Details
-
Bug
-
Status: Closed
-
Minor - P4
-
Resolution: Duplicate
-
2.6.4
-
None
-
None
-
ALL
Description
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
Attachments
Issue Links
- duplicates
-
SERVER-12281 When choosing multikey index bounds, never choose a superset if a subset is available
-
- Backlog
-