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planCacheListPlans

Definition

planCacheListPlans

Note

MongoDB 4.2 adds a new aggregation pipeline stage $planCacheStats that provides plan cache information for a collection.

The $planCacheStats aggregation stage is preferred over the PlanCache.getPlansByQuery() method/planCacheListPlans command and PlanCache.listQueryShapes() method/planCacheListQueryShapes command.

Displays the cached query plans for the specified query shape.

To help identify slow queries with the same query shape, starting in MongoDB 4.2, each query shape is associated with a queryHash. The queryHash is a hexadecimal string that represents a hash of the query shape and is dependent only on the 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.

The query optimizer only caches the plans for those query shapes that can have more than one viable plan.

The mongo shell provides the wrapper PlanCache.getPlansByQuery() for this command.

The planCacheListPlans command has the following syntax:

db.runCommand(
   {
      planCacheListPlans: <collection>,
      query: <query>,
      sort: <sort>,
      projection: <projection>
   }

The planCacheListPlans command has the following field:

Field Type Description
query document The query predicate of the query shape. Only the structure of the predicate, including the field names, are significant to the shape; the values in the query predicate are insignificant.
projection document Optional. The projection associated with the query shape.
sort document Optional. The sort associated with the query shape.

To see the query shapes for which cached query plans exist, use the planCacheListQueryShapes command.

New in version 3.6: The planCacheListPlans database command returns the same output as the PlanCache.getPlansByQuery() method.

Required Access

On systems running with authorization, a user must have access that includes the planCacheRead action.

Example

If a collection orders has the following query shape:

{
  "query" : { "qty" : { "$gt" : 10 } },
  "sort" : { "ord_date" : 1 },
  "projection" : { },
  "queryHash" : "9AAD95BE"   // Available starting in MongoDB 4.2
}

The following operation displays the query plan cached for the shape:

db.runCommand(
   {
      planCacheListPlans: "orders",
      query: { "qty" : { "$gt" : 10 } },
      sort: { "ord_date" : 1 }
   }
)