Firestore Query on large collection performance

Question

I am making an app where users can follow each other. To decide how to model it in firestore I would like to know how does collection size affect query performance. I first thought of making it like this:

relationships(coll.)
----{userId_1}(document)
--------following(coll)
------------{someId1}(document)
------------{someId2}(document)
.....
--------followers(coll)
------------{someId5}(document)
------------{someId7}(document)
.....
----{userId_2}(document)
--------following(coll)
------------{someId11}(document)
------------{someId24}(document)
.....
--------followers(coll)
------------{someId56}(document)
------------{someId72}(document)
.....

So I would have main collection relationships, then each document would represent one user and he would have two collections - following and followers, and in those collections I would store documents with data like id,name,email,.. Then when user1 wants to see his followers, I would get all documents under relationships/userId_1/followers, and if he would like to see who he follows I would get documents under relationships/userId_1/following

I also thought about doing it like this:

relationships(coll)
----{user5id_user4id}(document)
--------user1:"user5id" (field)
--------user2:"user4id" (field)
.........(other fields)
----{user4_user5}(document)
--------user1:"user4id" (field)
--------user2:"user5id" (field)
.........(other fields)

I would have one main collection relationships where each document would represent one following relationship, document name would be firstUserId_secondUSerId (means firstUserId follows secondUserId) and I would also have two fields user1 and user2 that would store ids of two users where user1 follows user2 So if I am {myUserId} and I would like to get all the people who I follow I would do a query on relationships collection where user1 = myUserId And if I would like to get all the people who follow me I would do a query on relationships collection where user2 = myUserId since each document represents relation user1 follows user2.

So my question is which way would be more efficient with querying the data. In first case each user would have collection of his followers/following and I would just get the documents, in second case relationship would have many document representing user1->follows->user2 relation. I know that I would be billed by number of documents that query function returns, but how fast would it be if it would need to search through large collection.

Doug Stevenson Doug Stevenson · Accepted Answer · 2019-04-28T11:51:06

Collection size has no bearing on the performance or cost of a query. Both are determined entirely by size of the result size (number of documents). So, a query for 10 documents out of 100 performs and costs the same as a query for 10 documents out of 100,000. The size of 10 is the only thing that matters here.

See also: Queries scale with the size of your result set, not the size of your data set

Firestore Query on large collection performance

1 Answers