dbms: chaining support for defaulted columns when corresponding columns files are absent. [#METR-12739]

dbms/include/DB/Core/Block.h

@@ -68,8 +68,10 @@ public:
 	void addDefaults(const NamesAndTypesList & required_columns);
 	void addDefaults(const NamesAndTypesList & required_columns,
 		const ColumnDefaults & column_defaults,
-		const Context & context,
-		bool only_explicitly_defaulted = false);
+		const Context & context);
+	void addAllDefaults(const NamesAndTypesList & required_columns,
+		const ColumnDefaults & column_defaults,
+		const Context & context);
 
 	ColumnWithNameAndType & getByPosition(size_t position);
 	const ColumnWithNameAndType & getByPosition(size_t position) const;

dbms/include/DB/Storages/MergeTree/MergeTreeReader.h

@@ -161,6 +161,7 @@ public:
 			size_t pos = 0;	/// Позиция, куда надо вставить недостающий столбец.
 			for (NamesAndTypesList::const_iterator it = columns.begin(); it != columns.end(); ++it, ++pos)
 			{
+				/// insert default values only for columns without default expressions
 				if (!res.has(it->name) && storage.column_defaults.count(it->name) == 0)
 				{
 					ColumnWithNameAndType column;
@@ -193,8 +194,8 @@ public:
 				}
 			}
 
-			/// evaluate defaulted values
-			res.addDefaults(columns, storage.column_defaults, storage.context, true);
+			/// evaluate defaulted columns
+			res.addAllDefaults(columns, storage.column_defaults, storage.context);
 		}
 		catch (const Exception & e)
 		{

dbms/src/Core/Block.cpp

@@ -34,8 +34,7 @@ void Block::addDefaults(const NamesAndTypesList & required_columns)
 
 void Block::addDefaults(const NamesAndTypesList & required_columns,
 	const ColumnDefaults & column_defaults,
-	const Context & context,
-	const bool only_explicitly_defaulted)
+	const Context & context)
 {
 	ASTPtr default_expr_list{stdext::make_unique<ASTExpressionList>().release()};
 
@@ -47,10 +46,10 @@ void Block::addDefaults(const NamesAndTypesList & required_columns,
 		const auto it = column_defaults.find(column.name);
 
 		/// expressions must be cloned to prevent modification by the ExpressionAnalyzer
-		if (it != column_defaults.end())
-			default_expr_list->children.emplace_back(it->second.expression->clone());
-		else if (only_explicitly_defaulted)
+		if (it == column_defaults.end())
 			insertDefault(column.name, column.type);
+		else
+			default_expr_list->children.emplace_back(it->second.expression->clone());
 	}
 
 	/// nothing to evaluate
@@ -68,6 +67,38 @@ void Block::addDefaults(const NamesAndTypesList & required_columns,
 		insert(std::move(column_name_type));
 }
 
+void Block::addAllDefaults(const NamesAndTypesList & required_columns,
+	const ColumnDefaults & column_defaults,
+	const Context & context)
+{
+	ASTPtr default_expr_list{stdext::make_unique<ASTExpressionList>().release()};
+
+	for (const auto & column_default : column_defaults)
+	{
+		if (has(column_default.first))
+			continue;
+
+		/// expressions must be cloned to prevent modification by the ExpressionAnalyzer
+		default_expr_list->children.emplace_back(column_default.second.expression->clone());
+	}
+
+	/// nothing to evaluate
+	if (default_expr_list->children.empty())
+		return;
+
+	/** ExpressionAnalyzer eliminates "unused" columns, in order to ensure their safety
+	 *	we are going to operate on a copy instead of  the original block */
+	Block copy_block{*this};
+	/// evaluate default values for defaulted columns
+	ExpressionAnalyzer{default_expr_list, context, required_columns}.getActions(true)->execute(copy_block);
+
+	/// move evaluated columns to the original block only if required
+	for (auto & column_name_type : required_columns) {
+		if (copy_block.has(column_name_type.name))
+			insert(std::move(copy_block.getByName(column_name_type.name)));
+	}
+}
+
 Block & Block::operator= (const Block & other)
 {
 	data = other.data;

dbms/src/Storages/AlterCommands.cpp

@@ -123,7 +123,7 @@ namespace DB
 				column_defaults.erase(column_name);
 			else if (default_expression && !had_default_expr)
 				column_defaults.emplace(column_name, ColumnDefault{default_type, default_expression});
-			else
+			else if (had_default_expr)
 				column_defaults[column_name].expression = default_expression;
 		}
 		else