Why MongoDB doesn't use Index Intersection?

Question

I am trying to reproduce the first example of index intersection instruction (http://docs.mongodb.org/manual/core/index-intersection/) but facing a problem: mongo doesn't uses both indexes

My steps:

1. Download mongo (3.0.3) and install it
2. Run mongod: mongod.exe --dbpath d:\data (folder is empty)
3. Run mongo: mongo.exe

4. Add indexes:

   db.orders.ensureIndex({ qty: 1 })
   db.orders.ensureIndex({ item: 1 })
   db.orders.getIndexes()
   [{
           "v" : 1,
           "key" : {
                   "_id" : 1
           },
           "name" : "_id_",
           "ns" : "test.orders"
   },
   {
           "v" : 1,
           "key" : {
                   "qty" : 1
           },
           "name" : "qty_1",
           "ns" : "test.orders"
   },
   {
           "v" : 1,
           "key" : {
                   "item" : 1
           },
           "name" : "item_1",
           "ns" : "test.orders"
   }]
   

5. Check query explain:

   db.orders.find( { item: "abc123", qty: { $gt: 15 } } ).explain()
   {
       "queryPlanner" : {
               "plannerVersion" : 1,
               "namespace" : "test.orders",
               "indexFilterSet" : false,
               "parsedQuery" : {
                       "$and" : [
                               {
                                       "item" : {
                                               "$eq" : "abc123"
                                       }
                               },
                               {
                                       "qty" : {
                                               "$gt" : 15
                                       }
                               }
                       ]
               },
               "winningPlan" : {
                       "stage" : "KEEP_MUTATIONS",
                       "inputStage" : {
                               "stage" : "FETCH",
                               "filter" : {
                                       "qty" : {
                                               "$gt" : 15
                                       }
                               },
                               "inputStage" : {
                                       "stage" : "IXSCAN",
                                       "keyPattern" : {
                                               "item" : 1
                                       },
                                       "indexName" : "item_1",
                                       "isMultiKey" : false,
                                       "direction" : "forward",
                                       "indexBounds" : {
                                               "item" : [
                                                       "[\"abc123\", \"abc123\"]"
                                               ]
                                       }
                               }
                       }
               },
               "rejectedPlans" : [
                       {
                               "stage" : "KEEP_MUTATIONS",
                               "inputStage" : {
                                       "stage" : "FETCH",
                                       "filter" : {
                                               "item" : {
                                                       "$eq" : "abc123"
                                               }
                                       },
                                       "inputStage" : {
                                               "stage" : "IXSCAN",
                                               "keyPattern" : {
                                                       "qty" : 1
                                               },
                                               "indexName" : "qty_1",
                                               "isMultiKey" : false,
                                               "direction" : "forward",
                                               "indexBounds" : {
                                                       "qty" : [
                                                               "(15.0, 1.#INF]"
                                                       ]
                                               }
                                       }
                               }
                       }
               ]
       },
       "serverInfo" : {
               "host" : "localhost",
               "port" : 27017,
               "version" : "3.0.3",
               "gitVersion" : "b40106b36eecd1b4407eb1ad1af6bc60593c6105"
       },
       "ok" : 1
   }
   

As you can see winningPlan contains only item_1 index. There is rejectedPlans which contains qty_1 index. But there is no plans which contains index intersection. I know that there are a lot of conditions to select specific index. But in my case mongo doesn't even plans it!

Could anybody help me?

Answer 1

There are some details about the index selection in the SERVER-3071 JIRA issue but I cannot say if all is still relevant for 3.0. Anyway:

MongoDB 3.0.2 seems not consider index interaction for range query. But it will for point intervals:

> db.orders.find( { item: {$eq : "abc123"}, qty: { $eq: 15 } } ).explain()
...

        {
            "stage" : "FETCH",
            "inputStage" : {
                "stage" : "KEEP_MUTATIONS",
                "inputStage" : {
                    "stage" : "AND_SORTED",
                    "inputStages" : [
                        {
                            "stage" : "IXSCAN",
                            "keyPattern" : {
                                "qty" : 1
                            },
                            "indexName" : "qty_1",
                            "isMultiKey" : false,
                            "direction" : "forward",
                            "indexBounds" : {
                                "qty" : [
                                    "[15.0, 15.0]"
                                ]
                            }
                        },
                        {
                            "stage" : "IXSCAN",
                            "keyPattern" : {
                                "item" : 1
                            },
                            "indexName" : "item_1",
                            "isMultiKey" : false,
                            "direction" : "forward",
                            "indexBounds" : {
                                "item" : [
                                    "[\"abc123\", \"abc123\"]"
                                ]
                            }
                        }
                    ]
                }