Add unionWith to SortedSet.

firebase-database-collection/src/main/java/com/google/firebase/database/collection/ImmutableSortedSet.java

@@ -87,6 +87,22 @@ public ImmutableSortedSet<T> insert(T entry) {
     return new ImmutableSortedSet<T>(map.insert(entry, null));
   }
 
+  public ImmutableSortedSet<T> unionWith(ImmutableSortedSet<T> other) {
+    ImmutableSortedSet<T> result = this;
+
+    // Make sure `result` always refers to the larger one of the two sets.
+    if (result.size() < other.size()) {
+      result = other;
+      other = this;
+    }
+
+    for(T elem : other) {
+      result = result.insert(elem);
+    }
+
+    return result;
+  }
+
   public T getMinEntry() {
     return this.map.getMinKey();
   }

firebase-firestore/CHANGELOG.md

@@ -1,4 +1,6 @@
 # Unreleased 
+- [fixed] Fixed a performance regression introduced by the addition of
+  `Query.limitToLast(n: long)` in Firestore 23.3.1.
 - [changed] Changed the in-memory representation of Firestore documents to
   reduce memory allocations and improve performance. Calls to 
   `DocumentSnapshot.getData()` and `DocumentSnapshot.toObject()` will see

firebase-firestore/src/main/java/com/google/firebase/firestore/core/SyncEngine.java

@@ -354,17 +354,17 @@ public ImmutableSortedSet<DocumentKey> getRemoteKeysForTarget(int targetId) {
     if (limboResolution != null && limboResolution.receivedDocument) {
       return DocumentKey.emptyKeySet().insert(limboResolution.key);
     } else {
-      List<DocumentKey> remoteKeys = Lists.newArrayList();
+      ImmutableSortedSet<DocumentKey> remoteKeys = DocumentKey.emptyKeySet();
       if (queriesByTarget.containsKey(targetId)) {
         for (Query query : queriesByTarget.get(targetId)) {
           if (queryViewsByQuery.containsKey(query)) {
-            remoteKeys.addAll(
-                Lists.newArrayList(queryViewsByQuery.get(query).getView().getSyncedDocuments()));
+            remoteKeys =
+                remoteKeys.unionWith(queryViewsByQuery.get(query).getView().getSyncedDocuments());
           }
         }
       }
 
-      return new ImmutableSortedSet(remoteKeys, DocumentKey.comparator());
+      return remoteKeys;
     }
   }