diff --git a/src/Interpreters/InterpreterSelectQueryAnalyzer.cpp b/src/Interpreters/InterpreterSelectQueryAnalyzer.cpp
index 4f2f05dc7eb..8db1d27c073 100644
--- a/src/Interpreters/InterpreterSelectQueryAnalyzer.cpp
+++ b/src/Interpreters/InterpreterSelectQueryAnalyzer.cpp
@@ -135,6 +135,8 @@ QueryTreeNodePtr buildQueryTreeAndRunPasses(const ASTPtr & query,
     QueryTreePassManager query_tree_pass_manager(context);
     addQueryTreePasses(query_tree_pass_manager);
 
+    /// We should not apply any query tree level optimizations on shards
+    /// because it can lead to a changed header.
     if (select_query_options.ignore_ast_optimizations
         || context->getClientInfo().query_kind == ClientInfo::QueryKind::SECONDARY_QUERY)
         query_tree_pass_manager.run(query_tree, 1 /*up_to_pass_index*/);