Adding documents (add vs set) in Firebase - Performance Comparison

Time Complexity: Adding documents (add vs set)

O(n)

Understanding Time Complexity

When adding documents in Firebase, it's important to know how the time to complete the operation changes as you add more documents.

We want to understand how the number of documents affects the work Firebase does when using add() versus set().

Scenario Under Consideration

Analyze the time complexity of adding multiple documents using add() and set().

const collectionRef = firestore.collection('users');

// Using add() to add a new document with auto ID
collectionRef.add({ name: 'Alice', age: 30 });

// Using set() to add or overwrite a document with a known ID
collectionRef.doc('user123').set({ name: 'Bob', age: 25 });

// Adding multiple documents in a loop
for (let i = 0; i < n; i++) {
  collectionRef.add({ index: i });
}
    

This code adds documents to a collection, either letting Firebase create an ID or specifying one, repeated n times.

Identify Repeating Operations

Look at what happens repeatedly when adding documents.

• Primary operation: Each call to add() or set() sends one write request to Firebase.
• How many times: The write request happens once per document added, so n times for n documents.

How Execution Grows With Input

As you add more documents, the number of write requests grows directly with the number of documents.

Input Size (n)	Approx. Api Calls/Operations
10	10 write requests
100	100 write requests
1000	1000 write requests

Pattern observation: The work grows in a straight line with the number of documents added.

Final Time Complexity

Time Complexity: O(n)

This means the time to add documents grows directly in proportion to how many documents you add.

Common Mistake

[X] Wrong: "Using add() is faster than set() because it auto-generates IDs."

[OK] Correct: Both add() and set() send one write request per document, so their time grows the same way with the number of documents.

Interview Connect

Understanding how adding documents scales helps you design apps that handle data efficiently and predict costs as your app grows.

Self-Check

"What if we batch multiple document writes together? How would that change the time complexity?"

Practice

1. What is the main difference between add and set when adding documents in Firebase?

easy

A. add creates a document with an automatic unique ID, set uses a specified ID.

B. add updates existing documents, set only creates new documents.

C. add deletes documents, set reads documents.

D. add requires a document ID, set generates one automatically.

Solution

1. Step 1: Understand add behavior

   add creates a new document with a unique ID generated by Firebase automatically.

2. Step 2: Understand set behavior

   set creates or updates a document at a specified ID you provide.

3. Final Answer:

   add creates a document with an automatic unique ID, set uses a specified ID. -> Option A

4. Quick Check:

   add auto ID, set specified ID = A [OK]

Hint: Remember: add auto-generates ID, set needs your ID [OK]

Common Mistakes:

• Thinking set always creates new documents only
• Confusing which method generates IDs automatically
• Assuming add can update existing documents

2. Which of the following is the correct syntax to add a new document with an automatic ID in Firebase Firestore?

easy

A. db.collection('users').add({name: 'Alice'})

B. db.collection('users').set({name: 'Alice'})

C. db.collection('users').doc('user1').add({name: 'Alice'})

D. db.collection('users').doc().add({name: 'Alice'})

Solution

1. Step 1: Identify method for automatic ID

   add creates a new document with an automatic ID inside a collection.

2. Step 2: Check syntax correctness

   db.collection('users').add({name: 'Alice'}) is the correct syntax to add a new document with auto ID.

3. Final Answer:

   db.collection('users').add({name: 'Alice'}) -> Option A

4. Quick Check:

   Use add for auto ID = B [OK]

Hint: Use add() on collection to auto-generate document ID [OK]

Common Mistakes:

• Using set() without specifying a document ID
• Calling add() on a document reference instead of collection
• Using doc() without an ID then calling add()

3. What will be the result of this code snippet?

db.collection('products').doc('p1').set({name: 'Book', price: 10});
db.collection('products').doc('p1').set({price: 12});

medium

A. The document 'p1' will have {name: 'Book', price: 12}

B. The document 'p1' will have {name: 'Book', price: 10, price: 12}

C. The document 'p1' will have only {price: 12}, name is removed

D. An error occurs because set is called twice

Solution

1. Step 1: Analyze first set call

   The first set creates or overwrites document 'p1' with {name: 'Book', price: 10}.

2. Step 2: Analyze second set call without merge

   The second set overwrites the entire document with {price: 12}, removing 'name' because merge option is not used.

3. Final Answer:

   The document 'p1' will have only {price: 12}, name is removed -> Option C

4. Quick Check:

   set without merge overwrites whole document = C [OK]

Hint: set without merge replaces whole document, removing old fields [OK]

Common Mistakes:

• Assuming set merges fields by default
• Expecting both name and price to remain
• Thinking calling set twice causes error

4. Identify the error in this code snippet and how to fix it:

db.collection('orders').add({id: 'order1', total: 50});
db.collection('orders').doc('order1').set({total: 60});

medium

A. No error; both lines work fine together.

B. The first line creates a document with a random ID, so the second line updates a different document.

C. The second line should use add() instead of set().

D. The first line should specify the document ID using doc('order1').set() instead of add().

Solution

1. Step 1: Understand add() behavior

   The first line adds a document with a random ID, ignoring the 'id' field inside data.

2. Step 2: Understand doc('order1').set() behavior

   The second line sets data on a document with ID 'order1', which is different from the random ID created before.

3. Step 3: Identify mismatch

   Because IDs differ, the second line updates a different document than the one created by add().

4. Final Answer:

   The first line creates a document with a random ID, so the second line updates a different document. -> Option B

5. Quick Check:

   add() random ID ≠ doc('order1') ID = A [OK]

Hint: add() ignores id field; doc(id).set() targets specific document [OK]

Common Mistakes:

• Assuming add() uses the id field as document ID
• Using set() on a doc without knowing if it exists
• Expecting add() and set() to affect same document automatically

5. You want to update only the 'price' field of a document without removing other fields. Which Firebase method and option should you use?

hard

A. Use add() with the new price data.

B. Use set() without any options.

C. Use update() without any options.

D. Use set() with {merge: true} option.

Solution

1. Step 1: Understand add() behavior

   add() creates a new document with a random ID; it cannot update existing documents.

2. Step 2: Understand set() without merge

   set() without options overwrites the whole document, removing other fields.

3. Step 3: Understand set() with merge option

   set() with {merge: true} updates only specified fields, keeping others intact.

4. Step 4: Consider update()

   update() also updates fields but requires the document to exist; set() with merge works even if document is new.

5. Final Answer:

   Use set() with {merge: true} option. -> Option D

6. Quick Check:

   Partial update without overwrite = set() + merge true = D [OK]

Hint: Use set() with merge:true to update fields without deleting others [OK]

Common Mistakes:

• Using add() to update existing documents
• Using set() without merge and losing data
• Confusing update() with set() merge behavior