streams: adding const column, important for virtual columns [METR-9172]

dbms/include/DB/DataStreams/AddingConstColumnBlockInputStream.h

@@ -0,0 +1,57 @@
+#pragma once
+
+#include <Poco/SharedPtr.h>
+
+#include <DB/DataStreams/IProfilingBlockInputStream.h>
+#include <DB/Columns/ColumnConst.h>
+#include <DB/Core/ColumnWithNameAndType.h>
+
+namespace DB
+{
+
+/** Добавляет в блок const column с заданным значением.
+  */
+template <typename ColumnType>
+class AddingConstColumnBlockInputStream : public IProfilingBlockInputStream
+{
+public:
+	AddingConstColumnBlockInputStream(
+		BlockInputStreamPtr input_,
+		DataTypePtr data_type_,
+		ColumnType value_,
+		String column_name_)
+		: data_type(data_type_), value(value_), column_name(column_name_)
+	{
+		std :: cerr << column_name << " " << value << std :: endl;
+		children.push_back(input_);
+	}
+
+	String getName() const { return "AddingConstColumnBlockInputStream"; }
+
+	String getID() const
+	{
+		std::stringstream res;
+		res << "AddingConstColumn(" << children.back()->getID() << ")";
+		return res.str();
+	}
+
+protected:
+	Block readImpl()
+	{
+		Block res = children.back()->read();
+		if (!res)
+			return res;
+		ColumnPtr column_ptr = new ColumnConst<ColumnType> (res.rows(), value, data_type);
+		ColumnWithNameAndType column(column_ptr, data_type, column_name);
+
+		res.insert(column);
+		return res;
+	}
+
+private:
+	DataTypePtr data_type;
+	ColumnType value;
+	String column_name;
+};
+
+}