feat: warn when orderBy+limit or lazy join falls back to full load due to missing index

Adds console warnings when no index is found for:
- orderBy + limit queries (order-by.ts)
- lazy join loading (joins.ts)

Both suggest creating an explicit index or enabling autoIndex: 'eager'.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kevin-dp

packages/db/src/query/compiler/joins.ts

@@ -312,6 +312,14 @@ function processJoin(
 
           if (!loaded) {
             // Snapshot wasn't sent because it could not be loaded from the indexes
+            const collectionId = followRefCollection.id
+            const fieldPath = followRefResult.path.join(`.`)
+            console.warn(
+              `[TanStack DB]${collectionId ? ` [${collectionId}]` : ``} Join requires an index on "${fieldPath}" for efficient loading. ` +
+                `Falling back to loading all data. ` +
+                `Consider creating an index on the collection with collection.createIndex((row) => row.${fieldPath}) ` +
+                `or enable auto-indexing with autoIndex: 'eager' and a defaultIndexType.`,
+            )
             lazySourceSubscription.requestSnapshot()
           }
         }),

packages/db/src/query/compiler/order-by.ts

@@ -191,6 +191,17 @@ export function processOrderBy(
           index = undefined
         }
 
+        if (!index) {
+          const collectionId = followRefCollection.id
+          const fieldPath = followRefResult.path.join(`.`)
+          console.warn(
+            `[TanStack DB]${collectionId ? ` [${collectionId}]` : ``} orderBy with limit requires an index on "${fieldPath}" for efficient lazy loading. ` +
+              `Falling back to loading all data. ` +
+              `Consider creating an index on the collection with collection.createIndex((row) => row.${fieldPath}) ` +
+              `or enable auto-indexing with autoIndex: 'eager' and a defaultIndexType.`,
+          )
+        }
+
         orderByAlias =
           firstOrderByExpression.path.length > 1
             ? String(firstOrderByExpression.path[0])