MongoDB $查询运算符忽略索引？

Question

我创建了一个测试MongoDB集合"sampleCollection",其文档如下所示:

 "_id" : ObjectId("510929e041cb2179b41ace1c"),
 "stringField" : "Random string0",
 "longField" : NumberLong(886)

并在字段"stringField"上有索引.当我执行

db.sampleCollection.find({"stringField":"Random string0"}).explain()

一切都好:

 "cursor" : "BtreeCursor stringField_1",
 "isMultiKey" : false,
 "n" : 2,
 "nscannedObjects" : 2,
 "nscanned" : 2,
 "nscannedObjectsAllPlans" : 2,
 "nscannedAllPlans" : 2,
 "scanAndOrder" : false,
 "indexOnly" : false,
 "nYields" : 0,
 "nChunkSkips" : 0,
 "millis" : 0,
 "indexBounds" : {
         "stringField" : [
                 [
                         "Random string0",
                         "Random string0"
                 ]
         ]
 }

但

db.sampleCollection.find({$query:{"stringField":"Random string0"}}).explain()

抓住我

"cursor" : "BasicCursor",
"isMultiKey" : false,
"n" : 0,
"nscannedObjects" : 4,
"nscanned" : 4,
"nscannedObjectsAllPlans" : 4,
"nscannedAllPlans" : 4,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {

}

这看起来不是问题,但我在生产和使用查询结构时使用org.springframework.data.mongodb框架是编写存储库的唯一方法.因此我有一个完全忽略索引数据的数据库.

这是对的吗？或者我误解了什么？

Answer 1

这很有趣我不能决定说它是一个错误或不是由你决定:

有两种可用的语法:http://docs.mongodb.org/manual/reference/operator/query/

当你使用:

db.collection.find( { age : 25 } )

也会

db.collection.find( { age : 25 } ).explain()
db.collection.find( { age : 25 } ).hint(someindex)

工作得很好.

当您使用解决方案时(其他语法):

db.collection.find( { $query: { age : 25 } } )

的输出

db.sampleCollection.find({$query:{"stringField":"Random string0"}}).explain()

将显示像查询不使用索引

如果您还使用.hint作为索引,它将省略结果.:)(那是我真的不明白)

幸运的是,这些操作还有另一种语法:您可以使用:

db.sampleCollection.find({$query:{"stringField":"Random string0"}, $explain:1})

它将具有正确的输出并向我显示索引的用法.$ hint也有类似的语法.

您可以在此处查看文档:http://docs.mongodb.org/manual/reference/meta-query-operators/

我发现这很有趣所以我打开了探查器:

我制作了一个测试集合(queryTst),其中包含大约250k个文档,每个文档只有_id和结构中的年龄字段,并带有年龄索引.

对于此查询:

db.queryTst.find({$query:{"age":16},$explain:1})

我有:

{
    "cursor" : "BtreeCursor age_1",
    "isMultiKey" : false,
    "n" : 2,
    "nscannedObjects" : 2,
    "nscanned" : 2,
    "nscannedObjectsAllPlans" : 2,
    "nscannedAllPlans" : 2,
    "scanAndOrder" : false,
    "indexOnly" : false,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "millis" : 0,
    "indexBounds" : {
        "age" : [
            [
                16,
                16
            ]
        ]
    },
    "allPlans" : [
        {
            "cursor" : "BtreeCursor age_1",
            "n" : 2,
            "nscannedObjects" : 2,
            "nscanned" : 2,
            "indexBounds" : {
                "age" : [
                    [
                        16,
                        16
                    ]
                ]
            }
        }
    ],
    "oldPlan" : {
        "cursor" : "BtreeCursor age_1",
        "indexBounds" : {
            "age" : [
                [
                    16,
                    16
                ]
            ]
        }
    },
    "server" : ""
}

为了这:

 db.queryTst.find({$query:{"age":16},$explain:1}).explain()

我有:

"cursor" : "BasicCursor",
    "isMultiKey" : false,
    "n" : 0,
    "nscannedObjects" : 250011,
    "nscanned" : 250011,
    "nscannedObjectsAllPlans" : 250011,
    "nscannedAllPlans" : 250011,
    "scanAndOrder" : false,
    "indexOnly" : false,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "millis" : 103,
    "indexBounds" : {

    },

在探查器日志中:第一个

{
    "ts" : ISODate("2013-01-30T20:35:40.526Z"),
    "op" : "query",
    "ns" : "test.queryTst",
    "query" : {
        "$query" : {
            "age" : 16
        },
        "$explain" : 1
    },
    "ntoreturn" : 0,
    "ntoskip" : 0,
    "nscanned" : 2,
    "keyUpdates" : 0,
    "numYield" : 0,
    "lockStats" : {
        "timeLockedMicros" : {
            "r" : NumberLong(368),
            "w" : NumberLong(0)
        },
        "timeAcquiringMicros" : {
            "r" : NumberLong(8),
            "w" : NumberLong(5)
        }
    },
    "nreturned" : 1,
    "responseLength" : 567,
    "millis" : 0,
    "client" : "127.0.0.1",
    "user" : ""
}

对于第二个:

{
    "ts" : ISODate("2013-01-30T20:35:47.715Z"),
    "op" : "query",
    "ns" : "test.queryTst",
    "query" : {
        "query" : {
            "$query" : {
                "age" : 16
            },
            "$explain" : 1
        },
        "$explain" : true
    },
    "ntoreturn" : 0,
    "ntoskip" : 0,
    "nscanned" : 250011,
    "keyUpdates" : 0,
    "numYield" : 0,
    "lockStats" : {
        "timeLockedMicros" : {
            "r" : NumberLong(104092),
            "w" : NumberLong(0)
        },
        "timeAcquiringMicros" : {
            "r" : NumberLong(13),
            "w" : NumberLong(5)
        }
    },
    "nreturned" : 1,
    "responseLength" : 373,
    "millis" : 104,
    "client" : "127.0.0.1",
    "user" : ""
}

以某种方式对我说的是explain()导致混合语法中的表扫描.