add docs & minor fix for lwd-aware merge settings

docs/en/operations/settings/merge-tree-settings.md

@@ -867,3 +867,31 @@ Default value: `Never`
 Persists virtual column `_block_number` on merges.
 
 Default value: false.
+
+## exclude_deleted_rows_for_part_size_in_merge {#exclude_deleted_rows_for_part_size_in_merge}
+
+If enabled, estimated actual size of data parts (i.e., excluding those rows that have been deleted through `DELETE FROM`) will be used when selecting parts to merge. Note that this behavior is only triggered for data parts affected by `DELETE FROM` executed after this setting is enabled.
+
+Possible values:
+
+- true, false
+
+Default value: false
+
+**See Also**
+
+- [load_existing_rows_count_for_old_parts](#load_existing_rows_count_for_old_parts) setting
+
+## load_existing_rows_count_for_old_parts {#load_existing_rows_count_for_old_parts}
+
+If enabled along with [exclude_deleted_rows_for_part_size_in_merge](#exclude_deleted_rows_for_part_size_in_merge), deleted rows count for existing data parts will be calculated during table starting up.
+
+Possible values:
+
+- true, false
+
+Default value: false
+
+**See Also**
+
+- [exclude_deleted_rows_for_part_size_in_merge](#exclude_deleted_rows_for_part_size_in_merge) setting

src/Storages/MergeTree/IMergeTreeDataPart.cpp

@@ -1349,8 +1349,9 @@ void IMergeTreeDataPart::loadExistingRowsCount()
     if (existing_rows_count.has_value())
         return;
 
-    if (!rows_count || !storage.getSettings()->load_existing_rows_count_for_old_parts || !supportLightweightDeleteMutate()
-        || !hasLightweightDelete())
+    if (!rows_count || !supportLightweightDeleteMutate() || !hasLightweightDelete()
+        || !storage.getSettings()->exclude_deleted_rows_for_part_size_in_merge
+        || !storage.getSettings()->load_existing_rows_count_for_old_parts)
         existing_rows_count = rows_count;
     else
         existing_rows_count = readExistingRowsCount();

src/Storages/MergeTree/IMergeTreeDataPart.h

@@ -674,7 +674,8 @@ private:
     /// For the older format version calculates rows count from the size of a column with a fixed size.
     void loadRowsCount();
 
-    /// Load existing rows count from _row_exists column if load_existing_rows_count_for_old_parts is true.
+    /// Load existing rows count from _row_exists column
+    /// if load_existing_rows_count_for_old_parts and exclude_deleted_rows_for_part_size_in_merge are both enabled.
     void loadExistingRowsCount();
 
     static void appendFilesOfRowsCount(Strings & files);