Change comments and test instances in QueryEngine (#3818)

Author: cherylEnkidu

firebase-firestore/src/main/java/com/google/firebase/firestore/local/QueryEngine.java

@@ -54,17 +54,9 @@
  * least once and allow the query engine to only read documents that previously matched a query plus
  * any documents that were edited after the query was last listened to.
  *
- * <p>There are some cases where this specific optimization is not guaranteed to produce the same
- * results as full collection scans. In these cases, query processing falls back to full scans.
- * These cases are:
- *
- * <ol>
- *   <li>Limit queries where a document that matched the query previously no longer matches the
- *       query.
- *   <li>Limit queries where a document edit may cause the document to sort below another document
- *       that is in the local cache.
- *   <li>Queries that have never been CURRENT or free of limbo documents.
- * </ol>
+ * <p>For queries that have never been CURRENT or free of limbo documents, this specific
+ * optimization is not guaranteed to produce the same results as full collection scans. So in these
+ * cases, query processing falls back to full scans.
  */
 public class QueryEngine {
   private static final String LOG_TAG = "QueryEngine";

firebase-firestore/src/test/java/com/google/firebase/firestore/local/QueryEngineTestCase.java

@@ -67,6 +67,10 @@ public abstract class QueryEngineTestCase {
       doc("coll/a", 1, map("matches", true, "order", 1));
   private static final MutableDocument NON_MATCHING_DOC_A =
       doc("coll/a", 1, map("matches", false, "order", 1));
+  private static final MutableDocument PENDING_MATCHING_DOC_A =
+      doc("coll/a", 1, map("matches", true, "order", 1)).setHasLocalMutations();
+  private static final MutableDocument PENDING_NON_MATCHING_DOC_A =
+      doc("coll/a", 1, map("matches", false, "order", 1)).setHasLocalMutations();
   private static final MutableDocument UPDATED_DOC_A =
       doc("coll/a", 11, map("matches", true, "order", 1));
   private static final MutableDocument MATCHING_DOC_B =
@@ -230,7 +234,7 @@ public void filtersNonMatchingInitialResults() throws Exception {
     persistQueryMapping(MATCHING_DOC_A.getKey(), MATCHING_DOC_B.getKey());
 
     // Add a mutated document that is not yet part of query's set of remote keys.
-    addDocumentWithEventVersion(version(1), NON_MATCHING_DOC_A);
+    addDocumentWithEventVersion(version(1), PENDING_NON_MATCHING_DOC_A);
 
     DocumentSet docs =
         expectOptimizedCollectionScan(() -> runQuery(query, LAST_LIMBO_FREE_SNAPSHOT));
@@ -314,9 +318,9 @@ public void doesNotUseInitialResultsForLimitQueryWhenLastDocumentHasPendingWrite
 
     // Add a query mapping for a document that matches, but that sorts below another document due to
     // a pending write.
-    addDocumentWithEventVersion(version(1), MATCHING_DOC_A);
+    addDocumentWithEventVersion(version(1), PENDING_MATCHING_DOC_A);
     addMutation(DOC_A_EMPTY_PATCH);
-    persistQueryMapping(MATCHING_DOC_A.getKey());
+    persistQueryMapping(PENDING_MATCHING_DOC_A.getKey());
 
     addDocument(MATCHING_DOC_B);
 
@@ -335,9 +339,9 @@ public void doesNotUseInitialResultsForLimitToLastQueryWhenLastDocumentHasPendin
 
     // Add a query mapping for a document that matches, but that sorts below another document due to
     // a pending write.
-    addDocumentWithEventVersion(version(1), MATCHING_DOC_A);
+    addDocumentWithEventVersion(version(1), PENDING_MATCHING_DOC_A);
     addMutation(DOC_A_EMPTY_PATCH);
-    persistQueryMapping(MATCHING_DOC_A.getKey());
+    persistQueryMapping(PENDING_MATCHING_DOC_A.getKey());
 
     addDocument(MATCHING_DOC_B);