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Type:
Improvement
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Resolution: Unresolved
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Priority:
Major - P3
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None
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Affects Version/s: None
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Component/s: Aggregation Framework
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None
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Query Optimization
Currently, it is hard to compute stats / transform data on a nested array of objects where a subset of the fields make up the key (hash) of the object. It is possible to do via $unwind and $group, but that is an issue when operating on multiple fields in the documents at the same time. The other way is to $map using $concat on the key fields -> $setUnion -> $map -> $reduce using $cond with $in, but that is way too slow.
It would be helpful if the set operations allowed specifying the comparison function. When providing a custom comparison function, the $setUnion, $setIntersection, and $setDifference could have a mandatory reduce function to merge duplicates.
{ $setEqual: {
input: [
[{a: 1, b: 1, c: 1}, {a: 2, b: 1, c: 3},{a: 2, b: 1, c: 4}]
[{a: 1, b: 1, c: 2}, {a: 2, b: 1, c: 5}]
]
],
as: ["item1", "item2"], // name the two objects being compared at a time
cond: {
$and: [
{$eq: ["$$item1.a", "$$item2.a"]},
{$eq: ["$$item1.b", "$$item2.b"]}
]
}
}
Result: true
{ $setUnion: {
input: [
[{a: 1, b: 1, c: 1}, {a: 2, b: 1, c: 3}, {a: 2, b: 1, c: 4}]
[{a: 1, b: 1, c: 2}, {a: 2, b: 1, c: 5}]
]
],
as: ["item1", "item2"], // name the two objects being compared at a time
cond: {
$and: [
{$eq: ["$$item1.a", "$$item2.a"]},
{$eq: ["$$item1.b", "$$item2.b"]}
]
},
reduce: {
initialValue: { c: 0 },
in: {
a: "$$this.a",
b: "$$this.b",
c: { $add: ["$$value.c", "$$this.c"]
}
}
}
Results: [{a: 1, b: 1, c: 3}, {a: 2: b: 1, c: 12}]
- is related to
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SERVER-31991 Allow n-ary aggregation expressions to compute their array of arguments dynamically
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- Backlog
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