diff --git a/dbms/src/Storages/MergeTree/ReplicatedMergeTreeLogEntry.h b/dbms/src/Storages/MergeTree/ReplicatedMergeTreeLogEntry.h
index 4797922d837..1d7d39f1b9e 100644
--- a/dbms/src/Storages/MergeTree/ReplicatedMergeTreeLogEntry.h
+++ b/dbms/src/Storages/MergeTree/ReplicatedMergeTreeLogEntry.h
@@ -77,6 +77,8 @@ struct ReplicatedMergeTreeLogEntryData
     bool deduplicate = false; /// Do deduplicate on merge
     String column_name;
 
+    /// Force filter by TTL in 'OPTIMIZE ... FINAL' query to remove expired values from old parts
+    ///  without TTL infos or with outdated TTL infos, e.g. after 'ALTER ... MODIFY TTL' query.
     bool force_ttl = false;
 
     /// For DROP_RANGE, true means that the parts need not be deleted, but moved to the `detached` directory.
diff --git a/dbms/src/Storages/StorageMergeTree.cpp b/dbms/src/Storages/StorageMergeTree.cpp
index 589a5ad2af2..69020023df5 100644
--- a/dbms/src/Storages/StorageMergeTree.cpp
+++ b/dbms/src/Storages/StorageMergeTree.cpp
@@ -590,7 +590,10 @@ bool StorageMergeTree::merge(
 
     try
     {
+        /// Force filter by TTL in 'OPTIMIZE ... FINAL' query to remove expired values from old parts
+        ///  without TTL infos or with outdated TTL infos, e.g. after 'ALTER ... MODIFY TTL' query.
         bool force_ttl = (final && (hasTableTTL() || hasAnyColumnTTL()));
+
         new_part = merger_mutator.mergePartsToTemporaryPart(
             future_part, *merge_entry, time(nullptr),
             merging_tagger->reserved_space.get(), deduplicate, force_ttl);