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$maxN (aggregation accumulator)

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  • Definition
  • Syntax
  • Behavior
  • Restrictions
  • Examples
$maxN

New in version 5.2.

Returns an aggregation of the maxmimum value n elements within a group. If the group contains fewer than n elements, $maxN returns all elements in the group.

{
$maxN:
{
input: <expression>,
n: <expression>
}
}
  • input specifies an expression that is the input to $maxN. It is evaluated for each element in the group and $maxN preserves the maximum n values.
  • n limits the number of results per group and n has to be a positive integral expression that is either a constant or depends on the _id value for $group.
  • $maxN filters out null and missing values.

Consider the following aggregation that returns the maximum n documents from a group:

db.aggregate( [
{
$documents: [
{ playerId: "PlayerA", gameId: "G1", score: 1 },
{ playerId: "PlayerB", gameId: "G1", score: 2 },
{ playerId: "PlayerC", gameId: "G1", score: 3 },
{ playerId: "PlayerD", gameId: "G1" },
{ playerId: "PlayerE", gameId: "G1", score: null }
]
},
{
$group:
{
_id: "$gameId",
maximumThreeScores:
{
$maxN:
{
input: "$score",
n: 4
}
}
}
}
] )

In this example:

  • $documents creates the literal documents that contain player scores.
  • $group groups the documents by gameId. This example has only one gameId, G1.
  • PlayerD has a missing score and PlayerE has a null score. These values are both considered as null.
  • The maximumThreeScores field is specified as $maxN with input : "$score" and returned as an array.
  • Since there are only 3 documents with scores maxN returns the maximum 3 score fields even though n = 4.
[
{
_id: 'G1',
maximumThreeScores: [ 3, 2, 1 ]
}
]

Both $maxN and $topN accumulators can accomplish similar results.

In general:

  • $maxN has the advantage of finding maximum values in no particular sort order. If you want to know the maximum values for n documents use $maxN.
  • If guaranteing a particular sort order is a requirement use $topN.
  • Use $topN if you don't intend on sorting on the output values.

You can use $maxN as an accumulator.

$maxN is supported as an aggregation expression.

$maxN is supported as a window operator.

Aggregation pipelines which call $maxN are subject to the 100 MB limit. If this limit is exceeded for an individual group, the aggregation fails with an error.

Consider a gamescores collection with the following documents:

db.gamescores.insertMany([
{ playerId: "PlayerA", gameId: "G1", score: 31 },
{ playerId: "PlayerB", gameId: "G1", score: 33 },
{ playerId: "PlayerC", gameId: "G1", score: 99 },
{ playerId: "PlayerD", gameId: "G1", score: 1 },
{ playerId: "PlayerA", gameId: "G2", score: 10 },
{ playerId: "PlayerB", gameId: "G2", score: 14 },
{ playerId: "PlayerC", gameId: "G2", score: 66 },
{ playerId: "PlayerD", gameId: "G2", score: 80 }
])

You can use the $maxN accumulator to find the maximum three scores in a single game.

db.gamescores.aggregate( [
{
$match : { gameId : "G1" }
},
{
$group:
{
_id: "$gameId",
maxThreeScores:
{
$maxN:
{
input: ["$score","$playerId"],
n:3
}
}
}
}
] )

The example pipeline:

  • Uses $match to filter the results on a single gameId. In this case, G1.
  • Uses $group to group the results by gameId. In this case, G1.
  • Specifies the fields that are input for $maxN with input : ["$score","$playerId"].
  • Uses $maxN to return the maximum three score elements for the G1 game with n : 3.

The operation returns the following results:

[
{
_id: 'G1',
maxThreeScores: [ [ 99, 'PlayerC' ], [ 33, 'PlayerB' ], [ 31, 'PlayerA' ] ]
}
]

You can use the $maxN accumulator to find the maximum n scores in each game.

db.gamescores.aggregate( [
{
$group:
{
_id: "$gameId",
maxScores:
{
$maxN:
{
input: ["$score","$playerId"],
n: 3
}
}
}
}
] )

The example pipeline:

  • Uses $group to group the results by gameId.
  • Uses $maxN to return the maximum three score elements for each game with n: 3.
  • Specifies the fields that are input for $maxN with input: ["$score","$playerId"].

The operation returns the following results:

[
{
_id: 'G1',
maxScores: [ [ 99, 'PlayerC' ], [ 33, 'PlayerB' ], [ 31, 'PlayerA' ] ]
},
{
_id: 'G2',
maxScores: [ [ 80, 'PlayerD' ], [ 66, 'PlayerC' ], [ 14, 'PlayerB' ] ]
}
]

You can also assign the value of n dynamically. In this example, the $cond expression is used on the gameId field.

db.gamescores.aggregate([
{
$group:
{
_id: {"gameId": "$gameId"},
gamescores:
{
$maxN:
{
input: ["$score","$playerId"],
n: { $cond: { if: {$eq: ["$gameId","G2"] }, then: 1, else: 3 } }
}
}
}
}
] )

The example pipeline:

  • Uses $group to group the results by gameId.
  • Specifies the fields that input for $maxN with input : ["$score","$playerId"].
  • If the gameId is G2 then n is 1, otherwise n is 3.

The operation returns the following results:

[
{ _id: { gameId: 'G2' }, gamescores: [ [ 80, 'PlayerD' ] ] },
{
_id: { gameId: 'G1' },
gamescores: [ [ 99, 'PlayerC' ], [ 33, 'PlayerB' ], [ 31, 'PlayerA' ] ]
}
]
←  $max (aggregation)$maxN (array operator) →
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