fix:[2.4]GrowingDataGetter get the wrong string data

Signed-off-by: cqy123456 <qianya.cheng@zilliz.com>

configs/milvus.yaml

@@ -412,15 +412,15 @@ queryNode:
     # 2. If set to "disable" original vector data will only be loaded into the chunk cache during search/query.
     warmup: disable
   mmap:
-    vectorField: false # Enable mmap for loading vector data
-    vectorIndex: false # Enable mmap for loading vector index
-    scalarField: false # Enable mmap for loading scalar data
-    scalarIndex: false # Enable mmap for loading scalar index
+    vectorField: true # Enable mmap for loading vector data
+    vectorIndex: true # Enable mmap for loading vector index
+    scalarField: true # Enable mmap for loading scalar data
+    scalarIndex: true # Enable mmap for loading scalar index
     chunkCache: true # Enable mmap for chunk cache (raw vector retrieving).
     # Enable memory mapping (mmap) to optimize the handling of growing raw data. 
     # By activating this feature, the memory overhead associated with newly added or modified data will be significantly minimized. 
     # However, this optimization may come at the cost of a slight decrease in query latency for the affected data segments.
-    growingMmapEnabled: false
+    growingMmapEnabled: true
     fixedFileSizeForMmapAlloc: 1 # tmp file size for mmap chunk manager
     maxDiskUsagePercentageForMmapAlloc: 50 # disk percentage used in mmap chunk manager
   lazyload:

internal/core/src/query/GroupByOperator.h

@@ -50,7 +50,11 @@ class GrowingDataGetter : public DataGetter<T> {
 
     T
     Get(int64_t idx) const {
-        return growing_raw_data_->operator[](idx);
+        if constexpr (std::is_same_v<std::string, T>) {
+            return T(growing_raw_data_->view_element(idx));
+        } else {
+            return growing_raw_data_->operator[](idx);
+        }
     }
 };