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Aggregation Pipeline Stages

$planCacheStats

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  • Definition
  • Considerations
  • Output
  • Examples

Definition

$planCacheStats

New in version 4.2.

Returns plan cache information for a collection. The stage returns a document for each plan cache entry.

The $planCacheStats stage must be the first stage in the pipeline. The stage takes an empty document as a parameter and has the following syntax:

{ $planCacheStats: { } }

Note

4.4 Changes

Starting in version 4.4,

Tip

See also:

Query Plans

Considerations

Pipeline

$planCacheStats must be the first stage in an aggregation pipeline.

Restrictions

Access Control

On systems running with authorization, the user must have the planCacheRead privilege for the collection.

Read Preference

$planCacheStats observes the read preference in selecting the host(s) from which to return the plan cache information.

Applications may target different members of a replica set. As such, each replica set member might receive different read commands and have plan cache information that differs from other members. Nevertheless, running $planCacheStats on a replica set or a sharded cluster obeys the normal read preference rules. That is, on a replica set, the operation gathers plan cache information from just one member of replica set, and on a sharded cluster, the operation gathers plan cache information from just one member of each shard replica set.

Output

For each plan cache entry, the $planCacheStats stage returns a document similar to the following:

{
   "createdFromQuery" : <document>,
   "queryHash" : <hexadecimal string>,
   "planCacheKey" : <hexadecimal string>,
   "isActive" :  <boolean>,
   "works" : <NumberLong>,
   "cachedPlan" : {
      "stage" : <STAGE1>,
      "filter" : <document>,
      "inputStage" : {
         "stage" : <STAGE2>,
         ...
      }
   },
   "timeOfCreation" : <date>,
   "creationExecStats" : [   // Exec Stats Document for each candidate plan
      {
         "nReturned" : <num>,
         "executionTimeMillisEstimate" : <num>
         "totalKeysExamined" : <num>
         "totalDocsExamined" :<num>
         "executionStages" : {
            "stage" : <STAGE A>,
            ...
            "inputStage" : {
               "stage" : <STAGE B>,
               ...
            }
         }
      },
      ...
   ],
   "candidatePlanScores" : [
      <number>,
      ...
   ],
   "indexFilterSet" : <boolean>,
   "estimatedSizeBytes" : <num>, // Available starting in MongoDB 5.0, 4.4.3, 4.2.12
   "host" : <string>,   // Available starting in MongoDB 4.4
   "shard" : <string>   // Available starting in MongoDB 4.4 if run on sharded cluster
}

Each document includes various query plan and execution stats, including:

Field
Description

createdFromQuery

A document that contains the specific query that resulted in this cache entry; i.e.

{
  "query" : <document>,
  "sort" : <document>,
  "projection" : <document>
}

isActive

A boolean that indicates whether the entry is active or inactive.

  • If active, the query planner is currently using the entry to generate query plans.

  • If inactive, the query planner is not currently using the entry to generate query plans.

Tip

See also:

Plan Cache Entry State

queryHash

A hexadecimal string that represents the hash of the query shape. For more information, see explain.queryPlanner.queryHash

planCacheKey

A hexadecimal string that represents the hash of the key used to find the plan cache entry associated with this query. The plan cache key is a function of both the query shape and the currently available indexes for that shape. For more information, see explain.queryPlanner.planCacheKey

cachedPlan

The details of the cached plan. See explain.queryPlanner.

works

The number of "work units" performed by the query execution plan during the trial period when the query planner evaluates candidate plans. For more information, see explain.executionStats.executionStages.works

timeOfCreation

Time of creation for the entry.

creationExecStats

An array of execution stats documents. The array contains a document for each candidate plan.

For details on the execution stats, see explain.executionStats.

candidatePlanScores

An array of scores for the candidate plans listed in the creationExecStats array.

indexFilterSet

A boolean that indicates whether the an index filter exists for the query shape.

estimatedSizeBytes

A number that describes the estimated size in bytes of a plan cache entry.

New in version 5.0.

Starting in MongoDB 5.0, 4.4.3, and 4.2.12, this field is available.

host

The hostname and port of the mongod instance from which the plan cache information was returned.

When run on a sharded cluster, the operation returns plan cache entry information from a single member in each shard replica set. This member is identified with the shard and host fields. See also Read Preference.

New in version 4.4.

shard

The name of the shard from which $planCacheStats retrieved the cache entry.

Only available if run on a sharded cluster.

New in version 4.4.

Examples

The examples in this section use the following orders collection:

db.orders.insertMany( [
   { "_id" : 1, "item" : "abc", "price" : NumberDecimal("12"), "quantity" : 2, "type": "apparel" },
   { "_id" : 2, "item" : "jkl", "price" : NumberDecimal("20"), "quantity" : 1, "type": "electronics" },
   { "_id" : 3, "item" : "abc", "price" : NumberDecimal("10"), "quantity" : 5, "type": "apparel" },
   { "_id" : 4, "item" : "abc", "price" : NumberDecimal("8"), "quantity" : 10, "type": "apparel" },
   { "_id" : 5, "item" : "jkl", "price" : NumberDecimal("15"), "quantity" : 15, "type": "electronics" }
] )

Create the following indexes on the collection:

db.orders.createIndex( { item: 1 } );
db.orders.createIndex( { item: 1, quantity: 1 } );
db.orders.createIndex( { quantity: 1 } );
db.orders.createIndex( { quantity: 1, type: 1 } );
db.orders.createIndex(
   { item: 1, price: 1 },
   { partialFilterExpression: { price: { $gte: NumberDecimal("10")} } }
);

Note

Index { item: 1, price: 1 } is a partial index and only indexes documents with price field greater than or equal to NumberDecimal("10").

Run some queries against the collection:

db.orders.find( { item: "abc", price: { $gte: NumberDecimal("10") } } )
db.orders.find( { item: "abc", price: { $gte: NumberDecimal("5") } } )
db.orders.find( { quantity: { $gte: 20 } } )
db.orders.find( { quantity: { $gte: 5 }, type: "apparel" } )

Return Information for All Entries in the Query Cache

The following aggregation pipeline uses $planCacheStats to return information on the plan cache entries for the collection:

db.orders.aggregate( [
   { $planCacheStats: { } }
] )

The operation returns all entries in the cache:

{                                               // Plan Cache Entry 1
   "createdFromQuery" : {
      "query" : { "quantity" : { "$gte" : 5 }, "type" : "apparel" },
      "sort" : { },
      "projection" : { }
   },
   "queryHash" : "4D151C4C",
   "planCacheKey" : "DD67E353",
   "isActive" : false,
   "works" : NumberLong(4),
   "cachedPlan" : {
      ...
   },
   "timeOfCreation" : ISODate("2020-02-06T18:15:44.849Z"),
   "creationExecStats" : [
      {
         ... // Exec Stats for Candidate 1
      },
      {
         ... // Exec Stats for Candidate 2
      }
   ],
   "candidatePlanScores" : [
      1.5002,
      1.5002
   ],
   "indexFilterSet" : false,
   "estimatedSizeBytes" : NumberLong(3160),     // Available starting in MongoDB 5.0, 4.4.3, 4.2.12
   "host" : "mongodb1.example.net:27018",       // Available starting in MongoDB 4.4
   "shard" : "shardA"                           // Available starting in MongoDB 4.4 if run on sharded cluster
}
{                                               // Plan Cache Entry 2
   "createdFromQuery" : {
      "query" : { "quantity" : { "$gte" : 20 } },
      "sort" : { },
      "projection" : { }
   },
   "queryHash" : "23B19B75",
   "planCacheKey" : "6F23F858",
   "isActive" : false,
   "works" : NumberLong(1),
   "cachedPlan" : {
      ...
   },
   "timeOfCreation" : ISODate("2020-02-06T18:15:44.454Z"),
   "creationExecStats" : [
      {
         ... // Exec Stats for Candidate 1
      },
      {
         ... // Exec Stats for Candidate 2
      }
   ],
   "candidatePlanScores" : [
      1.0002,
      1.0002
   ],
   "indexFilterSet" : false,
   "estimatedSizeBytes" : NumberLong(2539),     // Available starting in MongoDB 5.0, 4.4.3, 4.2.12
   "host" : "mongodb1.example.net:27018",       // Available starting in MongoDB 4.4
   "shard" : "shardA"                           // Available starting in MongoDB 4.4 if run on sharded cluster
}
{                                               // Plan Cache Entry 3
   "createdFromQuery" : {
      "query" : { "item" : "abc", "price" : { "$gte" : NumberDecimal("5") } },
      "sort" : { },
      "projection" : { }
   },
   "queryHash" : "117A6B10",
   "planCacheKey" : "A1824628",
   "isActive" : true,
   "works" : NumberLong(4),
   "cachedPlan" : {
      ...
   },
   "timeOfCreation" : ISODate("2020-02-06T18:15:44.452Z"),
   "creationExecStats" : [
      {
         ... // Exec Stats for Candidate 1
      },
      {
         ... // Exec Stats for Candidate 2
      }
   ],
   "candidatePlanScores" : [
      1.7502,
      1.7502
   ],
   "indexFilterSet" : false,
   "estimatedSizeBytes" : NumberLong(3183),     // Available starting in MongoDB 5.0, 4.4.3, 4.2.12
   "host" : "mongodb1.example.net:27018",       // Available starting in MongoDB 4.4
   "shard" : "shardA"                           // Available starting in MongoDB 4.4 if run on sharded cluster
}
{                                               // Plan Cache Entry 4
   "createdFromQuery" : {
      "query" : { "item" : "abc", "price" : { "$gte" : NumberDecimal("10") } },
      "sort" : { },
      "projection" : { }
   },
   "queryHash" : "117A6B10",
   "planCacheKey" : "2E6E536B",
   "isActive" : true,
   "works" : NumberLong(3),
   "cachedPlan" : {
      ...
   },
   "timeOfCreation" : ISODate("2020-02-06T18:15:44.449Z"),
   "creationExecStats" : [
      {
         ... // Exec Stats for Candidate 1
      },
      {
         ... // Exec Stats for Candidate 2
      },
      {
         ... // Exec Stats for Candidate 3
      }
   ],
   "candidatePlanScores" : [
      1.6668666666666665,
      1.6668666666666665,
      1.6668666666666665
   ],
   "indexFilterSet" : false,
   "estimatedSizeBytes" : NumberLong(4653),     // Available starting in MongoDB 5.0, 4.4.3, 4.2.12
   "host" : "mongodb1.example.net:27018",       // Available starting in MongoDB 4.4
   "shard" : "shardA"                           // Available starting in MongoDB 4.4 if run on sharded cluster
}

See also planCacheKey.

List Query Shapes

MongoDB 4.4 removes the deprecated planCacheListQueryShapes command and its helper method PlanCache.listQueryShapes().

As an alternative, you can use the $planCacheStats stage to obtain a list of all of the query shapes for which there is a cached plan.

For example, the following uses the $project stage to only output the createdFromQuery field and the queryHash field.

db.orders.aggregate( [ { $planCacheStats: { } } , { $project: {createdFromQuery: 1, queryHash: 1 } } ] )

The operation returns the following query shapes:

{ "createdFromQuery" : { "query" : { "item" : "abc", "price" : { "$gte" : NumberDecimal("5") } }, "sort" : {  }, "projection" : {  } }, "queryHash" : "117A6B10" }
{ "createdFromQuery" : { "query" : { "quantity" : { "$gte" : 5 }, "type" : "apparel" }, "sort" : {  }, "projection" : {  } }, "queryHash" : "4D151C4C" }
{ "createdFromQuery" : { "query" : { "quantity" : { "$gte" : 20 } }, "sort" : {  }, "projection" : {  } }, "queryHash" : "23B19B75" }
{ "createdFromQuery" : { "query" : { "item" : "abc", "price" : { "$gte" : NumberDecimal("10") } }, "sort" : {  }, "projection" : {  } }, "queryHash" : "117A6B10" }

Find Cache Entry Details for a Query Shape

To return plan cache information for a particular query shape, the $planCacheStats stage can be followed by a $match on the planCacheKey field.

The following aggregation pipeline uses $planCacheStats followed by a $match and $project to return specific information for a particular query shape:

db.orders.aggregate( [
   { $planCacheStats: { } },
   { $match: { planCacheKey: "DD67E353"} }
] )

The operation returns the following:

{
   "createdFromQuery" : {
      "query" : { "quantity" : { "$gte" : 5 }, "type" : "apparel" },
      "sort" : { },
      "projection" : { }
   },
   "queryHash" : "4D151C4C",
   "planCacheKey" : "DD67E353",
   "isActive" : false,
   "works" : NumberLong(4),
   "cachedPlan" : {
      "stage" : "FETCH",
      "filter" : {
         "type" : {
            "$eq" : "apparel"
         }
      },
      "inputStage" : {
         "stage" : "IXSCAN",
         "keyPattern" : {
            "quantity" : 1
         },
         "indexName" : "quantity_1",
         "isMultiKey" : false,
         "multiKeyPaths" : {
            "quantity" : [ ]
         },
         "isUnique" : false,
         "isSparse" : false,
         "isPartial" : false,
         "indexVersion" : 2,
         "direction" : "forward",
         "indexBounds" : {
            "quantity" : [
               "[5.0, inf.0]"
            ]
         }
      }
   },
   "timeOfCreation" : ISODate("2020-02-06T18:15:44.849Z"),
   "creationExecStats" : [
      {
         "nReturned" : 2,
         "executionTimeMillisEstimate" : 0,
         "totalKeysExamined" : 3,
         "totalDocsExamined" : 3,
         "executionStages" : {
            "stage" : "FETCH",
            "filter" : {
               "type" : {
                  "$eq" : "apparel"
               }
            },
            "nReturned" : 2,
            "executionTimeMillisEstimate" : 0,
            "works" : 4,
            "advanced" : 2,
            "needTime" : 1,
            "needYield" : 0,
            "saveState" : 0,
            "restoreState" : 0,
            "isEOF" : 1,
            "docsExamined" : 3,
            "alreadyHasObj" : 0,
            "inputStage" : {
               "stage" : "IXSCAN",
               "nReturned" : 3,
               "executionTimeMillisEstimate" : 0,
               "works" : 4,
               "advanced" : 3,
               "needTime" : 0,
               "needYield" : 0,
               "saveState" : 0,
               "restoreState" : 0,
               "isEOF" : 1,
               "keyPattern" : {
                  "quantity" : 1
               },
               "indexName" : "quantity_1",
               "isMultiKey" : false,
               "multiKeyPaths" : {
                  "quantity" : [ ]
               },
               "isUnique" : false,
               "isSparse" : false,
               "isPartial" : false,
               "indexVersion" : 2,
               "direction" : "forward",
               "indexBounds" : {
                  "quantity" : [
                     "[5.0, inf.0]"
                  ]
               },
               "keysExamined" : 3,
               "seeks" : 1,
               "dupsTested" : 0,
               "dupsDropped" : 0
            }
         }
      },
      {
         "nReturned" : 2,
         "executionTimeMillisEstimate" : 0,
         "totalKeysExamined" : 3,
         "totalDocsExamined" : 2,
         "executionStages" : {
            "stage" : "FETCH",
            "nReturned" : 2,
            "executionTimeMillisEstimate" : 0,
            "works" : 4,
            "advanced" : 2,
            "needTime" : 1,
            "needYield" : 0,
            "saveState" : 0,
            "restoreState" : 0,
            "isEOF" : 1,
            "docsExamined" : 2,
            "alreadyHasObj" : 0,
            "inputStage" : {
               "stage" : "IXSCAN",
               "nReturned" : 2,
               "executionTimeMillisEstimate" : 0,
               "works" : 4,
               "advanced" : 2,
               "needTime" : 1,
               "needYield" : 0,
               "saveState" : 0,
               "restoreState" : 0,
               "isEOF" : 1,
               "keyPattern" : {
                  "quantity" : 1,
                  "type" : 1
               },
               "indexName" : "quantity_1_type_1",
               "isMultiKey" : false,
               "multiKeyPaths" : {
                  "quantity" : [ ],
                  "type" : [ ]
               },
               "isUnique" : false,
               "isSparse" : false,
               "isPartial" : false,
               "indexVersion" : 2,
               "direction" : "forward",
               "indexBounds" : {
                  "quantity" : [
                     "[5.0, inf.0]"
                  ],
                  "type" : [
                     "[\"apparel\", \"apparel\"]"
                  ]
               },
               "keysExamined" : 3,
               "seeks" : 2,
               "dupsTested" : 0,
               "dupsDropped" : 0
            }
         }
      }
   ],
   "candidatePlanScores" : [
      1.5002,
      1.5002
   ],
   "indexFilterSet" : false,
   "estimatedSizeBytes" : NumberLong(3160),     // Available starting in MongoDB 5.0, 4.4.3, 4.2.12
   "host" : "mongodb1.example.net:27018",       // Available starting in MongoDB 4.4
   "shard" : "shardA"                           // Available starting in MongoDB 4.4 if run on sharded cluster
}

See also planCacheKey and queryHash.

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