Very Slow aggregation with lookup

Working with Data
1 
db.getCollection('room').aggregate(
[
  {
    $lookup: {
      from: "messages",
      localField: "_id",
      foreignField: "room.id",
      as: "firstMessage",
      pipeline: [
        {
          $sort: {
            "createdAt": 1
          },
        },
        {
          $limit: 1
        },
        {
          $project:{
            _id: 0,
            createdAt: 1
          }
        }
      ]
    }
  },
  {
    $lookup: {
      from: "messages",
      localField: "_id",
      foreignField: "room.id",
      as: "lastMessage",
      pipeline: [
        {
          $sort: {
            "createdAt": -1
          },
        },
        {
          $limit: 1
        },
        {
          $project: {
            _id: 0,
            createdAt: 1
          }
        }
      ]
    }
  },
  {
    $unwind: "$firstMessage"
  },
  {
    $unwind: "$lastMessage"
  },
  {
    $project: {
      _id:1,
      firstMessage: "$firstMessage.createdAt",
      lastMessage: "$lastMessage.createdAt",
      roomTime: {
        $dateDiff: {
          startDate: "$firstMessage.createdAt",
          endDate: "$lastMessage.createdAt",
          unit: 'second'
        }
      }
    }
  },
  {
    $group: {
      _id: null,
      totalRoomTime: {
        $avg: "$roomTime"
      }
    }
  }
]
)

Here’s my aggregation looks like

2

Hey @T_RAZIN,

Thank you for reaching out to the MongoDB Community forums.

To better understand the issue, could you please share the following information:

Looking forward to hearing from you.

Regards,
Kushagra

3

Hi @Kushagra_Kesav, Thanks for responding
I’m using MongoDB version v6.0.6
room stats

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    "capped": false,
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    "indexBuilds": [],
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      "createdAt_1": 86016,
      "createdAt_-1": 98304
    },
    "nindexes": 3,
    "ns": "chat.room",
    "numOrphanDocs": 0,
    "ok": 1,
    "scaleFactor": 1,
    "size": 1609816,
    "storageSize": 626688,
    "totalIndexSize": 335872,
    "totalSize": 962560,
    "wiredTiger": {
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      },
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      "uri": "statistics:table:collection-29--6292082429505695878",
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        "highest merge generation in the LSM tree": 0,
        "queries that could have benefited from a Bloom filter that did not exist": 0,
        "sleep for LSM checkpoint throttle": 0,
        "sleep for LSM merge throttle": 0,
        "total size of bloom filters": 0
      },
      "autocommit": {
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      },
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        "file magic number": 120897,
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        "minor version number": 0
      },
      "btree": {
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        "eviction walks started from root of tree": 28,
        "eviction walks started from saved location in tree": 0,
        "hazard pointer blocked page eviction": 0,
        "history store table insert calls": 0,
        "history store table insert calls that returned restart": 0,
        "history store table out-of-order resolved updates that lose their durable timestamp": 0,
        "history store table out-of-order updates that were fixed up by reinserting with the fixed timestamp": 0,
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        "history store table reads missed": 0,
        "history store table reads requiring squashed modifies": 0,
        "history store table truncation by rollback to stable to remove an unstable update": 0,
        "history store table truncation by rollback to stable to remove an update": 0,
        "history store table truncation to remove an update": 0,
        "history store table truncation to remove range of updates due to key being removed from the data page during reconciliation": 0,
        "history store table truncation to remove range of updates due to out-of-order timestamp update on data page": 0,
        "history store table writes requiring squashed modifies": 0,
        "in-memory page passed criteria to be split": 0,
        "in-memory page splits": 0,
        "internal pages evicted": 0,
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        "modify": 0,
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        "next calls": 198586487,
        "open cursor count": 0,
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        "reset calls": 320801,
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        "update key and value bytes": 0,
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      },
      "reconciliation": {
        "approximate byte size of timestamps in pages written": 0,
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        "dictionary matches": 0,
        "fast-path pages deleted": 0,
        "internal page key bytes discarded using suffix compression": 7827,
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        "maximum blocks required for a page": 1,
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        "page reconciliation calls": 1647,
        "page reconciliation calls for eviction": 1,
        "pages deleted": 0,
        "pages written including an aggregated newest start durable timestamp ": 0,
        "pages written including an aggregated newest stop durable timestamp ": 0,
        "pages written including an aggregated newest stop timestamp ": 0,
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        "pages written including an aggregated newest transaction ID ": 0,
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        "rollback to stable skipping delete rle": 0,
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        "transaction checkpoints due to obsolete pages": 0,
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      }
    }
  }
]

messages stats

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        "Pages that could not be queued for eviction": 0,
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        "number of blocks with compress ratio greater than 64": 0,
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        "Total number of entries skipped to position the history store cursor": 0,
        "Total number of times a search near has exited due to prefix config": 0,
        "bulk loaded cursor insert calls": 0,
        "cache cursors reuse count": 227645,
        "close calls that result in cache": 227646,
        "create calls": 164,
        "cursor next calls that skip due to a globally visible history store tombstone": 0,
        "cursor next calls that skip greater than or equal to 100 entries": 0,
        "cursor next calls that skip less than 100 entries": 10713915444,
        "cursor prev calls that skip due to a globally visible history store tombstone": 0,
        "cursor prev calls that skip greater than or equal to 100 entries": 0,
        "cursor prev calls that skip less than 100 entries": 1,
        "insert calls": 64468,
        "insert key and value bytes": 42207910,
        "modify": 0,
        "modify key and value bytes affected": 0,
        "modify value bytes modified": 0,
        "next calls": 10713915444,
        "open cursor count": 0,
        "operation restarted": 1,
        "prev calls": 1,
        "remove calls": 0,
        "remove key bytes removed": 0,
        "reserve calls": 0,
        "reset calls": 17113357,
        "search calls": 5285704847,
        "search history store calls": 0,
        "search near calls": 11000725,
        "truncate calls": 0,
        "update calls": 0,
        "update key and value bytes": 0,
        "update value size change": 0
      },
      "reconciliation": {
        "approximate byte size of timestamps in pages written": 0,
        "approximate byte size of transaction IDs in pages written": 0,
        "dictionary matches": 0,
        "fast-path pages deleted": 0,
        "internal page key bytes discarded using suffix compression": 80959,
        "internal page multi-block writes": 963,
        "leaf page key bytes discarded using prefix compression": 0,
        "leaf page multi-block writes": 1520,
        "leaf-page overflow keys": 0,
        "maximum blocks required for a page": 1,
        "overflow values written": 0,
        "page checksum matches": 0,
        "page reconciliation calls": 4025,
        "page reconciliation calls for eviction": 1,
        "pages deleted": 0,
        "pages written including an aggregated newest start durable timestamp ": 0,
        "pages written including an aggregated newest stop durable timestamp ": 0,
        "pages written including an aggregated newest stop timestamp ": 0,
        "pages written including an aggregated newest stop transaction ID": 0,
        "pages written including an aggregated newest transaction ID ": 1,
        "pages written including an aggregated oldest start timestamp ": 0,
        "pages written including an aggregated prepare": 0,
        "pages written including at least one prepare": 0,
        "pages written including at least one start durable timestamp": 0,
        "pages written including at least one start timestamp": 0,
        "pages written including at least one start transaction ID": 0,
        "pages written including at least one stop durable timestamp": 0,
        "pages written including at least one stop timestamp": 0,
        "pages written including at least one stop transaction ID": 0,
        "records written including a prepare": 0,
        "records written including a start durable timestamp": 0,
        "records written including a start timestamp": 0,
        "records written including a start transaction ID": 0,
        "records written including a stop durable timestamp": 0,
        "records written including a stop timestamp": 0,
        "records written including a stop transaction ID": 0
      },
      "session": {
        "object compaction": 0,
        "tiered operations dequeued and processed": 0,
        "tiered operations scheduled": 0,
        "tiered storage local retention time (secs)": 0
      },
      "transaction": {
        "race to read prepared update retry": 0,
        "rollback to stable history store records with stop timestamps older than newer records": 0,
        "rollback to stable inconsistent checkpoint": 0,
        "rollback to stable keys removed": 0,
        "rollback to stable keys restored": 0,
        "rollback to stable restored tombstones from history store": 0,
        "rollback to stable restored updates from history store": 0,
        "rollback to stable skipping delete rle": 0,
        "rollback to stable skipping stable rle": 0,
        "rollback to stable sweeping history store keys": 0,
        "rollback to stable updates removed from history store": 0,
        "transaction checkpoints due to obsolete pages": 0,
        "update conflicts": 0
      }
    }
  }
]
4

Here’s db.room.explain(‘executionStats’).aggregate(…) result

[
  {
    "command": {
      "aggregate": "room",
      "pipeline": [
        {
          "$lookup": {
            "from": "messages",
            "localField": "_id",
            "foreignField": "room.id",
            "as": "firstMessage",
            "pipeline": [
              {
                "$sort": {
                  "createdAt": 1
                }
              },
              {
                "$limit": 1
              },
              {
                "$project": {
                  "_id": 0,
                  "createdAt": 1
                }
              }
            ]
          }
        },
        {
          "$lookup": {
            "from": "messages",
            "localField": "_id",
            "foreignField": "room.id",
            "as": "lastMessage",
            "pipeline": [
              {
                "$sort": {
                  "createdAt": -1
                }
              },
              {
                "$limit": 1
              },
              {
                "$project": {
                  "_id": 0,
                  "createdAt": 1
                }
              }
            ]
          }
        },
        {
          "$unwind": "$firstMessage"
        },
        {
          "$unwind": "$lastMessage"
        },
        {
          "$project": {
            "_id": 1,
            "firstMessage": "$firstMessage.createdAt",
            "lastMessage": "$lastMessage.createdAt",
            "roomTime": {
              "$dateDiff": {
                "startDate": "$firstMessage.createdAt",
                "endDate": "$lastMessage.createdAt",
                "unit": "second"
              }
            }
          }
        },
        {
          "$group": {
            "_id": null,
            "totalRoomTime": {
              "$avg": "$roomTime"
            }
          }
        }
      ],
      "explain": true,
      "$db": "chat",
      "lsid": {
        "id": {"$binary": {"base64": "JgYIiH8eQZ2zdGWV78w5hw==", "subType": "04"}}
      }
    },
    "explainVersion": "1",
    "ok": 1,
    "serverInfo": {
      "host": "chat-engin-db",
      "port": 27017,
      "version": "6.0.6",
      "gitVersion": "26b4851a412cc8b9b4a18cdb6cd0f9f642e06aa7"
    },
    "serverParameters": {
      "internalQueryFacetBufferSizeBytes": 104857600,
      "internalQueryFacetMaxOutputDocSizeBytes": 104857600,
      "internalLookupStageIntermediateDocumentMaxSizeBytes": 104857600,
      "internalDocumentSourceGroupMaxMemoryBytes": 104857600,
      "internalQueryMaxBlockingSortMemoryUsageBytes": 104857600,
      "internalQueryProhibitBlockingMergeOnMongoS": 0,
      "internalQueryMaxAddToSetBytes": 104857600,
      "internalDocumentSourceSetWindowFieldsMaxMemoryBytes": 104857600
    },
    "stages": [
      {
        "$cursor": {
          "queryPlanner": {
            "namespace": "chat.room",
            "indexFilterSet": false,
            "parsedQuery": {
            },
            "queryHash": "180FC727",
            "planCacheKey": "180FC727",
            "maxIndexedOrSolutionsReached": false,
            "maxIndexedAndSolutionsReached": false,
            "maxScansToExplodeReached": false,
            "winningPlan": {
              "stage": "PROJECTION_SIMPLE",
              "transformBy": {
                "_id": 1,
                "firstMessage": 1,
                "lastMessage": 1
              },
              "inputStage": {
                "stage": "COLLSCAN",
                "direction": "forward"
              }
            },
            "rejectedPlans": []
          }
        }
      },
      {
        "$lookup": {
          "from": "messages",
          "as": "firstMessage",
          "localField": "_id",
          "foreignField": "room.id",
          "let": {
          },
          "pipeline": [
            {
              "$sort": {
                "createdAt": 1
              }
            },
            {
              "$limit": 1
            },
            {
              "$project": {
                "_id": 0,
                "createdAt": 1
              }
            }
          ]
        }
      },
      {
        "$lookup": {
          "from": "messages",
          "as": "lastMessage",
          "localField": "_id",
          "foreignField": "room.id",
          "let": {
          },
          "pipeline": [
            {
              "$sort": {
                "createdAt": -1
              }
            },
            {
              "$limit": 1
            },
            {
              "$project": {
                "_id": 0,
                "createdAt": 1
              }
            }
          ]
        }
      },
      {
        "$unwind": {
          "path": "$firstMessage"
        }
      },
      {
        "$unwind": {
          "path": "$lastMessage"
        }
      },
      {
        "$project": {
          "_id": true,
          "firstMessage": "$firstMessage.createdAt",
          "lastMessage": "$lastMessage.createdAt",
          "roomTime": {
            "$dateDiff": {
              "startDate": "$firstMessage.createdAt",
              "endDate": "$lastMessage.createdAt",
              "unit": {
                "$const": "second"
              }
            }
          }
        }
      },
      {
        "$group": {
          "_id": {
            "$const": null
          },
          "totalRoomTime": {
            "$avg": "$roomTime"
          }
        }
      }
    ]
  }
]
5

sample document room

{
  "_id": {
    "$oid": "64819ba918af3d0059e059e5"
  },
  "userOne": {
    "id": {
      "$oid": "64255842bf3c9200517015e2"
    },
    "name": "...",
    "email": "...",
    "userId": "6c3ca9cc-ab0b-4816-9575-1e20c8c0d606",
    "createdAt": {
      "$date": "2023-03-30T09:37:06.507Z"
    },
    "updatedAt": {
      "$date": "2023-03-30T09:37:06.507Z"
    }
  },
  "userTwo": {
    "id": {
      "$oid": "640386d6cb61685af3798355"
    },
    "name": "...",
    "email": "...",
    "createdAt": {
      "$date": "2023-03-04T17:58:46.602Z"
    },
    "updatedAt": {
      "$date": "2023-03-04T17:58:46.602Z"
    }
  },
  "createdAt": {
    "$date": "2023-06-08T09:13:13.968Z"
  },
  "updatedAt": {
    "$date": "2023-06-08T09:13:13.968Z"
  },
  "ratings": 4,
  "feedbackMessage": "Good"
}

sample document messages

{
  "_id": {
    "$oid": "6481d2f218af3d0059e059ea"
  },
  "isSeen": true,
  "message": "Hi Nesya",
  "room": {
    "id": {
      "$oid": "6481d2f118af3d0059e059e9"
    },
    "userOne": {
      "name": "...",
      "email": "...",
      "createdAt": {
        "$date": "2023-03-30T09:37:06.507Z"
      },
      "updatedAt": {
        "$date": "2023-03-30T09:37:06.507Z"
      },
      "id": [
        {
          "$oid": "64255842bf3c9200517015e2"
        }
      ]
    },
    "userTwo": {
      "name": "...",
      "email": "...",
      "createdAt": {
        "$date": "2023-03-04T17:58:46.602Z"
      },
      "updatedAt": {
        "$date": "2023-03-04T17:58:46.602Z"
      },
      "id": [
        {
          "$oid": "640386d6cb61685af3798355"
        }
      ]
    },
    "createdAt": {
      "$date": "2023-06-08T13:09:05.791Z"
    },
    "updatedAt": {
      "$date": "2023-06-08T13:09:05.791Z"
    }
  },
  "author": {
    "id": {
      "$oid": "64255842bf3c9200517015e2"
    },
    "name": "...",
    "email": "...",
    "userId": "6c3ca9cc-ab0b-4816-9575-1e20c8c0d606",
    "createdAt": {
      "$date": "2023-03-30T09:37:06.507Z"
    },
    "updatedAt": {
      "$date": "2023-03-30T09:37:06.507Z"
    }
  },
  "createdAt": {
    "$date": "2023-06-08T13:09:06.452Z"
  },
  "updatedAt": {
    "$date": "2023-06-08T13:09:06.452Z"
  }
}
6

Here’s indexes in messages

Screen Shot 2023-06-30 at 00.45.43
Screen Shot 2023-06-30 at 00.45.432134×602 106 KB

7

indexes in room

Screen Shot 2023-06-30 at 00.46.53
Screen Shot 2023-06-30 at 00.46.532322×634 85.4 KB

8

I noticed that you $lookup twice in messages to get the first and last createdAt. You could achieve the same with a single $lookup if you $group with _id:null and use $first and $last accumulators.

Since you $lookup on room.id and $sort on createdAt, you definitively need a compound index in messages like

{ "room.id" : 1 , "createdAt" : 1 }

Also, I do not think you will get totalRoomTime if you accumulate using $avg.

1 Like
9

One thing I forgot to mentioned.

If the query is a frequent use-case, it might be worth the effort to update your model and logic to store in the original collect the lastMessageCreatedAt and firstMessageCreatedAt in the room documents. You simply update them when you successfully insert a new document in messages.

10

Hi @steevey, thanks for your advice, it solved my problem. I’m new with mongoDB and didn’t know to much about compound index. Thanks

1 Like
11