Reading Data from Firestore - flutter

Question

I am creating an app where a user can create an event and see other events created by other users only if there are in a 10Km Radius. I am storing all the data in firestore.
This is how the app works, from the user side all the events are fetched and only those events are displayed whose distance is less than 10km.
The problem is if there are 10,000 events in the database and the user is in the 10km radius of only 10 events then obviously it will count as 10,000 reads which is too expensive.
Any suggestions for this problem?
One solution that I have in mind is to store data according to the geographical area but how to implement it is another problem.

Answer 1

You won't be charge for 10 000 reads but only the documents retrieved by your query, ten in your example.
Here's a good video from Firebase explaining their billing system : https://www.youtube.com/watch?time_continue=224&v=6NegFl9p_sE
Also keep in mind that for queries other than document reads, such as a request for a list of collection IDs, you are billed for one document read.

Answer 2

I suggest computing min and max longitude corresponding to 10 km distance from user longitude. You do the same with latitude and use those limits in Firestore query. Doing so You will have less events/reads and you can compute the exact distance to suppress events between 10 and 14 km radius... if necessary.
To compute your limit you can use the following formula from https://www.movable-type.co.uk/scripts/latlong.html (Destination point given distance and bearing from start point) and there's an online javascript calculator you can study.
φ2 = asin( sin φ1 ⋅ cos δ + cos φ1 ⋅ sin δ ⋅ cos θ )
λ2 = λ1 + atan2( sin θ ⋅ sin δ ⋅ cos φ1, cos δ − sin φ1 ⋅ sin φ2 )
where φ is latitude, λ is longitude, θ is the bearing (clockwise from north), δ is the angular distance d/R; d being the distance traveled in meters, R the earth’s radius (6371e3).
With bearing 0° you obtain LatitudeMax
With bearing 90° you obtain longitudeEast
With bearing 180° you obtain LatitudeMin
With bearing 270° you obtain longitudewest
Since earth is a sphere with longitude between -180° and 180°
longitudeMin = Min(longitudeEast, longitudeWest)
longitudeMax = Max(longitudeEast, longitudeWest)
And The Firestore part is like :
CollectionReference col = Firestore.instance.collection("mycollection");
Query latitudeMinQuery = col.where('latitude', isGreaterThan: LatitudeMin);
Query latitudeMaxQuery = latitudeMinQuery.where('latitude', isLessThan: LatitudeMin);
Query longitudeMinQuery = latitudeMaxQuery.where('longitude', isGreaterThan: LongitudeMin);
Query longitudeMaxQuery = longitudeMinQuery.where('latitude', isLessThan: LongitudeMax);

Answer 3

https://stackoverflow.com/a/43804487/9139407 (answers by #alex-mamo)
Hopes it helps!