ClickHouse/src/DataStreams/ConvertingBlockInputStream.h

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#pragma once
#include <unordered_map>
#include <DataStreams/IBlockInputStream.h>
namespace DB
{
/** Convert one block structure to another:
*
* Leaves only necessary columns;
*
* Columns are searched in source first by name;
* and if there is no column with same name, then by position.
*
* Converting types of matching columns (with CAST function).
*
* Materializing columns which are const in source and non-const in result,
* throw if they are const in result and non const in source,
* or if they are const and have different values.
*/
class ConvertingBlockInputStream : public IBlockInputStream
{
public:
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enum class MatchColumnsMode
{
/// Require same number of columns in source and result. Match columns by corresponding positions, regardless to names.
Position,
/// Find columns in source by their names. Allow excessive columns in source.
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Name
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};
ConvertingBlockInputStream(
const BlockInputStreamPtr & input,
const Block & result_header,
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MatchColumnsMode mode);
String getName() const override { return "Converting"; }
Block getHeader() const override { return header; }
private:
Block readImpl() override;
Block header;
/// How to construct result block. Position in source block, where to get each column.
using Conversion = std::vector<size_t>;
Conversion conversion;
};
}