How to fix PERMISSION_DENIED: Missing index for this query in Firebase

When you encounter the missing index error, Firestore (usually) provides a direct link in the error message or Firebase console. The error will look like:

FirebaseError: PERMISSION_DENIED: Missing index for this query
[cloud_firestore/failed-precondition]

If a link is present in the error message, click it to automatically navigate to the Firebase console with the required index configuration pre-filled.

If no link appears, manually create the index:

1. Go to the Firebase Console (https://console.firebase.google.com)
2. Select your project
3. Navigate to Firestore Database
4. Click the "Indexes" tab
5. Click "Create Index"
6. Enter the collection name (e.g., "users")
7. Add the fields used in your query (both filters and orderBy fields)
8. Set the sort direction (Ascending/Descending) for each field
9. Click "Create Index"

Firestore will begin building the index, which may take a few minutes depending on your data size.

For production and IaC (Infrastructure as Code), deploy indexes via CLI:

# Initialize Firestore in your project
firebase init firestore

# This creates a firestore.indexes.json file
# Edit it to include your custom composite indexes:

{
  "indexes": [
    {
      "collectionGroup": "users",
      "queryScope": "COLLECTION",
      "fields": [
        { "fieldPath": "status", "order": "ASCENDING" },
        { "fieldPath": "createdAt", "order": "DESCENDING" }
      ]
    }
  ],
  "fieldOverrides": []
}

# Deploy the indexes
firebase deploy --only firestore:indexes

Wait for the index to complete building before testing your query:

1. In the Firestore Console, go to Indexes tab
2. Look for your index in the "Composite Indexes" section
3. Wait for the status to change from "Building" to "Enabled" (green checkmark)
4. Once enabled, your query will execute successfully

Check your browser console for confirmation:

document.addEventListener('firestore-index-ready', (e) => {
  console.log('Index is ready:', e.detail);
  // Now safe to run your complex queries
});

Once the index is built, test that your query now works:

import { collection, query, where, orderBy, getDocs } from 'firebase/firestore';

const q = query(
  collection(db, 'users'),
  where('status', '==', 'active'),
  orderBy('createdAt', 'desc')
);

const snapshot = await getDocs(q);
console.log('Query successful! Documents:', snapshot.docs.length);

If the query still fails, verify:
- Index status is "Enabled" (not "Building")
- Query fields exactly match the index fields
- Field ordering in your query matches the index configuration