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It's technically possible to nest an $or under $elemMatch with the intent of finding array elements that match at least one criteria. However, in 2.4 the index is not used for that search. The new query engine on 2.6.5 handles the case as expected.
Here is a repro from 2.4:
db.ggg.drop()
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db.ggg.insert({party:[{a:5,b:10},{a:3,b:2}]})
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db.ggg.insert({party:[{a:5,b:10},{a:4,b:3}]})
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db.ggg.insert({party:[{a:5,b:0},{a:4,b:3}]})
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db.ggg.insert({party:[{a:1,b:0},{a:4,b:3}]})
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db.ggg.insert({party:[{a:0,b:1},{a:2,b:0}]})
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db.ggg.ensureIndex({"party.a":1,"party.b":1})
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Output from 2.4:
> db.ggg.find({ "party": { $elemMatch: { $or: [ { "a" : 5, "b" : 10 },{ "a" : 3, "b" : 2 } ]} } }).explain()
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{
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"cursor" : "BasicCursor",
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"isMultiKey" : false,
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"n" : 2,
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"nscannedObjects" : 5,
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"nscanned" : 5,
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"nscannedObjectsAllPlans" : 5,
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"nscannedAllPlans" : 5,
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"scanAndOrder" : false,
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"indexOnly" : false,
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"nYields" : 0,
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"nChunkSkips" : 0,
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"millis" : 0,
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"indexBounds" : {
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},
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"server" : "AD-MAC10G.local:27017"
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}
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Output from 2.6 for comparison:
db.ggg.find({ "party": { $elemMatch: { $or: [ { "a" : 5, "b" : 10 },{ "a" : 3, "b" : 2 } ]} } }).explain()
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{
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"clauses" : [
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{
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"cursor" : "BtreeCursor party.a_1_party.b_1",
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"isMultiKey" : true,
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"n" : 2,
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"nscannedObjects" : 2,
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"nscanned" : 2,
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"scanAndOrder" : false,
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"indexOnly" : false,
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"nChunkSkips" : 0,
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"indexBounds" : {
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"party.a" : [
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[
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5,
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5
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]
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],
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"party.b" : [
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[
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10,
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10
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]
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]
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}
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},
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{
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"cursor" : "BtreeCursor party.a_1_party.b_1",
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"isMultiKey" : true,
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"n" : 1,
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"nscannedObjects" : 1,
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"nscanned" : 1,
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"scanAndOrder" : false,
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"indexOnly" : false,
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"nChunkSkips" : 0,
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"indexBounds" : {
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"party.a" : [
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[
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3,
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3
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]
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],
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"party.b" : [
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[
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2,
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2
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]
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]
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}
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}
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],
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"cursor" : "QueryOptimizerCursor",
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"n" : 2,
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"nscannedObjects" : 3,
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"nscanned" : 3,
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"nscannedObjectsAllPlans" : 3,
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"nscannedAllPlans" : 3,
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"scanAndOrder" : false,
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"nYields" : 0,
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"nChunkSkips" : 0,
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"millis" : 0,
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"server" : "AD-MAC10G.local:37017",
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"filterSet" : false
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}
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