Query Convex data using index-backed reads (withIndex, order, take, first, unique) instead of full table scans

Documented steps

1. In a query or mutation handler, start a query with ctx.db.query("tableName").
2. Make sure a matching index exists in convex/schema.ts, e.g. .index("by_channel", ["channel"]) on that table.
3. Chain .withIndex("by_channel", (q) => q.eq("channel", channelId)) to scan only documents matching the index range instead of the full table.
4. For compound indexes, add range comparisons strictly in index field order: q.eq("channel", channelId).gt("_creationTime", startTime).lt("_creationTime", endTime) — supply zero or more leading .eq() calls, then at most one lower bound (.gt()/.gte()), then at most one upper bound (.lt()/.lte()); you cannot skip a field.
5. Control result order with .order("asc") or .order("desc"); default order is ascending by _creationTime (which Convex auto-appends to every index).
6. Retrieve results with .take(n) for the first n matches, .first() for one document or null, .unique() for exactly one document (throws if more than one matches), or .collect() for every matching document.
7. Use .filter() only to add extra conditions after the index has narrowed the scan set; it examines every already-scanned document and does not itself reduce how many documents get scanned.
8. Avoid calling .withIndex() with no range expression, or .filter() plus .collect(), on large tables — that becomes a full table scan; prefer .take(n)/.first()/.unique()/.paginate() to bound work.
9. Deploy with npx convex dev so the client has a deployment URL and generated api, then call the query from React with useQuery(api.module.functionName, args).
10. Official docs: https://docs.convex.dev/database/reading-data/indexes/ and https://docs.convex.dev/database/reading-data/