Firestore Queries & IndexesIntermediate
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javascriptHow do Firestore composite indexes work and why does Firestore reject unindexed compound queries?
Explanation & Concept
Firestore guarantees that query performance is proportional to the size of the result set, not the size of the database. For queries that filter or sort on multiple distinct fields (compound queries), Firestore requires a composite index.
If you run an unindexed query, Firestore returns an error containing a direct clickable URL to generate the required index in the Firebase Console.
Code Example
javascript
import { query, collection, where, orderBy, getDocs } from "firebase/firestore";
// Requires Composite Index: status (Ascending) + createdAt (Descending)
const q = query(
collection(db, "tickets"),
where("status", "==", "OPEN"),
where("priority", "==", "HIGH"),
orderBy("createdAt", "desc")
);
const querySnapshot = await getDocs(q);How It Works & Key Takeaways
Firestore constructs index entries combining all three fields, enabling single-scan index queries that return instantly regardless of collection size.
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