为什么MongoDB不使用Index Intersection？

Question

我正在尝试重现索引交集指令的第一个示例(http://docs.mongodb.org/manual/core/index-intersection/)但面临一个问题:mongo不使用这两个索引

我的步骤:

1. 下载mongo(3.0.3)并安装它
2. 运行mongod:mongod.exe --dbpath d:\ data(文件夹为空)
3. 运行mongo:mongo.exe

4. 添加索引:

   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. 检查查询说明:

   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
   }
   

如您所见,winnerPlan仅包含item_1索引.存在包含qty_1索引的rejectedPlans.但是没有包含索引交集的计划.我知道选择具体指数有很多条件.但在我的情况下,mongo甚至没有计划它!

有人能帮助我吗？

Answer 1

SERVER-3071 JIRA 问题中有一些关于索引选择的详细信息，但我不能说所有内容是否仍然与 3.0 相关。反正：

MongoDB 3.0.2 似乎没有考虑范围查询的索引交互。但对于点间隔来说是这样的：

> 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\"]"
                                ]
                            }
                        }
                    ]
                }