Don't push down where clauses that only touch the namespace of a source and not a prop (#600)

* add test

* the fix

* changeset

Author: samwillis

.changeset/curvy-mangos-yell.md

@@ -0,0 +1,5 @@
+---
+"@tanstack/db": patch
+---
+
+Prevent pushing down of where clauses that only touch the namespace of a source, rather than a prop on that namespace. This ensures that the semantics of the query are maintained for things such as `isUndefined(namespace)` after a join.

packages/db/src/query/optimizer.ts

@@ -142,6 +142,8 @@ export interface AnalyzedWhereClause {
   expression: BasicExpression<boolean>
   /** Set of table/source aliases that this WHERE clause touches */
   touchedSources: Set<string>
+  /** Whether this clause contains namespace-only references that prevent pushdown */
+  hasNamespaceOnlyRef: boolean
 }
 
 /**
@@ -486,19 +488,31 @@ function splitAndClausesRecursive(
  * This determines whether a clause can be pushed down to a specific table
  * or must remain in the main query (for multi-source clauses like join conditions).
  *
+ * Special handling for namespace-only references in outer joins:
+ * WHERE clauses that reference only a table namespace (e.g., isUndefined(special), eq(special, value))
+ * rather than specific properties (e.g., isUndefined(special.id), eq(special.id, value)) are treated as
+ * multi-source to prevent incorrect predicate pushdown that would change join semantics.
+ *
  * @param clause - The WHERE expression to analyze
  * @returns Analysis result with the expression and touched source aliases
  *
  * @example
  * ```typescript
- * // eq(users.department_id, 1) -> touches ['users']
- * // eq(users.id, posts.user_id) -> touches ['users', 'posts']
+ * // eq(users.department_id, 1) -> touches ['users'], hasNamespaceOnlyRef: false
+ * // eq(users.id, posts.user_id) -> touches ['users', 'posts'], hasNamespaceOnlyRef: false
+ * // isUndefined(special) -> touches ['special'], hasNamespaceOnlyRef: true (prevents pushdown)
+ * // eq(special, someValue) -> touches ['special'], hasNamespaceOnlyRef: true (prevents pushdown)
+ * // isUndefined(special.id) -> touches ['special'], hasNamespaceOnlyRef: false (allows pushdown)
+ * // eq(special.id, 5) -> touches ['special'], hasNamespaceOnlyRef: false (allows pushdown)
  * ```
  */
 function analyzeWhereClause(
   clause: BasicExpression<boolean>
 ): AnalyzedWhereClause {
+  // Track which table aliases this WHERE clause touches
   const touchedSources = new Set<string>()
+  // Track whether this clause contains namespace-only references that prevent pushdown
+  let hasNamespaceOnlyRef = false
 
   /**
    * Recursively collect all table aliases referenced in an expression
@@ -511,6 +525,13 @@ function analyzeWhereClause(
           const firstElement = expr.path[0]
           if (firstElement) {
             touchedSources.add(firstElement)
+
+            // If the path has only one element (just the namespace),
+            // this is a namespace-only reference that should not be pushed down
+            // This applies to ANY function, not just existence-checking functions
+            if (expr.path.length === 1) {
+              hasNamespaceOnlyRef = true
+            }
           }
         }
         break
@@ -537,6 +558,7 @@ function analyzeWhereClause(
   return {
     expression: clause,
     touchedSources,
+    hasNamespaceOnlyRef,
   }
 }
 
@@ -557,15 +579,15 @@ function groupWhereClauses(
 
   // Categorize each clause based on how many sources it touches
   for (const clause of analyzedClauses) {
-    if (clause.touchedSources.size === 1) {
-      // Single source clause - can be optimized
+    if (clause.touchedSources.size === 1 && !clause.hasNamespaceOnlyRef) {
+      // Single source clause without namespace-only references - can be optimized
       const source = Array.from(clause.touchedSources)[0]!
       if (!singleSource.has(source)) {
         singleSource.set(source, [])
       }
       singleSource.get(source)!.push(clause.expression)
-    } else if (clause.touchedSources.size > 1) {
-      // Multi-source clause - must stay in main query
+    } else if (clause.touchedSources.size > 1 || clause.hasNamespaceOnlyRef) {
+      // Multi-source clause or namespace-only reference - must stay in main query
       multiSource.push(clause.expression)
     }
     // Skip clauses that touch no sources (constants) - they don't need optimization

packages/db/tests/query/join.test.ts

@@ -1,5 +1,10 @@
 import { beforeEach, describe, expect, test } from "vitest"
-import { concat, createLiveQueryCollection, eq } from "../../src/query/index.js"
+import {
+  concat,
+  createLiveQueryCollection,
+  eq,
+  isUndefined,
+} from "../../src/query/index.js"
 import { createCollection } from "../../src/collection/index.js"
 import { mockSyncCollectionOptions } from "../utils.js"
 
@@ -1429,6 +1434,47 @@ function createJoinTests(autoIndex: `off` | `eager`): void {
         `Invalid join condition: both expressions refer to the same table "user"`
       )
     })
+
+    test(`should not push down isUndefined(namespace) to subquery`, () => {
+      const usersCollection = createUsersCollection()
+      const specialUsersCollection = createCollection(
+        mockSyncCollectionOptions({
+          id: `special-users`,
+          getKey: (user) => user.id,
+          initialData: [{ id: 1, special: true }],
+        })
+      )
+
+      const joinQuery = createLiveQueryCollection({
+        startSync: true,
+        query: (q) =>
+          q
+            .from({ user: usersCollection })
+            .leftJoin(
+              { special: specialUsersCollection },
+              ({ user, special }) => eq(user.id, special.id)
+            )
+            .where(({ special }) => isUndefined(special)),
+      })
+
+      // Should return users that don't have a matching special user
+      // This should be users with IDs 2, 3, 4 (since only user 1 has a special record)
+      const results = joinQuery.toArray
+      expect(results).toHaveLength(3)
+
+      // All results should have special as undefined
+      for (const row of results) {
+        expect(row.special).toBeUndefined()
+      }
+
+      // Should not include user 1 (Alice) since she has a special record
+      const aliceResult = results.find((r) => r.user.id === 1)
+      expect(aliceResult).toBeUndefined()
+
+      // Should include users 2, 3, 4 (Bob, Charlie, Dave)
+      const userIds = results.map((r) => r.user.id).sort()
+      expect(userIds).toEqual([2, 3, 4])
+    })
   })
 }