firestore : is it faster to retrieve a document by its id or by a query?

The documents in my collection have an id (of course), but also a field in which the same id is kept.
Question : is it faster to retrieve that document by its id

collection.doc(id).get().then(....)

or by a query

collection.where('id', '==', id).get().then(...)

I just refactored my functions-code. I used to use a query, but now I'm using the id of the document in the collection directly. But I have the impression that this direct method takes longer. I just wonder if that is just an impression ...

In theory, there is no difference in performance between the two. Firestore queries scale with the number of documents returned by the query. If both queries return 1 document, then they should perform the same. They are essentially both using an index to find a unique value.

See: https://stackoverflow.com/questions/46603205/queries-scale-with-the-size-of-your-result-set-not-the-size-of-your-data-set

If you really want a micro-benchmark on this, you should perform your own tests to find out which is faster. But I think you'll be spending a lot of time optimizing something that doesn't need to be optimized. You can decide for yourself if that's worth your time.

My opinionated advice: Instead of worrying about performance, you should worry about clarity of your code. Which one makes more sense to you when you read it in the context of your program? Usually a single get on a document reference is shorter and easier to read, and doesn't require you to look at an array of document results when you expect only one document, but your opinion might be different.