Query optimization in Firebase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a Firestore query that orders documents by the 'timestamp' field.

Firebase

const query = firestore.collection('messages').orderBy('[1]');

Drag options to blanks, or click blank then click option'

Atimestamp

Bname

Cage

Demail

Attempts:

3 left

💡 Hint

Common Mistakes

Using a field that does not exist in the collection.

Forgetting to specify the field name as a string.

The 'orderBy' method requires the field name to sort documents. Here, 'timestamp' is the correct field to order messages by time.

2fill in blank

medium

Complete the code to limit the number of documents returned by the query to 10.

Firebase

const limitedQuery = query.[1](10);

Drag options to blanks, or click blank then click option'

Afilter

BstartAt

CorderBy

Dlimit

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'filter' instead of 'limit' to restrict results.

Confusing 'orderBy' with limiting results.

The 'limit' method restricts the number of documents returned by the query. Here, it limits to 10 documents.

3fill in blank

hard

Fix the error in the query to filter documents where 'status' equals 'active'.

Firebase

const activeQuery = firestore.collection('users').where('status', '[1]', 'active');

Drag options to blanks, or click blank then click option'

A==

B=

Cis

Dequals

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' instead of '==' in the where clause.

Using non-supported operators like 'equals' or 'is'.

Firestore uses '==' as the operator for equality in queries. '=' is not valid here.

4fill in blank

hard

Fill both blanks to create a query that filters documents where 'age' is greater than 18 and orders by 'age'.

Firebase

const adultQuery = firestore.collection('users').where('age', '[1]', 18).[2]('age');

Drag options to blanks, or click blank then click option'

A>

B<

CorderBy

Dlimit

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' for filtering ages.

Using 'limit' instead of 'orderBy' to sort results.

The 'where' clause uses '>' to filter ages greater than 18, and 'orderBy' sorts the results by age.

5fill in blank

hard

Fill all three blanks to create a query that filters documents where 'score' is at least 50, orders by 'score', and limits results to 5.

Firebase

const topScores = firestore.collection('games').where('score', '[1]', 50).[2]('score').[3](5);

Drag options to blanks, or click blank then click option'

A>=

BorderBy

Climit

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '>=' for filtering scores.

Mixing up the order of 'orderBy' and 'limit' methods.

The query filters scores greater or equal to 50 using '>='. It orders by 'score' and limits results to 5 documents.

Practice

1. What is the main benefit of using query optimization in Firebase?

easy

A. It makes your app faster and reduces costs by fetching only needed data.

B. It automatically backs up your database.

C. It increases the size of your database.

D. It disables security rules temporarily.

Solution

1. Step 1: Understand query optimization purpose

   Query optimization aims to improve performance and reduce costs by limiting data fetched.

2. Step 2: Identify correct benefit

   Only "It makes your app faster and reduces costs by fetching only needed data." describes faster app and cost reduction by fetching needed data.

3. Final Answer:

   It makes your app faster and reduces costs by fetching only needed data. -> Option A

4. Quick Check:

   Query optimization = faster app and lower cost [OK]

Hint: Optimization means fetching only what you need [OK]

Common Mistakes:

• Thinking optimization backs up data
• Assuming optimization increases database size
• Confusing optimization with security settings

2. Which Firebase query syntax correctly limits results to 10 documents ordered by 'score'?

easy

A. db.collection('games').orderBy('score').limit(10)

B. db.collection('games').limit(10).orderBy('score')

C. db.collection('games').where('score', '==', 10).orderBy('score')

D. db.collection('games').orderBy('score', 10)

Solution

1. Step 1: Check correct method order

   In Firebase, orderBy must come before limit to sort then limit results.

2. Step 2: Validate syntax correctness

   db.collection('games').orderBy('score').limit(10) uses correct chaining: orderBy('score').limit(10). Others misuse order or parameters.

3. Final Answer:

   db.collection('games').orderBy('score').limit(10) -> Option A

4. Quick Check:

   orderBy then limit = correct syntax [OK]

Hint: Always orderBy before limit in Firebase queries [OK]

Common Mistakes:

• Placing limit before orderBy
• Using where instead of limit for count
• Passing number as second orderBy argument

3. Given this query:

db.collection('users').where('age', '>=', 18).orderBy('age').limit(3)

What will this query return?

medium

A. All users younger than 18.

B. An error because orderBy and where fields differ.

C. The first 3 users aged 18 or older, ordered by age ascending.

D. The last 3 users regardless of age.

Solution

1. Step 1: Analyze where and orderBy fields

   Both where and orderBy use 'age', which is allowed and required for range filters.

2. Step 2: Understand limit effect

   Limit(3) returns only first 3 documents matching criteria ordered by age ascending.

3. Final Answer:

   The first 3 users aged 18 or older, ordered by age ascending. -> Option C

4. Quick Check:

   where and orderBy on same field + limit = first 3 matching [OK]

Hint: where range and orderBy must use same field [OK]

Common Mistakes:

• Thinking it returns users younger than 18
• Assuming error due to different fields
• Ignoring limit effect

4. You wrote this query:

db.collection('orders').where('status', '==', 'shipped').orderBy('date')

But it throws an error about missing index. What should you do?

medium

A. Remove orderBy to fix the error.

B. Create a composite index on 'status' and 'date' fields in Firebase console.

C. Change '==' to '>=' in where clause.

D. Use limit(1) to avoid indexing.

Solution

1. Step 1: Understand composite index requirement

   Queries with where and orderBy on different fields need a composite index in Firebase.

2. Step 2: Fix error by creating index

   Creating a composite index on 'status' and 'date' fields resolves the error.

3. Final Answer:

   Create a composite index on 'status' and 'date' fields in Firebase console. -> Option B

4. Quick Check:

   Missing index error = create composite index [OK]

Hint: Missing index error means create composite index [OK]

Common Mistakes:

• Removing orderBy instead of indexing
• Changing operators incorrectly
• Using limit to bypass index requirement

5. You want to optimize a query to fetch only active users with scores above 50, ordered by score descending, limited to 5 results. Which query is best?

hard

A. db.collection('users').where('active', '==', true).orderBy('score').limit(5)

B. db.collection('users').orderBy('score', 'desc').limit(5).where('active', '==', true).where('score', '>', 50)

C. db.collection('users').where('score', '>', 50).limit(5).orderBy('score', 'desc').where('active', '==', true)

D. db.collection('users').where('active', '==', true).where('score', '>', 50).orderBy('score', 'desc').limit(5)

Solution

1. Step 1: Check correct chaining and order

   db.collection('users').where('active', '==', true).where('score', '>', 50).orderBy('score', 'desc').limit(5) correctly chains where filters first, then orderBy with descending, then limit.

2. Step 2: Validate query logic

   It filters active users with score > 50, orders by score descending, and limits to 5 results as required.

3. Final Answer:

   db.collection('users').where('active', '==', true).where('score', '>', 50).orderBy('score', 'desc').limit(5) -> Option D

4. Quick Check:

   Filter then orderBy desc then limit = optimized query [OK]

Hint: Chain where filters before orderBy and limit [OK]

Common Mistakes:

• Placing limit before where
• Incorrect orderBy direction
• Missing one of the filters