[{"key":["a","b","c"]},{"key":["b","d"]},{"key":["e"]}]

db.collection.aggregate([{"$project":{"keys":{"$map":{"input":{"$range":[0,{"$size":"$key"},1]},as:"index",in:{$slice:["$key","$$index",1]}}}}},{"$group":{"_id":null,"keys":{"$addToSet":{"$concatArrays":["$keys"]}},}}])