ClickHouse/dbms/include/DB/DataStreams/MaterializingBlockOutputStream.h

#pragma once

#include <DB/Columns/ColumnConst.h>
#include <DB/DataStreams/IBlockOutputStream.h>
#include <DB/DataTypes/DataTypeNullable.h>
#include <DB/DataTypes/DataTypesNumberFixed.h>
#include <ext/range.hpp>

namespace DB
{

/** Преобразует столбцы-константы в полноценные столбцы ("материализует" их).
  */
class MaterializingBlockOutputStream : public IBlockOutputStream
{
public:
	MaterializingBlockOutputStream(const BlockOutputStreamPtr & output)
		: output{output} {}

	void write(const Block & block) override
	{
			output->write(materialize(block));
	}

	void flush() 										override { output->flush(); }
	void writePrefix() 									override { output->writePrefix(); }
	void writeSuffix() 									override { output->writeSuffix(); }
	void setRowsBeforeLimit(size_t rows_before_limit) 	override { output->setRowsBeforeLimit(rows_before_limit); }
	void setTotals(const Block & totals) 				override { output->setTotals(materialize(totals)); }
	void setExtremes(const Block & extremes) 			override { output->setExtremes(materialize(extremes)); }
	void onProgress(const Progress & progress) 			override { output->onProgress(progress); }
	String getContentType() const 						override { return output->getContentType(); }

private:
	BlockOutputStreamPtr output;

	static Block materialize(const Block & original_block)
	{
		/// copy block to get rid of const
		auto block = original_block;

		for (const auto i : ext::range(0, block.columns()))
		{
			auto & element = block.safeGetByPosition(i);
			auto & src = element.column;
			ColumnPtr converted = src->convertToFullColumnIfConst();
			if (converted)
			{
				src = converted;
				auto & type = element.type;
				if (type->isNull())
				{
					/// A ColumnNull that is converted to a full column
					/// has the type DataTypeNullable(DataTypeUInt8).
					type = std::make_shared<DataTypeNullable>(std::make_shared<DataTypeUInt8>());
				}
			}
		}

		return block;
	}
};

}