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Fixed by new explain field "stats" field introduced in SERVER-10026. The full explain stats includes "keyPattern" information for each index scan stage.
> db.foo.find({a:1,b:1}).explain(true).stats
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{
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"type" : "KEEP_MUTATIONS",
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"works" : 1,
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"yields" : 0,
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"unyields" : 0,
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"invalidates" : 0,
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"advanced" : 0,
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"needTime" : 0,
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"needFetch" : 0,
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"isEOF" : 1,
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"children" : [
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{
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"type" : "FETCH",
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"works" : 1,
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"yields" : 0,
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"unyields" : 0,
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"invalidates" : 0,
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"advanced" : 0,
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"needTime" : 0,
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"needFetch" : 0,
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"isEOF" : 1,
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"alreadyHasObj" : 0,
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"forcedFetches" : 0,
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"matchTested" : 0,
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"children" : [
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{
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"type" : "IXSCAN",
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"works" : 1,
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"yields" : 0,
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"unyields" : 0,
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"invalidates" : 0,
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"advanced" : 0,
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"needTime" : 0,
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"needFetch" : 0,
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"isEOF" : 1,
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"keyPattern" : "{ a: 1.0 }",
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"boundsVerbose" : "field #0['a']: [1.0, 1.0]",
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"isMultiKey" : 0,
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"yieldMovedCursor" : 0,
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"dupsTested" : 0,
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"dupsDropped" : 0,
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"seenInvalidated" : 0,
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"matchTested" : 0,
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"keysExamined" : 0,
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"children" : [ ]
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}
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]
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}
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]
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}
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>
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