consists of information specific to the stage. For instance, an IXSCAN stage will include the index bounds along with other data specific to the index scan.

Starting in MongoDB 5.2, MongoDB can execute some $group stages using the slot-based execution query engine. When the slot-based engine is used for $group, this field value is GROUP.

explain.queryPlanner.winningPlan.queryPlan.planNodeId

Unique integer field that identifies each stage in the execution plan. Field is included in all stages throughout the explain results.

New in version 5.1.

explain.queryPlanner.winningPlan.queryPlan.inputStage

See explain.queryPlanner.winningPlan.inputStage.

explain.queryPlanner.winningPlan.slotBasedPlan

Document with information about the slot based query execution plan tree and stages.

New in version 5.1.

explain.queryPlanner.rejectedPlans

Array of candidate plans considered and rejected by the query optimizer. The array can be empty if there were no other candidate plans.

The returned explain.executionStats information details the execution of the winning plan. In order to include executionStats in the results, you must run the explain in either:

These examples may combine the output structures of MongoDB's classic and slot-based execution engines. They are not meant to be representative. Your output may differ significantly.

explain.executionStats

Contains statistics that describe the completed query execution for the winning plan. For write operations, completed query execution refers to the modifications that would be performed, but does not apply the modifications to the database.

explain.executionStats.nReturned

Number of documents that match the query condition. nReturned corresponds to the n field returned by cursor.explain() in earlier versions of MongoDB.

explain.executionStats.executionTimeMillis

Total time in milliseconds required for query plan selection and query execution. explain.executionStats.executionTimeMillis corresponds to the millis field returned by cursor.explain() in earlier versions of MongoDB.

explain.executionStats.totalKeysExamined

Number of index entries scanned. explain.executionStats.totalKeysExamined corresponds to the nscanned field returned by cursor.explain() in earlier versions of MongoDB.

explain.executionStats.totalDocsExamined

Number of documents examined during query execution. Common query execution stages that examine documents are COLLSCAN and FETCH.

Note

explain.executionStats.totalDocsExamined refers to the total number of documents examined and not to the number of documents returned. For example, a stage can examine a document in order to apply a filter. If the document is filtered out, then it has been examined but will not be returned as part of the query result set.

If a document is examined multiple times during query execution, explain.executionStats.totalDocsExamined counts each examination. That is, explain.executionStats.totalDocsExamined is not a count of the total number of unique documents examined.

explain.executionStats.executionStages

Details the completed execution of the winning plan as a tree of stages; i.e. a stage can have an inputStage or multiple inputStages.

Starting in MongoDB 5.1, a stage can have these input stages:

  • thenStage

  • elseStage

  • innerStage

  • outerStage

Each stage consists of execution information specific to the stage.

explain.executionStats.executionStages.executionTimeMillisEstimate

The estimated amount of time in milliseconds for query execution.

explain.executionStats.executionStages.opens

Starting in MongoDB 5.1, the number of times a stage was opened during query execution.

explain.executionStats.executionStages.closes

Starting in MongoDB 5.1, the number of times a stage was closed during query execution.

explain.executionStats.executionStages.works

Specifies the number of "work units" performed by the query execution stage. Query execution divides its work into small units. A "work unit" might consist of examining a single index key, fetching a single document from the collection, applying a projection to a single document, or doing a piece of internal bookkeeping.

This field appears if the operation used the classic query execution engine.

explain.executionStats.executionStages.saveState

The number of times that the query stage suspended processing and saved its current execution state, for example in preparation for yielding its locks.

explain.executionStats.executionStages.restoreState

The number of times that the query stage restored a saved execution state, for example after recovering locks that it had previously yielded.

explain.executionStats.executionStages.isEOF

Specifies whether the execution stage has reached end of stream:

  • If true or 1, the execution stage has reached end-of-stream.

  • If false or 0, the stage may still have results to return. For example, consider a query with a limit whose execution stages consists of a LIMIT stage with an input stage of IXSCAN for the query. If the query returns more than the specified limit, the LIMIT stage will report isEOF: 1, but its underlying IXSCAN stage will report isEOF: 0.

explain.executionStats.executionStages.inputStage

Each inputStage can have different fields depending on the value of inputStage.stage. The following table describes possible fields and what stages they can appear in.

Each inputStage can have another inputStage as a field. See Explain Output Structure.

Field
Description
Applicable Stages
docsExamined
Specifies the number of documents scanned during the query execution stage.
COLLSCAN, FETCH
keysExamined
For query execution stages that scan an index keysExamined is the total number of in-bounds and out-of-bounds keys that are examined in the process of the index scan. If the index scan consists of a single contiguous range of keys, only in-bounds keys need to be examined. If the index bounds consists of several key ranges, the index scan execution process may examine out-of-bounds keys in order to skip from the end of one range to the beginning of the next.
IXSCAN
numReads

The number of documents scanned or index keys examined during the query execution stage.

New in version 5.1.

COLLSCAN, IXSCAN
seeks
The number of times that we had to seek the index cursor to a new position in order to complete the index scan.
IXSCAN
spilledBytesApprox

The approximate number of in-memory bytes spilled to disk in the stage.

New in version 5.3.

GROUP
spilledRecords

The number of produced records spilled to disk in the stage.

New in version 5.3.

GROUP
usedDisk

Whether the stage wrote to disk.

New in version 5.3.

GROUP
explain.executionStats.allPlansExecution

Contains partial execution information captured during the plan selection phase for both the winning and rejected plans. The field is present only if explain runs in allPlansExecution verbosity mode.

New in version 5.0.

The explain results can include execution statistics for queries that use a $lookup pipeline stage. To include those execution statistics, you must run the explain operation in one of these execution verbosity modes:

The following fields are included in the explain results for a $lookup query:

'$lookup': {
from: <string>,
as: <string>,
localField: <string>,
foreignField: <string>
},
totalDocsExamined: <long>,
totalKeysExamined: <long>,
collectionScans: <long>,
indexesUsed: [ <string_1>, <string_2>, ..., <string_n> ],
nReturned: <long>,
executionTimeMillisEstimate: <long>

To see the descriptions for the fields in the $lookup section, see the $lookup page.

The other fields are:

explain.totalDocsExamined

Number of documents examined during the query execution.

explain.totalKeysExamined

Number of index keys examined.

explain.collectionScans

Number of times a collection scan occurred during query execution. During a collection scan, each document in a collection is compared to the query predicate. Collection scans occur if no appropriate index exists that covers the query.

explain.indexesUsed

Array of strings with the names of the indexes used by the query.

explain.nReturned

Number of documents that match the query condition.

explain.executionTimeMillisEstimate

Estimated time in milliseconds for the query execution.

If the query planner selects a collection scan, the explain result includes a COLLSCAN stage.

If the query planner selects an index, the explain result includes a IXSCAN stage. The stage includes information such as the index key pattern, direction of traversal, and index bounds.

Starting in MongoDB 5.3, if the query planner selects a clustered index for a clustered collection, the explain result includes a CLUSTERED_IXSCAN stage. The stage includes information about the clustered index key and index bounds.

For more information on execution statistics of collection scans, see Analyze Query Performance.

When an index covers a query, MongoDB can both match the query conditions and return the results using only the index keys; i.e. MongoDB does not need to examine documents from the collection to return the results.

When an index covers a query, the explain result has an IXSCAN stage that is not a descendant of a FETCH stage, and in the executionStats, the explain.executionStats.totalDocsExamined is 0.

For an index intersection plan, the result will include either an AND_SORTED stage or an AND_HASH stage with an explain.queryPlanner.winningPlan.inputStages array that details the indexes; e.g.:

{
stage : 'AND_SORTED',
inputStages : [
{
stage : 'IXSCAN',
...
},
{
stage : 'IXSCAN',
...
}
]
}

If MongoDB uses indexes for an $or expression, the result will include the OR stage with an explain.queryPlanner.winningPlan.inputStages array that details the indexes; e.g.:

{
stage: 'OR',
inputStages: [
{
stage: 'IXSCAN',
...
},
{
stage : 'IXSCAN',
...
},
...
]
}

In previous versions of MongoDB, cursor.explain() returned the clauses array that detailed the indexes.

When explain is run in either executionStats or allPlansExecution verbosity mode, the $sort and $group stages have additional output.

Stage
Field
Type
Description
totalDataSizeSortedBytesEstimate
long
An estimated number of bytes processed in the $sort stage.
usedDisk
boolean
Whether the $sort stage wrote to disk.
totalOutputDataSizeBytes
long
An estimate of the total size of all documents output by the $group stage in bytes.
usedDisk
boolean
Whether the $group stage wrote to disk.

If MongoDB cannot use an index or indexes to obtain the sort order, the results include a SORT stage indicating a blocking sort operation. Blocking sorts do not block concurrent operations on the collection or database. The name refers to the requirement that the SORT stage reads all input documents before returning any output documents, blocking the flow of data for that specific query.

If MongoDB requires using more than 100 megabytes of system memory for the blocking sort operation, MongoDB returns an error unless the query specifies cursor.allowDiskUse() (New in MongoDB 4.4). cursor.allowDiskUse() allows MongoDB to use temporary files on disk to store data exceeding the 100 megabyte system memory limit while processing a blocking sort operation. If the explain plan does not contain an explicit SORT stage, then MongoDB can use an index to obtain the sort order.

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