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Mongodb – Is there performance issue if I use multiple "$set" in one update

The data in mongodb is like this

{
    "_id": "1",
    "a": 1,
    "b": 2,
    "c": 3
    "d": 4
}

I can update the document using a single "$set" operator.

db.collection.update({"_id": "1"}, {
        "$set": {
            "a": 100,
            "b": 200,
            "c": 300,
            "d": 400
        }
    })

I can also update the document using multiple "$set" (each field has a "$set").

db.collection.update({"_id": "1"}, [
        {
            "$set": { "a": 100 }
        },
        {
            "$set": { "b": 200 }
        },
        {
            "$set": { "c": 300 }
        },
        {
            "$set": { "d": 400 }
        }
    ])

I would like to know if there are any performance concerns on the mongodb end by using the "multi-set" version. Is it Ok to use the "multi-set" version if, for some cases, I need to.

Tags:

2

Answers


  1. 0

    For your case, I think there will be a trivial performance difference, if not none. The explain plans for them are nearly identical.

    explain output for single $set:

    {
  "$clusterTime": {
    "clusterTime": Timestamp(1734276869, 3),
    "signature": {
      "hash": BinData(0, "tRv1PQVgmouDxOtttOdSk2z0O1U="),
      "keyId": NumberLong(7394893876424605697)
    }
  },
  "command": {
    "$db": "75f38391413d12711811639fd9b97e4f",
    "filter": {},
    "find": "collection",
    "maxTimeMS": NumberLong(20000)
  },
  "executionStats": {
    "allPlansExecution": [],
    "executionStages": {
      "advanced": 1,
      "direction": "forward",
      "docsExamined": 1,
      "executionTimeMillisEstimate": 0,
      "isEOF": 1,
      "nReturned": 1,
      "needTime": 0,
      "needYield": 0,
      "restoreState": 0,
      "saveState": 0,
      "stage": "COLLSCAN",
      "works": 2
    },
    "executionSuccess": true,
    "executionTimeMillis": 0,
    "nReturned": 1,
    "totalDocsExamined": 1,
    "totalKeysExamined": 0
  },
  "explainVersion": "1",
  "operationTime": Timestamp(1734276869, 3),
  "queryPlanner": {
    "indexFilterSet": false,
    "maxIndexedAndSolutionsReached": false,
    "maxIndexedOrSolutionsReached": false,
    "maxScansToExplodeReached": false,
    "namespace": "75f38391413d12711811639fd9b97e4f.collection",
    "parsedQuery": {},
    "planCacheKey": "5F5FC979",
    "queryHash": "5F5FC979",
    "rejectedPlans": [],
    "winningPlan": {
      "direction": "forward",
      "stage": "COLLSCAN"
    }
  },
  "serverParameters": {
    "internalDocumentSourceGroupMaxMemoryBytes": 104857600,
    "internalDocumentSourceSetWindowFieldsMaxMemoryBytes": 104857600,
    "internalLookupStageIntermediateDocumentMaxSizeBytes": 104857600,
    "internalQueryFacetBufferSizeBytes": 104857600,
    "internalQueryFacetMaxOutputDocSizeBytes": 104857600,
    "internalQueryMaxAddToSetBytes": 104857600,
    "internalQueryMaxBlockingSortMemoryUsageBytes": 104857600,
    "internalQueryProhibitBlockingMergeOnMongoS": 0
  }
}

    Mongo Playground

    explain output for multiple $set:

    {
  "$clusterTime": {
    "clusterTime": Timestamp(1734276224, 3),
    "signature": {
      "hash": BinData(0, "QUsSlU1zBAvtbFAgbv3/fA7RENU="),
      "keyId": NumberLong(7394893876424605697)
    }
  },
  "command": {
    "$db": "3d58f286ab6b7455181163098d794665",
    "filter": {},
    "find": "collection",
    "maxTimeMS": NumberLong(20000)
  },
  "executionStats": {
    "allPlansExecution": [],
    "executionStages": {
      "advanced": 1,
      "direction": "forward",
      "docsExamined": 1,
      "executionTimeMillisEstimate": 0,
      "isEOF": 1,
      "nReturned": 1,
      "needTime": 0,
      "needYield": 0,
      "restoreState": 0,
      "saveState": 0,
      "stage": "COLLSCAN",
      "works": 2
    },
    "executionSuccess": true,
    "executionTimeMillis": 0,
    "nReturned": 1,
    "totalDocsExamined": 1,
    "totalKeysExamined": 0
  },
  "explainVersion": "1",
  "operationTime": Timestamp(1734276224, 3),
  "queryPlanner": {
    "indexFilterSet": false,
    "maxIndexedAndSolutionsReached": false,
    "maxIndexedOrSolutionsReached": false,
    "maxScansToExplodeReached": false,
    "namespace": "3d58f286ab6b7455181163098d794665.collection",
    "parsedQuery": {},
    "planCacheKey": "5F5FC979",
    "queryHash": "5F5FC979",
    "rejectedPlans": [],
    "winningPlan": {
      "direction": "forward",
      "stage": "COLLSCAN"
    }
  },
  "serverParameters": {
    "internalDocumentSourceGroupMaxMemoryBytes": 104857600,
    "internalDocumentSourceSetWindowFieldsMaxMemoryBytes": 104857600,
    "internalLookupStageIntermediateDocumentMaxSizeBytes": 104857600,
    "internalQueryFacetBufferSizeBytes": 104857600,
    "internalQueryFacetMaxOutputDocSizeBytes": 104857600,
    "internalQueryMaxAddToSetBytes": 104857600,
    "internalQueryMaxBlockingSortMemoryUsageBytes": 104857600,
    "internalQueryProhibitBlockingMergeOnMongoS": 0
  }
}

    Mongo Playground

    Given your query pattern in the example on updating on only 1 document and you are fetching by _id, I don’t think there will be any observable performance difference.

    There are 2 caveats outside of performance though:

    1. If in your real use case, you have a dependency on the $set operations, you need to break them into different stages

    e.g. you $set a to be c * d = 12, and b to be a + 1 = 12 + 1 = 13. You will need to do:

    db.collection.update({
  "_id": "1"
},
[
  {
    "$set": {
      "a": {
        "$multiply": [
          "$c",
          "$d"
        ]
      }
    }
  },
  {
    "$set": {
      "b": {
        "$add": [
          "$a",
          1
        ]
      }
    }
  }
])

    Mongo Playground

    A single $set won’t give you expected result of b = 13, but 2 because a is yet to be evaluated as 12(keeping 1 as value)

    db.collection.update({
  "_id": "1"
},
[
  {
    "$set": {
      "a": {
        "$multiply": [
          "$c",
          "$d"
        ]
      },
      "b": {
        "$add": [
          "$a",
          1
        ]
      }
    }
  }
])

    Mongo Playground

    1. readablility: In real-life and more complex pipelines, you may find breaking up $set statement more readable or manageable. But I will admit this is quite subjective and you may consider the other way round.
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  2. 0

    Why do you think, someone "needs to do so"? The operators are JSON objects, you can create an update for example like this:

    let values = { a: 100 };
values.b = 200;
values["c"] = 300;
values = Object.assign(values, { d: 400 });

db.collection.update({"_id": "1"}, { $set: values })

//or 
const operation = { $set: values };
db.collection.update({"_id": "1"}, operation)
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