firebase Firestore document read

Question

I am using google cloud FireStore as a database for our e-commerce mobile application. I am a bit confused about the document read count for firestore documents when we retrieve documents from cloud firestore.

Location of firebase doc: users/123/ This doc contains users information like userId, userName, wishlist, cart, addresses and so. I have used multiple sub-maps to save all user data in a single document to reduce firestore billing costs. Structure of document :

userId:"123",
userName:"Sunil",
addresses : {
  address1 : {
    "doorNo":"123",
    "city" : "Bengaluru"
  },
  address2 : {
    "doorNo":"456",
    "city" : "Bengaluru"
  },
  address3 : {
    "doorNo":"789",
    "city" : "Bengaluru"
  },
wishlistItems:{
  item1 : {
    "itemId":"1"
  },
  item2 : {
    "itemId":"2"
  },
  item3 : {
    "itemId":"3"
  },
  item4 : {
    "itemId":"4"
  },
 },
cartItems:{
  cartItem1:{
    "itemId":"1",
    "itemCount":"2",
  },
  cartItem2:{
    "itemId":"2",
    "itemCount":"5",
  },
  cartItem3:{
    "itemId":"3",
    "itemCount":"7",
  },
  cartItem1:{
    "itemId":"4",
    "itemCount":"8",
  },
 },
},

I retrieve the document as follow: firestore().collection("users").doc("123").get();

My Questions : how many reads will be counted for each retrieval? Will it be only one as I am retrieving only a single document or will it be 12 (as the document is having 11 sub-maps and 1 as normal count)?

Please, help me get clear on this. Thanking you in advance.

Answer 1

Since you are reading everything from a single document, it counts as 1 read.

Answer 2

Reads are charged on how many documents you get back in the result irrespective of content.

firestore().collection("users").doc("123").get();

The above read will cost only 1 read (that too if the document exists).

firestore().collection("users").where("age", ">=", 10).get();

For example, in above query if 57 docs are matched and returned then you'll be charged 57 reads.

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If you have offline persistence enabled and some docs are read from cache, you won't be charged for them.