Add documentation

Author: cherylEnkidu

firebase-firestore/src/androidTest/java/com/google/firebase/firestore/IndexingTest.java

@@ -117,6 +117,7 @@ public void testAutomaticIndexingSetSuccessfully() {
             .build();
     db.setFirestoreSettings(settings);
 
+    // Based on current heuristic, collection document counts (3) > 2 * resultSize (1).
     CollectionReference collection =
         testCollectionWithDocs(
             map(
@@ -135,13 +136,17 @@ public void testAutomaticIndexingSetSuccessfully() {
 
     results = waitFor(collection.whereEqualTo("match", true).get());
     assertEquals(1, results.size());
+
+    assertDoesNotThrow(() -> db.getPersistentCacheIndexManager().deleteAllIndexes());
+    assertEquals(1, results.size());
   }
 
   @Test
   public void testAutomaticIndexingSetSuccessfullyUseDefault() {
     // Use persistent disk cache (default)
     FirebaseFirestore db = testFirestore();
 
+    // Based on current heuristic, collection document counts (3) > 2 * resultSize (1).
     CollectionReference collection =
         testCollectionWithDocs(
             map(
@@ -160,5 +165,8 @@ public void testAutomaticIndexingSetSuccessfullyUseDefault() {
 
     results = waitFor(collection.whereEqualTo("match", true).get());
     assertEquals(1, results.size());
+
+    assertDoesNotThrow(() -> db.getPersistentCacheIndexManager().deleteAllIndexes());
+    assertEquals(1, results.size());
   }
 }

firebase-firestore/src/main/java/com/google/firebase/firestore/PersistentCacheIndexManager.java

@@ -53,6 +53,10 @@ public void disableIndexAutoCreation() {
     client.disableIndexAutoCreation();
   }
 
+  /**
+   * Removes all persistent cache indexes. Please note this function will also deletes indexes
+   * generated by {@link FirebaseFirestore#setIndexConfiguration(String)}.
+   */
   public void deleteAllIndexes() {
     client.deleteAllFieldIndexes();
   }

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

@@ -112,7 +112,8 @@ public ImmutableSortedMap<DocumentKey, Document> getDocumentsMatchingQuery(
   // TODO(csi): Auto experiment data.
   private void CreateCacheIndexes(Query query, QueryContext context, int resultSize) {
     String decisionStr = "";
-    // If evaluation is updated, please update tests in SQLiteLocalStoreTest.java
+    // If evaluation is updated, please update tests in SQLiteLocalStoreTest.java and
+    // IndexingTest.java.
     if (context.getDocumentReadCount() > 2 * resultSize) {
       indexManager.createTargetIndices(query.toTarget());
     } else {