Why Realtime Database differs from Firestore in Firebase - Performance Analysis

Time Complexity: Why Realtime Database differs from Firestore

O(n)

Understanding Time Complexity

We want to understand how the speed of data operations changes when using Realtime Database versus Firestore.

How does the number of operations grow as we work with more data?

Scenario Under Consideration

Analyze the time complexity of reading multiple records from each database.

// Realtime Database
const ref = firebase.database().ref('items');
ref.once('value').then(snapshot => {
  snapshot.forEach(child => {
    console.log(child.key, child.val());
  });
});

// Firestore
const collection = firebase.firestore().collection('items');
collection.get().then(querySnapshot => {
  querySnapshot.forEach(doc => {
    console.log(doc.id, doc.data());
  });
});
    

This code reads all items from a list in each database and logs their data.

Identify Repeating Operations

Look at what happens repeatedly when reading data.

• Primary operation: Processing each item's data.
• How many times: Once per item in the list.

How Execution Grows With Input

As the number of items grows, the number of operations grows too.

Input Size (n)	Approx. Operations
10	About 10 operations
100	About 100 operations
1000	About 1000 operations

Each item requires a separate operation, so the work grows directly with the number of items.

Final Time Complexity

Time Complexity: O(n)

This means the time to read all items grows in a straight line as the number of items increases.

Common Mistake

[X] Wrong: "Fetching data from Firestore or Realtime Database always takes the same time no matter how many items there are."

[OK] Correct: Each item requires a separate operation, so more items mean more work and longer time.

Interview Connect

Understanding how data operations scale helps you design apps that stay fast as they grow. This skill shows you can think about real-world app performance.

Self-Check

What if we used queries to limit the number of items fetched? How would that change the time complexity?

Practice

1. What is a key difference between Firebase Realtime Database and Firestore in how they store data?

easy

A. Realtime Database stores data as one large JSON tree, while Firestore uses collections and documents.

B. Realtime Database uses tables and rows, Firestore uses JSON files.

C. Realtime Database stores data in XML format, Firestore uses CSV files.

D. Realtime Database uses collections and documents, Firestore uses one big JSON tree.

Solution

1. Step 1: Understand Realtime Database data structure

   Realtime Database organizes data as a single large JSON tree, making it simple but less structured.

2. Step 2: Understand Firestore data structure

   Firestore organizes data into collections and documents, allowing more structure and flexibility.

3. Final Answer:

   Realtime Database stores data as one large JSON tree, while Firestore uses collections and documents. -> Option A

4. Quick Check:

   Data structure difference = Realtime Database stores data as one large JSON tree, while Firestore uses collections and documents. [OK]

Hint: Remember: JSON tree vs collections-documents [OK]

Common Mistakes:

• Confusing Firestore's collections with Realtime Database's JSON tree
• Thinking both use the same data structure
• Mixing up file formats like XML or CSV

2. Which of the following is the correct way to reference a document in Firestore?

easy

A. db.ref('users/user123')

B. db.collection('users').doc('user123')

C. db.table('users').row('user123')

D. db.get('users/user123')

Solution

1. Step 1: Identify Firestore syntax

   Firestore uses collections and documents accessed by db.collection('name').doc('id').

2. Step 2: Compare options

   db.collection('users').doc('user123') matches Firestore syntax; db.ref('users/user123') is Realtime Database style; Options C and D are invalid.

3. Final Answer:

   db.collection('users').doc('user123') -> Option B

4. Quick Check:

   Firestore document reference = db.collection('users').doc('user123') [OK]

Hint: Firestore uses collection().doc() syntax [OK]

Common Mistakes:

• Using Realtime Database syntax for Firestore
• Confusing doc() with ref()
• Trying to use SQL-like table and row methods

3. Consider this Firestore query code:

const users = await db.collection('users').where('age', '>', 18).get();
console.log(users.docs.length);

What will this code output compared to a similar Realtime Database query?

medium

A. Always zero because Firestore cannot filter by age.

B. Syntax error because Firestore does not support where queries.

C. Number of users older than 18, Firestore supports this query directly.

D. Number of users older than 18, but Realtime Database cannot do this query.

Solution

1. Step 1: Understand Firestore query capabilities

   Firestore supports advanced queries like filtering with where clauses.

2. Step 2: Compare with Realtime Database

   Realtime Database has limited querying and cannot filter by fields easily.

3. Final Answer:

   Number of users older than 18, Firestore supports this query directly. -> Option C

4. Quick Check:

   Firestore advanced queries = Number of users older than 18, Firestore supports this query directly. [OK]

Hint: Firestore supports where queries; Realtime Database limited [OK]

Common Mistakes:

• Thinking Firestore cannot filter data
• Assuming Realtime Database supports complex queries
• Confusing query syntax between databases

4. You wrote this Realtime Database code:

const ref = db.ref('users').orderByChild('age').equalTo(25);
ref.on('value', snapshot => { console.log(snapshot.val()); });

But it returns null even though users with age 25 exist. What is the likely issue?

medium

A. Realtime Database cannot query by child properties.

B. Firestore syntax was used instead of Realtime Database syntax.

C. The database path 'users' does not exist.

D. The query is invalid; Realtime Database requires indexes for orderByChild queries.

Solution

1. Step 1: Check Realtime Database query requirements

   Realtime Database needs proper indexing for orderByChild queries to work.

2. Step 2: Identify cause of null result

   Without indexes, the query returns null even if data exists.

3. Final Answer:

   The query is invalid; Realtime Database requires indexes for orderByChild queries. -> Option D

4. Quick Check:

   Missing index causes null result = The query is invalid; Realtime Database requires indexes for orderByChild queries. [OK]

Hint: Realtime DB needs indexes for orderByChild queries [OK]

Common Mistakes:

• Confusing Firestore and Realtime Database syntax
• Assuming data path is missing without checking
• Believing Realtime Database cannot query child properties

5. You want to build a chat app with many users and complex queries on messages. Which Firebase database should you choose and why?

hard

A. Use Firestore because it supports complex queries and scales well.

B. Use Realtime Database because it scales better for complex queries.

C. Use Realtime Database because it uses collections and documents.

D. Use Firestore because it stores data as one big JSON tree.

Solution

1. Step 1: Analyze app requirements

   A chat app with many users and complex queries needs good scaling and query support.

2. Step 2: Compare database features

   Firestore supports complex queries and scales better than Realtime Database for large apps.

3. Final Answer:

   Use Firestore because it supports complex queries and scales well. -> Option A

4. Quick Check:

   Complex queries + scaling = Firestore [OK]

Hint: Choose Firestore for complex queries and scaling [OK]

Common Mistakes:

• Choosing Realtime Database for complex queries
• Confusing data structure descriptions
• Ignoring scaling needs for large apps