consider the case of alter table add projection

src/Storages/MergeTree/MergeTreeData.cpp

@@ -3211,6 +3211,16 @@ void MergeTreeData::checkAlterIsPossible(const AlterCommands & commands, Context
                             queryToString(mutation_commands.ast()));
     }
 
+    /// Block the case of alter table add projection for special merge trees.
+    if (std::any_of(commands.begin(), commands.end(), [](const AlterCommand & c) { return c.type == AlterCommand::ADD_PROJECTION; }))
+    {
+        if (auto storage_name = getName(); storage_name != "MergeTree" && storage_name != "ReplicatedMergeTree"
+            && settings_from_storage->deduplicate_merge_projection_mode == DeduplicateMergeProjectionMode::THROW)
+            throw Exception(ErrorCodes::NOT_IMPLEMENTED,
+                "Projection is fully supported in (Replictaed)MergeTree, but also allowed in non-throw mode with other"
+                " mergetree family members. Consider drop or rebuild option of deduplicate_merge_projection_mode.");
+    }
+
     commands.apply(new_metadata, local_context);
 
     if (AlterCommands::hasFullTextIndex(new_metadata) && !settings.allow_experimental_full_text_index)

tests/queries/0_stateless/03206_projection_merge_special_mergetree.reference

@@ -0,0 +1 @@
+p

tests/queries/0_stateless/03206_projection_merge_special_mergetree.sql

@@ -59,8 +59,7 @@ OPTIMIZE TABLE tp FINAL;
 -- expecting no projection
 SYSTEM FLUSH LOGS;
 SELECT
-    name,
-    part_name
+    name
 FROM system.projection_parts
 WHERE (database = currentDatabase()) AND (`table` = 'tp') AND (active = 1);
 
@@ -81,4 +80,27 @@ ALTER TABLE tp MODIFY SETTING deduplicate_merge_projection_mode = 'throw';
 
 OPTIMIZE TABLE tp DEDUPLICATE;  -- { serverError NOT_IMPLEMENTED }
 
+DROP TABLE tp;
+
+-- test alter add projection case
+CREATE TABLE tp (
+    type Int32,
+    eventcnt UInt64
+) engine = ReplacingMergeTree order by type;
+
+ALTER TABLE tp ADD PROJECTION p (SELECT sum(eventcnt), type GROUP BY type);  -- { serverError NOT_IMPLEMENTED }
+
+ALTER TABLE tp MODIFY SETTING deduplicate_merge_projection_mode = 'drop';
+
+ALTER TABLE tp ADD PROJECTION p (SELECT sum(eventcnt), type GROUP BY type);
+
+INSERT INTO tp SELECT number%3, 1 FROM numbers(3);
+
+SYSTEM FLUSH LOGS;
+-- expecting projection p
+SELECT
+    name
+FROM system.projection_parts
+WHERE (database = currentDatabase()) AND (`table` = 'tp') AND (active = 1);
+
 DROP TABLE tp;