关于mongodb如何选择索引或$or查询的问题

背景

我有一个具有以下架构的集合：

{ appId, createdAt, lastSeen, lastHeard, deleted, ...otherFields }

以及以下指标：

{ appId: 1, deleted: 1, lastSeen: 1 }
{ appId: 1, deleted: 1, lastHeard: 1 }
{ appId: 1, createdAt: 1, deleted: 1 }
{ appId: 1, deleted: 1, createdAt: 1, lastSeen: 1, lastHeard: 1 }

在我的应用程序中，我有一个聚合：

 db.getCollection('client_users').aggregate([
  {
    $match: {
      deleted: false,
      appId: 'appid',
      $or: [
        { createdAt: { $gt: new Date('2020-10-19T17:00:00.000Z') } },
        { lastSeen: { $gt: new Date('2020-10-19T17:00:00.000Z') } },
        { lastHeard: { $gt: new Date('2020-10-19T17:00:00.000Z') } },
      ]
    }
  },
  {
    $group: {
      _id: '$geoLocation.city',
      count: {
        $sum: 1
      }
    }
  },
  {
    $sort: {
      count: -1
    }
  }
]);

我的意图是将上面的前三个索引用于此聚合，因为我知道$or查询被解析为3个单独的查询。但是，从解释输出中，获胜计划使用第四个索引（

{appId:1，deleted:1，createdAt:1，lastSeen:1，lastsheard:1}

）作为最后两个子句：

"winningPlan" : {
    "stage" : "FETCH",
    "inputStage" : {
        "stage" : "OR",
        "inputStages" : [
            {
                "stage" : "IXSCAN",
                "keyPattern" : {
                    "appId" : 1,
                    "createdAt" : 1,
                    "deleted" : 1
                },
                "indexName" : "appId_1_createdAt_1_deleted_1",
                "isMultiKey" : false,
                "multiKeyPaths" : {
                    "appId" : [],
                    "createdAt" : [],
                    "deleted" : []
                },
                "isUnique" : false,
                "isSparse" : false,
                "isPartial" : false,
                "indexVersion" : 2,
                "direction" : "forward",
                "indexBounds" : {
                    "appId" : [
                        "[\"appid\", \"appid\"]"
                    ],
                    "createdAt" : [
                        "(new Date(1603126800000), new Date(9223372036854775807)]"
                    ],
                    "deleted" : [
                        "[false, false]"
                    ]
                }
            },
            {
                "stage" : "IXSCAN",
                "keyPattern" : {
                    "appId" : 1,
                    "deleted" : 1,
                    "createdAt" : 1,
                    "lastSeen" : 1,
                    "lastHeard" : 1
                },
                "indexName" : "appId_1_deleted_1_createdAt_1_lastSeen_1_lastHeard_1",
                "isMultiKey" : false,
                "multiKeyPaths" : {
                    "appId" : [],
                    "deleted" : [],
                    "createdAt" : [],
                    "lastSeen" : [],
                    "lastHeard" : []
                },
                "isUnique" : false,
                "isSparse" : false,
                "isPartial" : false,
                "indexVersion" : 2,
                "direction" : "forward",
                "indexBounds" : {
                    "appId" : [
                        "[\"appid\", \"appid\"]"
                    ],
                    "deleted" : [
                        "[false, false]"
                    ],
                    "createdAt" : [
                        "[MinKey, MaxKey]"
                    ],
                    "lastSeen" : [
                        "[MinKey, MaxKey]"
                    ],
                    "lastHeard" : [
                        "(new Date(1603126800000), new Date(9223372036854775807)]"
                    ]
                }
            },
            {
                "stage" : "IXSCAN",
                "keyPattern" : {
                    "appId" : 1,
                    "deleted" : 1,
                    "createdAt" : 1,
                    "lastSeen" : 1,
                    "lastHeard" : 1
                },
                "indexName" : "appId_1_deleted_1_createdAt_1_lastSeen_1_lastHeard_1",
                "isMultiKey" : false,
                "multiKeyPaths" : {
                    "appId" : [],
                    "deleted" : [],
                    "createdAt" : [],
                    "lastSeen" : [],
                    "lastHeard" : []
                },
                "isUnique" : false,
                "isSparse" : false,
                "isPartial" : false,
                "indexVersion" : 2,
                "direction" : "forward",
                "indexBounds" : {
                    "appId" : [
                        "[\"appid\", \"appid\"]"
                    ],
                    "deleted" : [
                        "[false, false]"
                    ],
                    "createdAt" : [
                        "[MinKey, MaxKey]"
                    ],
                    "lastSeen" : [
                        "(new Date(1603126800000), new Date(9223372036854775807)]"
                    ],
                    "lastHeard" : [
                        "[MinKey, MaxKey]"
                    ]
                }
            }
        ]
    }
},

这不是我想要的。奇怪的是，当我只尝试其中一个条款时，就像在这个$match阶段：

$match: {
  deleted: false,
  appId: 'appid',
  lastSeen: {$gt: new Date('2020-10-19T17:00:00.000Z') },
}

它使用了正确的索引（

{appId:1，deleted:1，lastSeen:1}

）。我从explain输出和实际聚合的计时中知道这一点。具体地说，在没有提示或使用

hint:appId\u 1\u deleted\u 1\u lastSeen\u 1

运行它所需的时间比使用

hint:appId\u 1\u deleted\u 1\u createdAt\u 1\u lastSeen\u 1

的时间短三倍。这让我对mongodb如何选择索引感到非常困惑

---

有人能给我解释一下这种行为的原因吗？有没有办法强迫mongodb在这种情况下使用我想要的索引？谢谢。

我想出来了。这正是因为$or查询。Mongodb通过让它们彼此进行小规模竞争来选择查询计划。效率较低的计划幸运地获胜，因为第一个$or子句处理了一切（记住它是$or左右，只有一个子句就足够了）。我通过删除第四个索引来解决这个问题