Index management in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Firestore query with missing index?

Consider a Firestore collection users with fields age and city. You run this query:

db.collection('users').where('age', '>', 25).where('city', '==', 'NYC').get()

If the required composite index is missing, what will happen?

Firebase

db.collection('users').where('age', '>', 25).where('city', '==', 'NYC').get()

A The query returns no results but does not throw an error.

B The query returns users filtered only by age, ignoring city.

C The query throws an error indicating a missing index and provides a link to create it.

D The query returns all users older than 25 in NYC without error.

💡 Hint

Firestore requires composite indexes for multiple where filters on different fields.

Firestore requires a composite index for queries filtering on multiple fields. If missing, it throws an error with a link to create the index.

🧠 Conceptual

intermediate

Why are composite indexes important in Firestore?

Which of the following best explains why Firestore uses composite indexes?

A To allow queries filtering or sorting on multiple fields efficiently.

B To store data redundantly for backup purposes.

C To encrypt data fields for security.

D To automatically generate reports from data.

💡 Hint

Think about how Firestore handles queries with multiple conditions.

Composite indexes enable Firestore to quickly find documents matching multiple field conditions or sorting orders.

📝 Syntax

advanced

Which Firestore index definition is valid in index configuration file?

Given the Firestore index configuration JSON, which option correctly defines a composite index on age ascending and city descending for collection users?

Firebase

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

A { "collectionGroup": "users", "queryScope": "COLLECTION", "fields": [ {"fieldPath": "age", "direction": "ASC"}, {"fieldPath": "city", "direction": "DESC"} ] }

B { "collectionGroup": "users", "queryScope": "COLLECTION", "fields": [ {"fieldPath": "age", "order": "ASCENDING"}, {"fieldPath": "city", "order": "DESCENDING"} ] }

C { "collection": "users", "fields": [ {"fieldPath": "age", "order": "ASCENDING"}, {"fieldPath": "city", "order": "DESCENDING"} ] }

D { "collectionGroup": "users", "queryScope": "COLLECTION", "fields": [ {"fieldPath": "age", "order": "ASC"}, {"fieldPath": "city", "order": "DESC"} ] }

💡 Hint

Check the exact field names Firestore expects in index config.

Firestore index config uses 'collectionGroup', 'queryScope', and 'order' for field sorting. 'direction' and 'collection' keys are invalid here.

❓ optimization

advanced

How to optimize Firestore queries with multiple filters?

You have a Firestore query filtering on status and priority fields. Which approach best optimizes query performance?

A Use only one filter at a time to avoid needing indexes.

B Create separate single-field indexes on <code>status</code> and <code>priority</code>.

C Avoid indexes and filter results client-side after fetching all documents.

D Create a composite index on <code>status</code> and <code>priority</code> fields.

💡 Hint

Think about how Firestore uses indexes for multiple filters.

Composite indexes allow Firestore to efficiently execute queries with multiple filters without scanning all documents.

🔧 Debug

expert

Why does this Firestore query fail despite having indexes?

You have these indexes:
- Composite index on category ascending and date descending
- Single-field indexes on category and date

You run this query:

db.collection('items').where('category', '==', 'books').orderBy('date', 'asc').get()

Why does it fail?

A The composite index orders <code>date</code> descending but query orders ascending, causing mismatch.

B The single-field index on <code>date</code> is missing.

C Firestore does not support ordering by <code>date</code> after filtering by <code>category</code>.

D The query is missing a limit clause.

💡 Hint

Check if index order matches query order.

Firestore requires the index order to match the query's orderBy direction exactly. The composite index has date descending but query orders ascending, so it fails.