Details
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Task
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Resolution: Fixed
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Major - P3
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None
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None
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None
Description
Issue
Query with min/max modifier uses first index with related fields.
On fresh MongoDB secondary startup `id` index became last in order (this looks like a bug)Â and `find` operation with `min`/`max` modifier starts using first hashed index by `_id`.
When this secondary became primary, this shard returns incomplete results on query.
user:PRIMARY> db.socialTopics.getIndices()
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[
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{
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"v" : 2, |
"key" : { |
"_id" : "hashed" |
},
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"name" : "_id_hashed", |
"background" : true, |
"ns" : "user.socialTopics" |
},
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{
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"v" : 2, |
"key" : { |
"_id" : 1 |
},
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"name" : "_id_", |
"ns" : "user.socialTopics" |
}
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]
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user:PRIMARY> db.socialTopics.find({}, {_id: 1}).min({_id: ObjectId("5bb74d8cefa37173fe2cf692")}).sort({_id: 1}).limit(5).explain() |
{
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"queryPlanner" : { |
"plannerVersion" : 1, |
"namespace" : "user.socialTopics", |
"indexFilterSet" : false, |
"parsedQuery" : { |
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},
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"winningPlan" : { |
"stage" : "PROJECTION", |
"transformBy" : { |
"_id" : 1 |
},
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"inputStage" : { |
"stage" : "SORT", |
"sortPattern" : { |
"_id" : 1 |
},
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"limitAmount" : 5, |
"inputStage" : { |
"stage" : "SORT_KEY_GENERATOR", |
"inputStage" : { |
"stage" : "FETCH", |
"inputStage" : { |
"stage" : "IXSCAN", |
"keyPattern" : { |
"_id" : "hashed" |
},
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"indexName" : "_id_hashed", |
"isMultiKey" : false, |
"isUnique" : false, |
"isSparse" : false, |
"isPartial" : false, |
"indexVersion" : 2, |
"direction" : "forward", |
"indexBounds" : { |
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}
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}
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}
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}
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}
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},
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"rejectedPlans" : [ ] |
},
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...
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}
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Why we use min/max modifier
We use min/max modifier for iterating over collections, because MongoDB don't have operation like `after` or `before`.
We can't use `$gt`/`$gte`/`$lt`/`$lte` because:
- this operations always false for values with differ types;
- it's not trivial to make query with multiple field bounds.
Workaround
Looks like with `hint` we can set correct index directly.
Attachments
Issue Links
- depends on
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SERVER-39567 Query with min/max modifier uses hashed index
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- Closed
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- Closed
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- Closed
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