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queries on sharded collections run against primary can be covered. Behavior seems to have changed starting 2.6.4.
mongos limitation seem unchanged (until 2.8)
MongoDB shell version: 2.6.3
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connecting to: 127.0.0.1:27018/test
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shardA:PRIMARY> db.foo.find( { a: 1 }, { _id: 0 } ).explain()
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{
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"cursor" : "BtreeCursor a_1",
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"isMultiKey" : false,
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"n" : 1,
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"nscannedObjects" : 1,
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"nscanned" : 1,
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"nscannedObjectsAllPlans" : 1,
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"nscannedAllPlans" : 1,
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"scanAndOrder" : false,
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"indexOnly" : false,
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sharda-265:PRIMARY> db.foo.find( { a: 1}, { a: 1, _id: 0 } ).explain()
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{
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"cursor" : "BtreeCursor a_1_b_1",
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"isMultiKey" : false,
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"n" : 1,
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"nscannedObjects" : 0,
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"nscanned" : 1,
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"nscannedObjectsAllPlans" : 0,
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"nscannedAllPlans" : 2,
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"scanAndOrder" : false,
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"indexOnly" : true,
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