Question

Mongo 2.4.1，带有仲裁者的2成员副本

我认为这是一个非常简单的查询，返回不一致的结果：

objects = db.collection.find({'field.id': my_id, 'field1': None, 'field2': 'value'})

我收到77个对象，但集合中有248个与该查询匹配的对象。我怀疑查询优化器正在做一些我不期望的事情。收集中的所有索引都不稀疏。

在上述查询的.explain（）中，我得到了

{cursor: 'BtreeCursor field.id_1', n: 283, nscanned: 77}

为什么nscanned小于283？

我还应该注意，虽然77长度的结果非常一致，但偶尔完全相同的查询将返回包含238个文档的结果。

编辑：完整.explain（）包含在下面。我注意到它看了BtreeCursor field.id_1两次。这是正常的吗？

{
    "nYields": 0,
    "nscannedAllPlans": 1011,
    "allPlans": [
        {
            "cursor": "BtreeCursor field2_1",
            "indexBounds": {
                "field2": [
                    ["moment", "moment"]
                ]
            },
            "nscannedObjects": 78,
            "nscanned": 78,
            "n": 0
        },
        {
            "cursor": "BtreeCursor field.id_1_field3_1_field2_1_field1_1_created_at_-1",
            "indexBounds": {
                "field1": [
                    [null, null]
                ],
                "created_at": [
                    [
                        {
                            "$maxElement": 1
                        },
                        {
                            "$minElement": 1
                        }
                    ]
                ],
                "field3": [
                    [
                        {
                            "$minElement": 1
                        },
                        {
                            "$maxElement": 1
                        }
                    ]
                ],
                "field2": [
                    ["moment", "moment"]
                ],
                "field.id": [
                    ["oid-50eba1a2256b4738d0e5254d", "oid-50eba1a2256b4738d0e5254d"]
                ]
            },
            "nscannedObjects": 78,
            "nscanned": 78,
            "n": 78
        },
        {
            "cursor": "BtreeCursor field1_1_field3_1_field4_1_field5_-1_field2_1_created_at_-1",
            "indexBounds": {
                "field4": [
                    [
                        {
                            "$minElement": 1
                        },
                        {
                            "$maxElement": 1
                        }
                    ]
                ],
                "field3": [
                    [
                        {
                            "$minElement": 1
                        },
                        {
                            "$maxElement": 1
                        }
                    ]
                ],
                "field1": [
                    [null, null]
                ],
                "created_at": [
                    [
                        {
                            "$maxElement": 1
                        },
                        {
                            "$minElement": 1
                        }
                    ]
                ],
                "field5": [
                    [
                        {
                            "$maxElement": 1
                        },
                        {
                            "$minElement": 1
                        }
                    ]
                ],
                "field2": [
                    ["moment", "moment"]
                ]
            },
            "nscannedObjects": 78,
            "nscanned": 78,
            "n": 0
        },
        {
            "cursor": "BtreeCursor field1_1_field3_1_field4_1_field2_1_created_at_-1",
            "indexBounds": {
                "field1": [
                    [null, null]
                ],
                "created_at": [
                    [
                        {
                            "$maxElement": 1
                        },
                        {
                            "$minElement": 1
                        }
                    ]
                ],
                "field4": [
                    [
                        {
                            "$minElement": 1
                        },
                        {
                            "$maxElement": 1
                        }
                    ]
                ],
                "field2": [
                    ["moment", "moment"]
                ],
                "field3": [
                    [
                        {
                            "$minElement": 1
                        },
                        {
                            "$maxElement": 1
                        }
                    ]
                ]
            },
            "nscannedObjects": 78,
            "nscanned": 78,
            "n": 0
        },
        {
            "cursor": "BtreeCursor field1_-1_created_at_-1",
            "indexBounds": {
                "field1": [
                    [null, null]
                ],
                "created_at": [
                    [
                        {
                            "$maxElement": 1
                        },
                        {
                            "$minElement": 1
                        }
                    ]
                ]
            },
            "nscannedObjects": 78,
            "nscanned": 78,
            "n": 0
        },
        {
            "cursor": "BtreeCursor field.id_1_field3_1_field1_1_created_at_-1",
            "indexBounds": {
                "field1": [
                    [null, null]
                ],
                "created_at": [
                    [
                        {
                            "$maxElement": 1
                        },
                        {
                            "$minElement": 1
                        }
                    ]
                ],
                "field3": [
                    [
                        {
                            "$minElement": 1
                        },
                        {
                            "$maxElement": 1
                        }
                    ]
                ],
                "field.id": [
                    ["oid-50eba1a2256b4738d0e5254d", "oid-50eba1a2256b4738d0e5254d"]
                ]
            },
            "nscannedObjects": 78,
            "nscanned": 78,
            "n": 78
        },
        {
            "cursor": "BtreeCursor field.id_1_field6_1_field2_1",
            "indexBounds": {
                "field6": [
                    [
                        {
                            "$minElement": 1
                        },
                        {
                            "$maxElement": 1
                        }
                    ]
                ],
                "field2": [
                    ["moment", "moment"]
                ],
                "field.id": [
                    ["oid-50eba1a2256b4738d0e5254d", "oid-50eba1a2256b4738d0e5254d"]
                ]
            },
            "nscannedObjects": 76,
            "nscanned": 77,
            "n": 76
        },
        {
            "cursor": "BtreeCursor field.id_1_field7_1",
            "indexBounds": {
                "field7": [
                    [
                        {
                            "$minElement": 1
                        },
                        {
                            "$maxElement": 1
                        }
                    ]
                ],
                "field.id": [
                    ["oid-50eba1a2256b4738d0e5254d", "oid-50eba1a2256b4738d0e5254d"]
                ]
            },
            "nscannedObjects": 78,
            "nscanned": 78,
            "n": 71
        },
        {
            "cursor": "BtreeCursor field.id_1",
            "indexBounds": {
                "field.id": [
                    ["oid-50eba1a2256b4738d0e5254d", "oid-50eba1a2256b4738d0e5254d"]
                ]
            },
            "nscannedObjects": 78,
            "nscanned": 78,
            "n": 77
        },
        {
            "cursor": "BtreeCursor field.id_1",
            "indexBounds": {
                "field.id": [
                    ["oid-50eba1a2256b4738d0e5254d", "oid-50eba1a2256b4738d0e5254d"]
                ]
            },
            "nscannedObjects": 77,
            "nscanned": 77,
            "n": 72
        },
        {
            "cursor": "BasicCursor",
            "indexBounds": {},
            "nscannedObjects": 77,
            "nscanned": 77,
            "n": 0
        }
    ], 
    "millis": 14, 
    "nChunkSkips": 0, 
    "server": "c3:10003", 
    "n": 283, 
    "cursor": "BtreeCursor field.id_1",
    "oldPlan": {
        "cursor": "BtreeCursor field.id_1",
        "indexBounds": {
            "field.id": [
                ["oid-50eba1a2256b4738d0e5254d", "oid-50eba1a2256b4738d0e5254d"]
            ]
        }
    }, 
    "scanAndOrder": false, 
    "indexBounds": {
        "field.id": [
            ["oid-50eba1a2256b4738d0e5254d", "oid-50eba1a2256b4738d0e5254d"]
        ]
    }, 
    "nscannedObjectsAllPlans": 1000, 
    "isMultiKey": false, 
    "indexOnly": false, 
    "nscanned": 77, 
    "nscannedObjects": 77
}

Answer 1

可能是因为您有稀疏索引。

n =符合查询选择条件的文档数
nscanned =数据库操作期间扫描的文档或索引条目数

如果您有https://gist.github.com/mauritsvanrees/99cb4a25b622479e7dc3，则索引将不包含没有索引键的对象的任何条目。

从查询的外观来看，mongodb确实正在使用field1的索引，而您的查询正在搜索该字段的null值。因此，请检查您的索引是否稀疏 - 这很可能是原因。

MongoDB与n>的结果不一致nscanned in explain

1 个答案: