爲什麼MongoDB不使用索引相交？

Question

我試圖重現指數路口指令（http://docs.mongodb.org/manual/core/index-intersection/）的第一個例子，但面臨的一個問題：蒙戈不使用股指雙雙

我的步驟：

1. 下載蒙戈（3.0.3）和安裝
2. 運行的mongod：mongod.exe --dbpath d：\數據（文件夾爲空）
3. 運行蒙戈：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. 檢查查詢說明：

   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 
   } 
   

正如你可以看到winningPlan只包含ITEM_1指數。有包含qty_1索引的rejectPlans。但是沒有包含索引交集的計劃。 我知道有很多條件可以選擇特定的索引。但在我的情況下，mongo甚至沒有計劃！

有人能幫助我嗎？

Answer 1

有大約在SERVER-3071 JIRA issue指數選擇的一些細節，但我不能說，如果一切仍然是相關的3.0。無論如何：

MongoDB 3.0.2 似乎不考慮索引交互範圍查詢。但它會爲點間隔：

> 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\"]" 
           ] 
          } 
         } 
        ] 
       } 

Answer 2

我們承載一個多租戶網站。因此，有一個蒙戈集合持有所有客戶的一些信息，例如，

{customerId: 22, category: "category", region: "region", balance: 23434... } 

後，我們瞭解到索引交集，我們試圖建立多單字段索引，以支持不同的查詢條件，具有可變領域，如查詢作爲

{ customerId: 22, category: "1" } 
{ customerId: 22, region: "somewhere" } 
{ customerId: 22, balance: {$gt:0} } 
{ customerId: 22, region: {$in: ["R1", "R2"]},balance: {$gt:0} } 
.... 

但它簡單地通過學習「db.collection.explain」沒有發生交集。 由於字段有限，我們終於找到了一個解決方案來構建包含所有字段的複合索引。

{customerId: 1, category: 1, region: 1, balance:1...} 

然後事實證明，下面的所有查詢使用「大」複合索引，只要「客戶ID」是在查詢中。

{ customerId: 22, category: "1" } 
{ customerId: 22, region: "somewhere" } 
{ customerId: 22, balance: {$gt:0} } 
{ customerId: 22, region: {$in: ["R1", "R2"]},balance: {$gt:0} } 
....