我正在尝试找到一种有效的方法来搜索特定字段为null的项目。
在MongoDB中，我有folder架构，其父文件夹的parent字段ObjectId和parent已编入索引。对于根文件夹，父字段为null。
当我尝试使用parent:null查找所有根文件夹时，explain会显示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" : []
}
]
}
]
}
]
}
}
我尝试使用$ type查找父文件夹不是ObjectId的任何内容。但仍然是indexOnly:false。
db.folders.find({parent: {$ne: {$type: 7}}}, {parent: 1, _id: 0}).explain()
"indexOnly": false
有没有办法只使用索引搜索空值？如果没有，是否有更好的值存储而不是null能够使用索引进行搜索？
附加
根文件夹的示例
{
"_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
}
子文件夹的示例
{
"_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"
}
}
答案 0 :(得分：2)
评论后 - 更新： 消除docScan的方法是在父字段中具有值。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
}