The following queries generates an anomaly with nscanned: t = db.distinct_index1 t.drop(); for ( i=0; i < 100; ++i ) { t.insert( {a: i%10} ); } t.ensureIndex({a:1}); t.runCommand( "distinct" , { key : 'a' , query : { a : { $in : [2, 4, 6, 8] } } } ); { "values" : [ 2, 4, 6, 8 ], "stats" : { "n" : 40, "nscanned" : 43, "nscannedObjects" : 40, "timems" : 12, "cursor" : "BtreeCursor a_1" }, "ok" : 1 } t.runCommand( "distinct" , { key : 'a' , query :{ $or: [{a:3}, {a:5}] } } ); { "values" : [ 3, 5 ], "stats" : { "n" : 20, "nscanned" : 21, "nscannedObjects" : 20, "timems" : 0, "cursor" : "BtreeCursor a_1" }, "ok" : 1 }