Apply conditional estimatedDocumentCount & countDocuments (PR parse-community#5264)

Author: NicksonYap

src/Adapters/Storage/Mongo/MongoCollection.js

@@ -55,7 +55,18 @@ export default class MongoCollection {
   }
 
   count(query, { skip, limit, sort, maxTimeMS, readPreference } = {}) {
-    const countOperation = this._mongoCollection.count(query, { skip, limit, sort, maxTimeMS, readPreference });
+    //ref: https://github.com/parse-community/parse-server/pull/5264/files#diff-271c623e43af83bc3d858c5ad248451a
+    // If query is empty, then use estimatedDocumentCount instead.
+    // This is due to countDocuments performing a scan,
+    // which greatly increases execution time when being run on large collections.
+    // See https://github.com/Automattic/mongoose/issues/6713 for more info regarding this problem.
+    if (typeof query !== 'object' || !Object.keys(query).length) {
+      return this._mongoCollection.estimatedDocumentCount({
+        maxTimeMS,
+      });
+    }
+
+    const countOperation = this._mongoCollection.countDocuments(query, { skip, limit, sort, maxTimeMS, readPreference });
 
     return countOperation;
   }