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Type:
Task
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Resolution: Done
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Priority:
Major - P3
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Affects Version/s: None
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Component/s: None
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None
My setup:
class ImportQueue include Mongoid::Document include Mongoid::Timestamps include Mongoid::Attributes::Dynamic field :attrs, :type => Hash # attrs to import index({ :attrs => "hashed" }, { :background => true }) validates :attrs, :presence => true, :uniqueness => true end
The problem is in that the uniqueness validator issues the next query:
find({ attrs: { k: 'v' }).sort({ _id: 1 })
Explain shows the next:
{
"cursor" : "BtreeCursor _id_",
"isMultiKey" : false,
"n" : 1,
"nscannedObjects" : 175164,
"nscanned" : 175164,
"nscannedObjectsAllPlans" : 350328,
"nscannedAllPlans" : 350328,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 116,
"nChunkSkips" : 0,
"millis" : 1225,
"indexBounds" : {
"_id" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
]
},
"server" : "...:27017"
}
So the index on attrs is not used. But if we change the query to:
find({ attrs: { k: 'v' }).sort({ _id: 1 })._addSpecial( "$hint", { attrs: 'hashed' } )
Explain shows:
{
"cursor" : "BtreeCursor attrs_hashed",
"isMultiKey" : false,
"n" : 1,
"nscannedObjects" : 1,
"nscanned" : 1,
"nscannedObjectsAllPlans" : 1,
"nscannedAllPlans" : 1,
"scanAndOrder" : true,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"attrs" : [
[
NumberLong("5173886864922833935"),
NumberLong("5173886864922833935")
]
]
},
"server" : "...:27017"
}
I do not want to say this is a bug, just an observation. I think that could be solved by a custom validation. But perhaps there is something we can do with this, like if a field has an index and we validating uniqueness on that field the hint could be added automatically.