Major - P3 ISSUE SUMMARY
Negation predicates like $ne or $not did not make use of an index if the index was a "multikey" index (where the indexed fields contained arrays).
USER IMPACT
Queries with negation predicates were scanning the entire collection, which is inefficient and can cause disruption in the working set.
WORKAROUNDS
In some cases, the query can be rewritten (for example with $gt and $lt predicates) to eliminate the negation predicate.
RESOLUTION
Multikey index usage has been enabled for negation predicates.
AFFECTED VERSIONS
All recent production releases up to and including 2.6.0 are affected.
PATCHES
The patch is included in the 2.6.1 production release.
Original description
This query does not seem to use an index at all even when it exists:
db.Book.find({ "Ratings" : { "$ne" : null, "$not" : { "$size" : 0 } } } )
Notice the DB (referenced below), has an index:
{
"v" : 1,
"key" : {
"Ratings" : 1
},
"name" : "Ratings_1",
"ns" : "BookStore.Book"
}
Yet, when you explain the query, you see basic cursor:
> db.Book.find({ "Ratings" : { "$ne" : null, "$not" : { "$size" : 0 } } } ).expl
ain()
{
"cursor" : "BasicCursor",
"isMultiKey" : false,
"n" : 270171,
"nscannedObjects" : 271380,
"nscanned" : 271380,
"nscannedObjectsAllPlans" : 271380,
"nscannedAllPlans" : 271380,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 2120,
"nChunkSkips" : 0,
"millis" : 559,
"server" : "WIN-7S2IMPQ2TOE:27017",
"filterSet" : false
}
To see this in action, open the BookStore.zip file from this issue:
https://jira.mongodb.org/browse/SERVER-13065