和 MySQL 以及其他计算引擎类似，MongoDB 给我们提供了 explain 命令来查看某个查询的执行计划，其使用也比较简单，具体如下：

db.collection.explain().<method(...)>

explain 命令默认是打印出查询的 queryPlanner，也就是什么参数都不传递。从 3.5.5 版本开始，explain 命名还支持 executionStats 和 allPlansExecution 两种运行模式，参见 SERVER-19758。

比如下面我们查看 db.orders.find({"o_custkey" : 1439371}) 命令运行的运行详细的信息可以如下执行：

db.orders.find({"o_custkey" : 1439371}).explain('executionStats')

[
    {
        'queryPlanner': {
            'plannerVersion': 1,
            'namespace': "mongo_test.orders",
            'indexFilterSet': false,
            'parsedQuery': {
                'o_custkey': {
                    '$eq': 1439371
                }
            },
            'winningPlan': {
                'stage': "FETCH",
                'inputStage': {
                    'stage': "IXSCAN",
                    'keyPattern': {
                        'o_custkey': 1
                    },
                    'indexName': "o_custkey_1",
                    'isMultiKey': false,
                    'multiKeyPaths': {
                        'o_custkey': [
                        ]
                    },
                    'isUnique': false,
                    'isSparse': false,
                    'isPartial': false,
                    'indexVersion': 2,
                    'direction': "forward",
                    'indexBounds': {
                        'o_custkey': [
                            "[1439371, 1439371]"
                        ]
                    }
                }
            },
            'rejectedPlans': [
            ]
        },
        'executionStats': {
            'executionSuccess': true,
            'nReturned': 31,
            'executionTimeMillis': 1,
            'totalKeysExamined': 31,
            'totalDocsExamined': 31,
            'executionStages': {
                'stage': "FETCH",
                'nReturned': 31,
                'executionTimeMillisEstimate': 0,
                'works': 32,
                'advanced': 31,
                'needTime': 0,
                'needYield': 0,
                'saveState': 0,
                'restoreState': 0,
                'isEOF': 1,
                'docsExamined': 31,
                'alreadyHasObj': 0,
                'inputStage': {
                    'stage': "IXSCAN",
                    'nReturned': 31,
                    'executionTimeMillisEstimate': 0,
                    'works': 32,
                    'advanced': 31,
                    'needTime': 0,
                    'needYield': 0,
                    'saveState': 0,
                    'restoreState': 0,
                    'isEOF': 1,
                    'keyPattern': {
                        'o_custkey': 1
                    },
                    'indexName': "o_custkey_1",
                    'isMultiKey': false,
                    'multiKeyPaths': {
                        'o_custkey': [
                        ]
                    },
                    'isUnique': false,
                    'isSparse': false,
                    'isPartial': false,
                    'indexVersion': 2,
                    'direction': "forward",
                    'indexBounds': {
                        'o_custkey': [
                            "[1439371, 1439371]"
                        ]
                    },
                    'keysExamined': 31,
                    'seeks': 1,
                    'dupsTested': 0,
                    'dupsDropped': 0
                }
            }
        },
        'ok': 1.0
    }
]

从上面输出可以看出，executionStats 模式下可以看到详细的执行计划，比如数据返回多少行（nReturned），查询运行时间（executionTimeMillis）等信息。如果想看到 aggregate 命令相关的执行计划，也是可以使用这个命令的，具体如下：

db.orders.explain('executionStats').aggregate([
   { $match: { o_orderdate : { $gt: "1996-06-20" } } },
   { $group: { _id: "$o_custkey", total: { $sum: "$o_totalprice" } } },
   { $sort: { total: -1 } },
   { $limit: 10 }
],
{ cursor:{} })


[
    {
        'stages': [
            {
                '$cursor': {
                    'query': {
                        'o_orderdate': {
                            '$gt': "1996-06-20"
                        }
                    },
                    'fields': {
                        'o_custkey': 1,
                        'o_totalprice': 1,
                        '_id': 0
                    },
                    'queryPlanner': {
                        'plannerVersion': 1,
                        'namespace': "mongo_test.orders",
                        'indexFilterSet': false,
                        'parsedQuery': {
                            'o_orderdate': {
                                '$gt': "1996-06-20"
                            }
                        },
                        'queryHash': "52B420E9",
                        'planCacheKey': "487D9A58",
                        'winningPlan': {
                            'stage': "PROJECTION_COVERED",
                            'transformBy': {
                                'o_custkey': 1,
                                'o_totalprice': 1,
                                '_id': 0
                            },
                            'inputStage': {
                                'stage': "IXSCAN",
                                'keyPattern': {
                                    'o_orderdate': 1,
                                    'o_custkey': 1,
                                    'o_totalprice': 1,
                                    'o_orderkey': 1
                                },
                                'indexName': "o_orderdate_1_o_custkey_1_o_totalprice_1_o_orderkey_1",
                                'isMultiKey': false,
                                'multiKeyPaths': {
                                    'o_orderdate': [
                                    ],
                                    'o_custkey': [
                                    ],
                                    'o_totalprice': [
                                    ],
                                    'o_orderkey': [
                                    ]
                                },
                                'isUnique': false,
                                'isSparse': false,
                                'isPartial': false,
                                'indexVersion': 2,
                                'direction': "forward",
                                'indexBounds': {
                                    'o_orderdate': [
                                        "(\"1996-06-20\", {})"
                                    ],
                                    'o_custkey': [
                                        "[MinKey, MaxKey]"
                                    ],
                                    'o_totalprice': [
                                        "[MinKey, MaxKey]"
                                    ],
                                    'o_orderkey': [
                                        "[MinKey, MaxKey]"
                                    ]
                                }
                            }
                        },
                        'rejectedPlans': [
                        ]
                    },
                    'executionStats': {
                        'executionSuccess': true,
                        'nReturned': 4818876,
                        'executionTimeMillis': 13264,
                        'totalKeysExamined': 4818876,
                        'totalDocsExamined': 0,
                        'executionStages': {
                            'stage': "PROJECTION_COVERED",
                            'nReturned': 4818876,
                            'executionTimeMillisEstimate': 237,
                            'works': 4818877,
                            'advanced': 4818876,
                            'needTime': 0,
                            'needYield': 0,
                            'saveState': 38011,
                            'restoreState': 38011,
                            'isEOF': 1,
                            'transformBy': {
                                'o_custkey': 1,
                                'o_totalprice': 1,
                                '_id': 0
                            },
                            'inputStage': {
                                'stage': "IXSCAN",
                                'nReturned': 4818876,
                                'executionTimeMillisEstimate': 177,
                                'works': 4818877,
                                'advanced': 4818876,
                                'needTime': 0,
                                'needYield': 0,
                                'saveState': 38011,
                                'restoreState': 38011,
                                'isEOF': 1,
                                'keyPattern': {
                                    'o_orderdate': 1,
                                    'o_custkey': 1,
                                    'o_totalprice': 1,
                                    'o_orderkey': 1
                                },
                                'indexName': "o_orderdate_1_o_custkey_1_o_totalprice_1_o_orderkey_1",
                                'isMultiKey': false,
                                'multiKeyPaths': {
                                    'o_orderdate': [
                                    ],
                                    'o_custkey': [
                                    ],
                                    'o_totalprice': [
                                    ],
                                    'o_orderkey': [
                                    ]
                                },
                                'isUnique': false,
                                'isSparse': false,
                                'isPartial': false,
                                'indexVersion': 2,
                                'direction': "forward",
                                'indexBounds': {
                                    'o_orderdate': [
                                        "(\"1996-06-20\", {})"
                                    ],
                                    'o_custkey': [
                                        "[MinKey, MaxKey]"
                                    ],
                                    'o_totalprice': [
                                        "[MinKey, MaxKey]"
                                    ],
                                    'o_orderkey': [
                                        "[MinKey, MaxKey]"
                                    ]
                                },
                                'keysExamined': 4818876,
                                'seeks': 1,
                                'dupsTested': 0,
                                'dupsDropped': 0
                            }
                        }
                    }
                }
            },
            {
                '$group': {
                    '_id': "$o_custkey",
                    'total': {
                        '$sum': "$o_totalprice"
                    }
                }
            },
            {
                '$sort': {
                    'sortKey': {
                        'total': -1
                    },
                    'limit': NumberLong(10)
                }
            }
        ],
        'ok': 1.0
    }
]

注意，如果对于的 collection 数据比较多，explain 的 executionStats 模式下运行时间可能会比较长。