From 135cac6d7753b715d15929582bf8e3e562890a21 Mon Sep 17 00:00:00 2001
From: Alexey Arno <af-arno@yandex-team.ru>
Date: Sat, 11 Jul 2015 18:16:59 +0300
Subject: [PATCH] dbms: Server: Performance improvement. [#METR-16778]

---
 dbms/include/DB/Parsers/ASTSelectQuery.h      |  5 ++++-
 .../Interpreters/InterpreterSelectQuery.cpp   |  5 +----
 dbms/src/Parsers/ASTSelectQuery.cpp           | 22 ++++++++++++++-----
 3 files changed, 22 insertions(+), 10 deletions(-)

diff --git a/dbms/include/DB/Parsers/ASTSelectQuery.h b/dbms/include/DB/Parsers/ASTSelectQuery.h
index 244a53b45e2..f6edf7ebfb9 100644
--- a/dbms/include/DB/Parsers/ASTSelectQuery.h
+++ b/dbms/include/DB/Parsers/ASTSelectQuery.h
@@ -36,11 +36,14 @@ public:
 
 	ASTPtr clone() const override;
 
+	/// Получить глубокую копию дерева первого запроса SELECT.
+	ASTPtr cloneFirstSelect() const;
+
 	/// Возвращает указатель на формат из последнего SELECT'а цепочки UNION ALL.
 	const IAST * getFormat() const override;
 
 private:
-	ASTPtr cloneImpl() const;
+	ASTPtr cloneImpl(bool traverse_union_all) const;
 
 public:
 	bool distinct = false;
diff --git a/dbms/src/Interpreters/InterpreterSelectQuery.cpp b/dbms/src/Interpreters/InterpreterSelectQuery.cpp
index f781bc70510..cf5180f9e08 100644
--- a/dbms/src/Interpreters/InterpreterSelectQuery.cpp
+++ b/dbms/src/Interpreters/InterpreterSelectQuery.cpp
@@ -720,10 +720,7 @@ QueryProcessingStage::Enum InterpreterSelectQuery::executeFetchColumns(BlockInpu
 		if (storage->isRemote())
 		{
 			/// В случае удаленного запроса отправляем только SELECT, который выполнится.
-			actual_query_ptr = query_ptr->clone();
-			auto actual_select_query = static_cast<ASTSelectQuery *>(&*actual_query_ptr);
-			actual_select_query->prev_union_all = nullptr;
-			actual_select_query->next_union_all = nullptr;
+			actual_query_ptr = query.cloneFirstSelect();
 		}
 		else
 			actual_query_ptr = query_ptr;
diff --git a/dbms/src/Parsers/ASTSelectQuery.cpp b/dbms/src/Parsers/ASTSelectQuery.cpp
index a63202eeb72..36fa67a2cec 100644
--- a/dbms/src/Parsers/ASTSelectQuery.cpp
+++ b/dbms/src/Parsers/ASTSelectQuery.cpp
@@ -134,7 +134,7 @@ void ASTSelectQuery::rewriteSelectExpressionList(const Names & column_names)
 
 ASTPtr ASTSelectQuery::clone() const
 {
-	ASTPtr ptr = cloneImpl();
+	ASTPtr ptr = cloneImpl(true);
 
 	/// Установить указатели на предыдущие запросы SELECT.
 	ASTPtr current = ptr;
@@ -150,7 +150,14 @@ ASTPtr ASTSelectQuery::clone() const
 	return ptr;
 }
 
-ASTPtr ASTSelectQuery::cloneImpl() const
+ASTPtr ASTSelectQuery::cloneFirstSelect() const
+{
+	ASTPtr res = cloneImpl(false);
+	static_cast<ASTSelectQuery *>(&*res)->prev_union_all = nullptr;
+	return res;
+}
+
+ASTPtr ASTSelectQuery::cloneImpl(bool traverse_union_all) const
 {
 	ASTSelectQuery * res = new ASTSelectQuery(*this);
 	ASTPtr ptr{res};
@@ -187,11 +194,16 @@ ASTPtr ASTSelectQuery::cloneImpl() const
 
 #undef CLONE
 
-	if (next_union_all)
+	if (traverse_union_all)
 	{
-		res->next_union_all = static_cast<const ASTSelectQuery *>(&*next_union_all)->cloneImpl();
-		res->children.push_back(res->next_union_all);
+		if (next_union_all)
+		{
+			res->next_union_all = static_cast<const ASTSelectQuery *>(&*next_union_all)->cloneImpl(true);
+			res->children.push_back(res->next_union_all);
+		}
 	}
+	else
+		res->next_union_all = nullptr;
 
 	return ptr;
 }