MongoDB高效的方式来搜索objectId字段与null使用索引？

Question

我正试图找到一种有效的方式来搜索特定字段为null的项目。

在MongoDB中，我有folder架构，其parent字段的父文件夹的ObjectId和parent被索引。对于根文件夹，父项字段为null。

当我试图找到所有的根文件夹与parent:null，explain显示indexOnly: false

db.folders.find({parent: null}, {parent: 1, _id: 0}).explain() 
{ 
    "cursor" : "BtreeCursor parent_1", 
    "isMultiKey" : false, 
    "n" : 126, 
    "nscannedObjects" : 126, 
    "nscanned" : 126, 
    "nscannedObjectsAllPlans" : 126, 
    "nscannedAllPlans" : 126, 
    "scanAndOrder" : false, 
    "indexOnly" : false, 
    "nYields" : 1, 
    "nChunkSkips" : 0, 
    "millis" : 0, 
    "indexBounds" : { 
     "parent" : [ 
      [ 
       null, 
       null 
      ] 
     ] 
    }, 
    "server" : "c268.candidate.36:10268", 
    "filterSet" : false, 
    "stats" : { 
     "type" : "PROJECTION", 
     "works" : 128, 
     "yields" : 1, 
     "unyields" : 1, 
     "invalidates" : 0, 
     "advanced" : 126, 
     "needTime" : 0, 
     "needFetch" : 0, 
     "isEOF" : 1, 
     "children" : [ 
      { 
       "type" : "KEEP_MUTATIONS", 
       "works" : 128, 
       "yields" : 1, 
       "unyields" : 1, 
       "invalidates" : 0, 
       "advanced" : 126, 
       "needTime" : 1, 
       "needFetch" : 0, 
       "isEOF" : 1, 
       "children" : [ 
        { 
         "type" : "FETCH", 
         "works" : 128, 
         "yields" : 1, 
         "unyields" : 1, 
         "invalidates" : 0, 
         "advanced" : 126, 
         "needTime" : 1, 
         "needFetch" : 0, 
         "isEOF" : 1, 
         "alreadyHasObj" : 0, 
         "forcedFetches" : 0, 
         "matchTested" : 126, 
         "children" : [ 
          { 
           "type" : "IXSCAN", 
           "works" : 127, 
           "yields" : 1, 
           "unyields" : 1, 
           "invalidates" : 0, 
           "advanced" : 126, 
           "needTime" : 1, 
           "needFetch" : 0, 
           "isEOF" : 1, 
           "keyPattern" : "{ parent: 1.0 }", 
           "isMultiKey" : 0, 
           "boundsVerbose" : "field #0['parent']: [null, null]", 
           "yieldMovedCursor" : 0, 
           "dupsTested" : 0, 
           "dupsDropped" : 0, 
           "seenInvalidated" : 0, 
           "matchTested" : 0, 
           "keysExamined" : 126, 
           "children" : [] 
          } 
         ] 
        } 
       ] 
      } 
     ] 
    } 
} 

我试图用$类型找到任何的人，其父母文件夹不是ObjectId。但仍然indexOnly:false。

db.folders.find({parent: {$ne: {$type: 7}}}, {parent: 1, _id: 0}).explain() "indexOnly": false

有没有一种方法来搜索空值仅使用索引？如果没有，是否有更好的价值来存储，而不是null能够搜索索引？根文件夹的

---

附加

例如

{ 
    "_id" : ObjectId("55a04a2d754971030059b7ad"), 
    "active" : true, 
    "modified" : ISODate("2016-02-05T22:30:08.053Z"), 
    "created" : ISODate("2015-07-10T22:41:49.009Z"), 
    "user" : ObjectId("54d3ae187a738c0300f59e61"), 
    "name" : "2nd Grade", 
    "parent" : null, 
    "clientModified" : ISODate("2016-02-05T22:30:07.872Z"), 
    "userCreated" : ISODate("2015-07-10T22:41:48.328Z"), 
    "ancestors" : [], 
    "__v" : 2 
} 

示例子文件夹的

{ 
    "_id" : ObjectId("56d0b4edb6f05e03009bcabc"), 
    "active" : true, 
    "modified" : ISODate("2016-02-26T20:26:21.328Z"), 
    "created" : ISODate("2016-02-26T20:26:21.328Z"), 
    "user" : ObjectId("54d3ae187a738c0300f59e61"), 
    "name" : "music", 
    "parent" : ObjectId("55a04a2d754971030059b7ad"), 
    "clientModified" : ISODate("2016-02-26T20:26:20.398Z"), 
    "userCreated" : ISODate("2016-02-26T20:26:20.398Z"), 
    "ancestors" : [ 
    ObjectId("55a04a2d754971030059b7ad") 
    ], 
    "__v" : 0 
} 

db.folders.getIndexes（）

{ 
    "0" : { 
     "v" : 1, 
     "key" : { 
      "_id" : 1 
     }, 
     "name" : "_id_", 
     "ns" : "app29099188.folders" 
    }, 
    "1" : { 
     "v" : 1, 
     "key" : { 
      "user" : 1, 
      "_fts" : "text", 
      "_ftsx" : 1 
     }, 
     "name" : "user_1_name_text", 
     "ns" : "app29099188.folders", 
     "background" : true, 
     "safe" : null, 
     "weights" : { 
      "name" : 1 
     }, 
     "default_language" : "english", 
     "language_override" : "language", 
     "textIndexVersion" : 2 
    }, 
    "2" : { 
     "v" : 1, 
     "key" : { 
      "user" : 1, 
      "parent" : 1 
     }, 
     "name" : "user_1_parent_1", 
     "ns" : "app29099188.folders", 
     "background" : true, 
     "safe" : null 
    }, 
    "3" : { 
     "v" : 1, 
     "key" : { 
      "parent" : 1.0000000000000000 
     }, 
     "name" : "parent_1", 
     "ns" : "app29099188.folders" 
    } 
} 

Answer 1

评论 - 更新： 消除docScan的方法是在父字段中有一个值。它可以归零objectId或只是“/”作为根。

db.satoko.insert({"test":"sdsf", parent: "/"}) 
db.satoko.insert({"test":"sds33f", parent: "/"})   
db.satoko.insert({"parent":ObjectId("56d8b2879bd059e7247a6096"), "test":"sdsf"}) 

解释结果：

db.satoko.find({parent:{$eq:"/"} }, {parent: 1, _id: 0}).explain("allPlansExec 
ution") 
{ 
     "queryPlanner" : { 
       "plannerVersion" : 1, 
       "namespace" : "test.satoko", 
       "indexFilterSet" : false, 
       "parsedQuery" : { 
         "parent" : { 
           "$eq" : "/" 
         } 
       }, 
       "winningPlan" : { 
         "stage" : "PROJECTION", 
         "transformBy" : { 
           "parent" : 1, 
           "_id" : 0 
         }, 
         "inputStage" : { 
           "stage" : "IXSCAN", 
           "keyPattern" : { 
             "parent" : 1 
           }, 
           "indexName" : "parent_1", 
           "isMultiKey" : false, 
           "isUnique" : false, 
           "isSparse" : false, 
           "isPartial" : false, 
           "indexVersion" : 1, 
           "direction" : "forward", 
           "indexBounds" : { 
             "parent" : [ 
               "[\"/\", \"/\"]" 
             ] 
           } 
         } 
       }, 
       "rejectedPlans" : [ ] 
     }, 
     "executionStats" : { 
       "executionSuccess" : true, 
       "nReturned" : 2, 
       "executionTimeMillis" : 0, 
       "totalKeysExamined" : 2, 
       "totalDocsExamined" : 0, 
       "executionStages" : { 
         "stage" : "PROJECTION", 
         "nReturned" : 2, 
         "executionTimeMillisEstimate" : 0, 
         "works" : 3, 
         "advanced" : 2, 
         "needTime" : 0, 
         "needYield" : 0, 
         "saveState" : 0, 
         "restoreState" : 0, 
         "isEOF" : 1, 
         "invalidates" : 0, 
         "transformBy" : { 
           "parent" : 1, 
           "_id" : 0 
         }, 
         "inputStage" : { 
           "stage" : "IXSCAN", 
           "nReturned" : 2, 
           "executionTimeMillisEstimate" : 0, 
           "works" : 3, 
           "advanced" : 2, 
           "needTime" : 0, 
           "needYield" : 0, 
           "saveState" : 0, 
           "restoreState" : 0, 
           "isEOF" : 1, 
           "invalidates" : 0, 
           "keyPattern" : { 
             "parent" : 1 
           }, 
           "indexName" : "parent_1", 
           "isMultiKey" : false, 
           "isUnique" : false, 
           "isSparse" : false, 
           "isPartial" : false, 
           "indexVersion" : 1, 
           "direction" : "forward", 
           "indexBounds" : { 
             "parent" : [ 
               "[\"/\", \"/\"]" 
             ] 
           }, 
           "keysExamined" : 2, 
           "dupsTested" : 0, 
           "dupsDropped" : 0, 
           "seenInvalidated" : 0 
         } 
       }, 
       "allPlansExecution" : [ ] 
     }, 
     "serverInfo" : { 
       "host" : "greg", 
       "port" : 27017, 
       "version" : "3.2.3", 
       "gitVersion" : "b326ba837cf6f49d65c2f85e1b70f6f31ece7937" 
     }, 
     "ok" : 1 
}