planCacheListQueryShapes

planCacheListQueryShapes

Definition

planCacheListQueryShapes

Deprecated since version 4.2.

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 overthe following methods and commands, which have been deprecated in 4.2:

PlanCache.getPlansByQuery()method/planCacheListPlans command, and
PlanCache.listQueryShapes()method/planCacheListQueryShapes command.

Displays the query shapes for which cachedquery 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 witha queryHash. The queryHash is ahexadecimal string that represents a hash of the query shape andis dependent only on the query shape.

Note

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

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

The mongo shell provides the wrapperPlanCache.listQueryShapes() for this command.

The command has the following syntax:

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

The planCacheListQueryShapes command has the following field:

FieldTypeDescriptionplanCacheListQueryShapesstringThe 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 thatincludes the planCacheRead action.

Example

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

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

The command returns a document that contains the field shapes thatcontains an array of the query shapes currentlyin the cache. In the example, the orders collection had cachedquery 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
}