MongoDB：文档大小会影响查询性能吗？

Question

Assume a mobile game that is backed by a MongoDB database containing a User collection with several million documents.

Now assume several dozen properties that must be associated with the user - eg an array of _id values of Friend documents, their username, photo, an array of _id values of Game documents, last_login date, count of in-game currency, etc, etc, etc..

My concern is whether creating and updating large, growing arrays on many millions of User documents will add any 'weight' to each User document, and/or slowness to the overall system.

We will likely never eclipse 16mb per document, but we can safely say our documents will be 10-20x larger if we store these growing lists directly.

Question: is this even a problem in MongoDB? Does document size even matter if your queries are properly managed using projection and indexes, etc? Should we be actively pruning document size, eg with references to external lists vs. embedding lists of _id values directly?

In other words: if I want a user's last_login value, will a query that projects/selects only the last_login field be any different if my User documents are 100kb vs. 5mb?

Or: if I want to find all users with a specific last_login value, will document size affect that sort of query?

Answer 1

One way to rephrase the question is to say, does a 1 million document query take longer if documents are 16mb vs 16kb each.

Correct me if I'm wrong, from my own experience, the smaller the document size, the faster the query.

I've done queries on 500k documents vs 25k documents and the 25k query was noticeably faster - ranging anywhere from a few milliseconds to 1-3 seconds faster. On production the time difference is about 2x-10x more.

The one aspect where document size comes into play is in query sorting, in which case, document size will affect whether the query itself will run or not. I've reached this limit numerous times trying to sort as little as 2k documents.

More references with some solutions here: https://docs.mongodb.org/manual/reference/limits/#operations https://docs.mongodb.org/manual/reference/operator/aggregation/sort/#sort-memory-limit

At the end of the day, its the end user that suffers.

When I attempt to remedy large queries causing unacceptably slow performance. I usually find myself creating a new collection with a subset of data, and using a lot of query conditions along with a sort and a limit.

Hope this helps!

Answer 2

First of all you should spend a little time reading up on how MongoDB stores documents with reference to padding factors and powerof2sizes allocation:

http://docs.mongodb.org/manual/core/storage/ http://docs.mongodb.org/manual/reference/command/collStats/#collStats.paddingFactor

Put simply MongoDB tries to allocate some additional space when storing your original document to allow for growth. Powerof2sizes allocation became the default approach in version 2.6, where it will grow the document size in powers of 2.

Overall, performance will be much better if all updates fit within the original size allocation. The reason is that if they don't, the entire document needs to be moved someplace else with enough space, causing more reads and writes and in effect fragmenting your storage.

If your documents are really going to grow in size by a factor of 10X to 20X overtime that could mean multiple moves per document, which depending on your insert, update and read frequency could cause issues. If that is the case there are a couple of approaches you can consider:

1) Allocate enough space on initial insertion to cover most (let's say 90%) of normal documents lifetime growth. While this will be inefficient in space usage at the beginning, efficiency will increase with time as the documents grow without any performance reduction. In effect you will pay ahead of time for storage that you will eventually use later to get good performance over time.

2) Create "overflow" documents - let's say a typical 80-20 rule applies and 80% of your documents will fit in a certain size. Allocate for that amount and add an overflow collection that your document can point to if they have more than 100 friends or 100 Game documents for example. The overflow field points to a document in this new collection and your app only looks in the new collection if the overflow field exists. Allows for normal document processing for 80% of the users, and avoids wasting a lot of storage on the 80% of user documents that won't need it, at the expense of additional application complexity.

In either case I'd consider using covered queries by building the appropriate indexes:

A covered query is a query in which:

 all the fields in the query are part of an index, and all the fields returned in the results are in the same index.

Because the index “covers” the query, MongoDB can both match the query conditions and return the results using only the index; MongoDB does not need to look at the documents, only the index, to fulfill the query.

Querying only the index can be much faster than querying documents outside of the index. Index keys are typically smaller than the documents they catalog, and indexes are typically available in RAM or located sequentially on disk.

More on that approach here: http://docs.mongodb.org/manual/tutorial/create-indexes-to-support-queries/

Answer 3

Just wanted to share my experience when dealing with large documents in MongoDB... don't do it!

We made the mistake of allowing users to include files encoded in base64 (normally images and screenshots) in documents. We ended up with a collection of ~500k documents ranging from 2 Mb to 10 Mb each.

Doing a simple aggregate in this collection would bring down the cluster!

Aggregate queries can be very heavy in MongoDB, especially with large documents like these. Indexes in aggregates can only be used in some conditions and since we needed to $group , indexes were not being used and MongoDB would have to scan all the documents.

The exact same query in a collection with smaller sized documents was very fast to execute and the resource consumption was not very high.

Hence, querying in MongoDB with large documents can have a big impact in performance, especially aggregates.

Also, if you know that the document will continue to grow after it is created (eg like including log events in a given entity (document)) consider creating a collection for these child items because the size can also become a problem in the future.

Bruno.

Answer 4

Short answer: yes.

Long answer: how it will affect the queries depends on many factors, like the nature of the queries, the memory available and the indices sizes.

The best you can do is testing.

The code bellow will generate two collections named smallDocuments and bigDocuments, with 1024 documents each, being different only by a field 'c' containing a big string and the _id. The bigDocuments collection will have about 2GB, so be careful running it.

const numberOfDocuments = 1024;

// 2MB string x 1024 ~ 2GB collection
const bigString = 'a'.repeat(2 * 1024 * 1024);

// generate and insert documents in two collections: shortDocuments and
// largeDocuments;
for (let i = 0; i < numberOfDocuments; i++) {
  let doc = {};
  // field a: integer between 0 and 10, equal in both collections;
  doc.a = ~~(Math.random() * 10);

  // field b: single character between a to j, equal in both collections;
  doc.b = String.fromCharCode(97 + ~~(Math.random() * 10));

  //insert in smallDocuments collection
  db.smallDocuments.insert(doc);

  // field c: big string, present only in bigDocuments collection;
  doc.c = bigString;

  //insert in bigDocuments collection
  db.bigDocuments.insert(doc);
}

You can put this code in a file (eg create-test-data.js) and run it directly in the mongoshell, typing this command:

mongo testDb < create-test-data.js

It will take a while. After that you can execute some test queries, like these ones:

const numbersToQuery = [];

// generate 100 random numbers to query documents using field 'a':
for (let i = 0; i < 100; i++) {
  numbersToQuery.push(~~(Math.random() * 10));
}

const smallStart = Date.now();
numbersToQuery.forEach(number => {
  // query using inequality conditions: slower than equality
  const docs = db.smallDocuments
    .find({ a: { $ne: number } }, { a: 1, b: 1 })
    .toArray();
});
print('Small:' + (Date.now() - smallStart) + ' ms');

const bigStart = Date.now();
numbersToQuery.forEach(number => {
  // repeat the same queries in the bigDocuments collection; note that the big field 'c'
  // is ommited in the projection
  const docs = db.bigDocuments
    .find({ a: { $ne: number } }, { a: 1, b: 1 })
    .toArray();
});
print('Big: ' + (Date.now() - bigStart) + ' ms');

Here I got the following results:

Without index:

Small: 1976 ms
Big: 19835 ms

After indexing field 'a' in both collections, with .createIndex({ a: 1 }) :

Small: 2258 ms
Big: 4761 ms

This demonstrates that queries on big documents are slower. Using index, the result time from bigDocuments is more than 100% bigger than in smallDocuments.

My sugestions are:

1. Use equality conditions in queries ( https://docs.mongodb.com/manual/core/query-optimization/index.html#query-selectivity );
2. Use covered queries ( https://docs.mongodb.com/manual/core/query-optimization/index.html#covered-query );
3. Use indices that fit in memory ( https://docs.mongodb.com/manual/tutorial/ensure-indexes-fit-ram/ );
4. Keep documents small;
5. If you need phrase queries using text indices, make sure the entire collection fits in memory ( https://docs.mongodb.com/manual/core/index-text/#storage-requirements-and-performance-costs , last bullet);
6. Generate test data and make test queries, simulating your app use case; use random strings generators if needed.

I had problems with text queries in big documents, using MongoDB: Autocomplete and text search memory issues in apostrophe-cms: need ideas

Here there is some code I wrote to generate sample data, in ApostropheCMS, and some test results: https://github.com/souzabrs/misc/tree/master/big-pieces .

This is more a database design issue than a MongoDB internal one. I think MongoDB was made to behave this way. But, it would help a lot to have more obvious explanation in its documentation.