Revert "Revert "Add logging for query mode (#776)" (#797)"

This reverts commit 98d83b4.

Author: schmidt-sebastian

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

@@ -24,6 +24,7 @@
 import com.google.firebase.firestore.model.DocumentKey;
 import com.google.firebase.firestore.model.MaybeDocument;
 import com.google.firebase.firestore.model.SnapshotVersion;
+import com.google.firebase.firestore.util.Logger;
 import java.util.Collections;
 import java.util.Map;
 
@@ -51,6 +52,8 @@
  * </ol>
  */
 public class IndexFreeQueryEngine implements QueryEngine {
+  private static final String LOG_TAG = "IndexFreeQueryEngine";
+
   private LocalDocumentsView localDocumentsView;
 
   @Override
@@ -83,6 +86,14 @@ && needsRefill(previousResults, remoteKeys, queryData.getLastLimboFreeSnapshotVe
       return executeFullCollectionScan(query);
     }
 
+    if (Logger.isDebugEnabled()) {
+      Logger.debug(
+          LOG_TAG,
+          "Re-using previous result from %s to execute query: %s",
+          queryData.getLastLimboFreeSnapshotVersion().toString(),
+          query.toString());
+    }
+
     // Retrieve all results for documents that were updated since the last limbo-document free
     // remote snapshot.
     ImmutableSortedMap<DocumentKey, Document> updatedResults =
@@ -161,6 +172,9 @@ public void handleDocumentChange(MaybeDocument oldDocument, MaybeDocument newDoc
   }
 
   private ImmutableSortedMap<DocumentKey, Document> executeFullCollectionScan(Query query) {
+    if (Logger.isDebugEnabled()) {
+      Logger.debug(LOG_TAG, "Using full collection scan to execute query: %s", query.toString());
+    }
     return localDocumentsView.getDocumentsMatchingQuery(query, SnapshotVersion.NONE);
   }
 }