Sorting 레코드 용량 제한

포털의 정렬 기능이 에러가났다.

Encountered non-retryable error during query :: caused by :: Executor error during find command :: caused by :: Sort operation used more than the maximum 33554432 bytes of RAM. Add an index, or specify a smaller limit.:
	MongoError: Encountered non-retryable error during query :: caused by :: Executor error during find command :: caused by :: Sort operation used more than the maximum 33554432 bytes of RAM. Add an index, or specify a smaller limit.
    at MessageStream.messageHandler (/usr/src/app/node_modules/mongodb/lib/cmap/connection.js:272:20)
    at MessageStream.emit (events.js:400:28)
    at processIncomingData (/usr/src/app/node_modules/mongodb/lib/cmap/message_stream.js:144:12)
    at MessageStream._write (/usr/src/app/node_modules/mongodb/lib/cmap/message_stream.js:42:5)
    at writeOrBuffer (internal/streams/writable.js:358:12)
    at MessageStream.Writable.write (internal/streams/writable.js:303:10)
    at Socket.ondata (internal/streams/readable.js:726:22)
    at Socket.emit (events.js:400:28)
    at addChunk (internal/streams/readable.js:290:12)
    at readableAddChunk (internal/streams/readable.js:265:9)
    at Socket.Readable.push (internal/streams/readable.js:204:10)
    at TCP.onStreamRead (internal/stream_base_commons.js:188:23)
    at TCP.callbackTrampoline (internal/async_hooks.js:131:17)

해당 에러는 메모리 사용량이 많아 정렬을 할 수 없다는 뜻 이다.
메모리가 32MB를 초과하는 많은 양의 Document가 있는 경우 정렬을 하지 않는다.

=> createIndex()를 통해서 해결