less load under mutex

src/Storages/MergeTree/MergeTreeData.cpp

@@ -8608,13 +8608,14 @@ void MergeTreeData::unloadPrimaryKeys()
 
 size_t MergeTreeData::unloadPrimaryKeysOfOutdatedParts()
 {
-    size_t total_unloaded = 0;
-
     /// If the method is already called from another thread, then we don't need to do anything.
     std::unique_lock lock(unload_primary_key_mutex, std::defer_lock);
     if (!lock.try_lock())
-        return total_unloaded;
+        return 0;
 
+    DataPartsVector parts_to_unload_index;
+
+    {
         auto parts_lock = lockParts();
         auto parts_range = getDataPartsStateRange(DataPartState::Outdated);
 
@@ -8622,13 +8623,17 @@ size_t MergeTreeData::unloadPrimaryKeysOfOutdatedParts()
         {
             /// Outdated part may be hold by SELECT query and still needs the index.
             if (part.unique())
+                parts_to_unload_index.push_back(part);
+        }
+    }
+
+    for (const auto & part : parts_to_unload_index)
     {
-            ++total_unloaded;
         const_cast<IMergeTreeDataPart &>(*part).unloadIndex();
         LOG_TEST(log, "Unloaded primary key for outdated part {}", part->name);
     }
-    }
-    return total_unloaded;
+
+    return parts_to_unload_index.size();
 }
 
 void MergeTreeData::verifySortingKey(const KeyDescription & sorting_key)