Composite index requirements in Firebase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a composite index in Firebase?

A composite index in Firebase is an index that combines multiple fields from a collection to speed up queries that filter or sort by more than one field.

beginner

Why do you need composite indexes in Firebase?

You need composite indexes to efficiently run queries that use multiple where clauses or order by multiple fields, because single-field indexes alone cannot support these queries.

beginner

What happens if a required composite index is missing in Firebase?

Firebase will return an error with a direct link to create the missing composite index automatically in the Firebase console.

intermediate

Which query types require composite indexes in Firebase?

Queries that combine multiple filters (where clauses) on different fields, or queries that order by multiple fields, require composite indexes.

intermediate

How do you define a composite index in Firebase?

You define a composite index by specifying the collection and the fields with their sort order (ascending or descending) in the Firebase console or in the index configuration file.

What does a composite index in Firebase combine?

A Multiple fields from a collection

B Multiple collections

C Multiple databases

D Multiple users

A composite index combines multiple fields from a single collection to optimize queries.

When is a composite index required in Firebase?

A When reading a single document by ID

B When querying a single field only

C When querying multiple fields with filters or sorting

D When writing data

Composite indexes are needed when queries filter or sort by multiple fields.

What does Firebase do if a composite index is missing for a query?

A Automatically creates the index

B Ignores the query

C Runs the query slowly without an index

D Returns an error with a link to create the index

Firebase returns an error with a direct link to create the missing composite index.

How do you specify the order of fields in a composite index?

A By listing fields alphabetically

B By choosing ascending or descending for each field

C By the size of the field data

D By the number of documents

You specify ascending or descending order for each field in the composite index.

Which Firebase tool helps you create composite indexes?

A Firebase console

B Firebase CLI only

C Firebase Authentication

D Firebase Hosting

The Firebase console provides an interface to create and manage composite indexes.

Explain what a composite index is and why it is important in Firebase.

Think about how queries with multiple filters or sorts work.

Describe the process and tools used to create a composite index in Firebase.

Consider where you manage indexes in Firebase.

Practice

1. What is the main reason Firebase requires a composite index?

easy

A. To encrypt data for security

B. To store data in multiple locations

C. To backup data automatically

D. To efficiently query data using multiple fields together

Solution

1. Step 1: Understand composite index purpose

   Composite indexes allow Firebase to quickly search using more than one field at the same time.

2. Step 2: Compare options to purpose

   Only To efficiently query data using multiple fields together matches this purpose; others describe unrelated features.

3. Final Answer:

   To efficiently query data using multiple fields together -> Option D

4. Quick Check:

   Composite index = multi-field query speed [OK]

Hint: Composite indexes speed up queries with multiple fields [OK]

Common Mistakes:

• Confusing composite indexes with data backup
• Thinking composite indexes encrypt data
• Believing composite indexes store data in multiple places

2. Which of the following is the correct way to define a composite index in Firebase JSON?

easy

A. "indexes": [{"collectionGroup": "users", "queryScope": "COLLECTION", "fields": [{"fieldPath": "age", "order": "ASCENDING"}, {"fieldPath": "name", "order": "DESCENDING"}]}]

B. "indexes": [{"collection": "users", "fields": ["age", "name"]}]

C. "indexes": [{"collectionGroup": "users", "fields": [{"fieldPath": "age"}, {"fieldPath": "name"}]}]

D. "indexes": [{"collectionGroup": "users", "queryScope": "COLLECTION", "fields": ["age", "name"]}]

Solution

1. Step 1: Check JSON structure for composite index

   Correct composite index JSON includes collectionGroup, queryScope, and fields with fieldPath and order.

2. Step 2: Validate each option

   Only "indexes": [{"collectionGroup": "users", "queryScope": "COLLECTION", "fields": [{"fieldPath": "age", "order": "ASCENDING"}, {"fieldPath": "name", "order": "DESCENDING"}]}] has all required keys and correct field definitions with order specified.

3. Final Answer:

   "indexes": [{"collectionGroup": "users", "queryScope": "COLLECTION", "fields": [{"fieldPath": "age", "order": "ASCENDING"}, {"fieldPath": "name", "order": "DESCENDING"}]}] -> Option A

4. Quick Check:

   Composite index JSON needs fields with order [OK]

Hint: Composite index JSON needs fields with order and collectionGroup [OK]

Common Mistakes:

• Omitting order in fields
• Using 'collection' instead of 'collectionGroup'
• Listing fields as simple strings without objects

3. Given this query in Firebase: db.collection('products').where('category', '==', 'books').orderBy('price', 'asc').orderBy('rating', 'desc'), what composite index is required?

medium

A. Index on fields: category (DESC), price (ASC), rating (ASC)

B. Index on fields: price (ASC), rating (DESC)

C. Index on fields: category (ASC), price (ASC), rating (DESC)

D. No composite index needed

Solution

1. Step 1: Identify query filters and orderBy fields

   The query filters on 'category' and orders by 'price' ascending and 'rating' descending.

2. Step 2: Determine composite index fields and order

   Composite index must include 'category' (filter), then 'price' (asc), then 'rating' (desc) in that order.

3. Final Answer:

   Index on fields: category (ASC), price (ASC), rating (DESC) -> Option C

4. Quick Check:

   Filter + orderBy fields = composite index fields [OK]

Hint: Include all filter and orderBy fields with correct order [OK]

Common Mistakes:

• Ignoring filter field in index
• Wrong order or direction of fields
• Assuming no index needed for multiple orderBy

4. You get an error saying a composite index is missing for this query: db.collection('orders').where('status', '==', 'shipped').orderBy('date', 'desc'). Which fix is correct?

medium

A. Create a composite index on fields: status (ASC), date (DESC)

B. Create a composite index on fields: date (DESC), status (ASC)

C. Create a single-field index on date only

D. No index needed; fix query syntax

Solution

1. Step 1: Analyze query filter and orderBy

   The query filters on 'status' and orders by 'date' descending.

2. Step 2: Determine correct composite index order

   Composite index must start with filter field 'status' (ASC by default), then 'date' (DESC) as orderBy.

3. Final Answer:

   Create a composite index on fields: status (ASC), date (DESC) -> Option A

4. Quick Check:

   Filter field first, then orderBy fields in index [OK]

Hint: Filter field first in index, then orderBy fields [OK]

Common Mistakes:

• Reversing field order in index
• Creating single-field index only
• Ignoring error and changing query syntax

5. You want to query a collection 'events' filtering by 'location' and 'type', and sorting by 'date' ascending. Which composite index definition is correct?

hard

A. Index on fields: date (ASC), location (ASC), type (ASC)

B. Index on fields: location (ASC), type (ASC), date (ASC)

C. Index on fields: location (DESC), type (DESC), date (ASC)

D. No composite index needed if only filtering

Solution

1. Step 1: Identify query filter and sort fields

   The query filters on 'location' and 'type', and sorts by 'date' ascending.

2. Step 2: Determine composite index order

   Composite index must list filter fields first in any order, then the sort field last with ascending order.

3. Step 3: Validate options

   Index on fields: location (ASC), type (ASC), date (ASC) matches filter fields first (location, type) then date ascending. Index on fields: date (ASC), location (ASC), type (ASC) starts with date, which is incorrect. Index on fields: location (DESC), type (DESC), date (ASC) uses descending for filters, which is wrong. No composite index needed if only filtering is incorrect because sorting requires composite index.

4. Final Answer:

   Index on fields: location (ASC), type (ASC), date (ASC) -> Option B

5. Quick Check:

   Filters first, then sort field in index [OK]

Hint: Filter fields first, then sort field ascending in index [OK]

Common Mistakes:

• Placing sort field before filters in index
• Using descending order for filter fields
• Assuming no index needed when sorting