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Type:
Bug
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Resolution: Duplicate
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Priority:
Minor - P4
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None
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Affects Version/s: 2.6.4
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Component/s: Performance, Querying
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Labels:None
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ALL
When matching an attribute against both $in and $nin, Mongo doesn't use the index correctly.
If only $in is used, then index takes advantage of that:
db.assets.find({
tags: {
$in: ['blah']
}
}).explain()
{
"cursor" : "BtreeCursor tags_1",
"isMultiKey" : true,
"n" : 6,
"nscannedObjects" : 6,
"nscanned" : 6,
"nscannedObjectsAllPlans" : 6,
"nscannedAllPlans" : 6,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"tags" : [
[ "blah", "blah" ]
]
}
}
However, if $nin is involved, instead of finding documents that match $in and then filtering out those that don't pass the $nin condition, it scans all documents.
db.assets.find({
tags: {
$in: ['blah'],
$nin: ['cat']
}
}).explain()
{
"cursor" : "BtreeCursor tags_1",
"isMultiKey" : true,
"n" : 75760,
"nscannedObjects" : 79974,
"nscanned" : 1197016,
"nscannedObjectsAllPlans" : 79974,
"nscannedAllPlans" : 1197130,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 9351,
"nChunkSkips" : 0,
"millis" : 2331,
"indexBounds" : {
"tags" : [
[ {"$minElement" : 1}, "cat" ],
[ "cat", {"$maxElement" : 1} ]
]
}
}
Related SO issue: http://stackoverflow.com/questions/26351714/query-with-in-and-nin-doesnt-use-index
- duplicates
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SERVER-12281 When choosing multikey index bounds, never choose a superset if a subset is available
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- Backlog
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