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planCacheListQueryShapes

Definition

planCacheListQueryShapes

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 query shapes for which cached query plans exist for a collection.

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.listQueryShapes() for this command.

The command has the following syntax:

db.runCommand(
   {
      planCacheListQueryShapes: <collection>
   }
)

The planCacheListQueryShapes command has the following field:

Field Type Description
planCacheListQueryShapes string The name of the collection.
Returns:A document that contains an array of query shapes for which cached query plans exist.

Required Access

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

Example

The following returns the query shapes that have cached plans for the orders collection:

db.runCommand(
   {
      planCacheListQueryShapes: "orders"
   }
)

The command returns a document that contains the field shapes that contains an array of the query shapes currently in the cache. In the example, the orders collection had cached query plans associated with the following shapes:

{
  "shapes" : [
     {
       "query" : { "qty" : { "$gt" : 10 } },
        "sort" : { "ord_date" : 1 },
        "projection" : { },
        "queryHash" : "9AAD95BE"  // Available starting in MongoDB 4.2
     },
     {
        "query" : { "$or" : [ { "qty" : { "$gt" : 15 } }, { "status" : "A" } ] },
        "sort" : { },
        "projection" : { }
     },
     {
       "query" : { "$or" :
          [
            { "qty" : { "$gt" : 15 }, "item" : "xyz123" },
            { "status" : "A" }
          ]
        },
        "sort" : { },
        "projection" : { },
        "queryHash" : "0A087AD0"  // Available starting in MongoDB 4.2
      }
   ],
   "ok" : 1
}