ClickHouse/dbms/src/DataStreams/LimitBlockInputStream.cpp

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#include <algorithm>
#include <DB/DataStreams/LimitBlockInputStream.h>
namespace DB
{
LimitBlockInputStream::LimitBlockInputStream(BlockInputStreamPtr input_, size_t limit_, size_t offset_, bool always_read_till_end_)
: limit(limit_), offset(offset_), always_read_till_end(always_read_till_end_)
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{
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children.push_back(input_);
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}
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Block LimitBlockInputStream::readImpl()
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{
Block res;
size_t rows = 0;
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/// pos - сколько строк было прочитано, включая последний прочитанный блок
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if (pos >= offset + limit)
{
if (!always_read_till_end)
return res;
else
{
while (children.back()->read())
;
return res;
}
}
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do
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{
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res = children.back()->read();
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if (!res)
return res;
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rows = res.rows();
pos += rows;
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} while (pos <= offset);
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/// отдать целый блок
if (pos >= offset + rows && pos <= offset + limit)
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return res;
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/// отдать кусок блока
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size_t start = std::max(
static_cast<Int64>(0),
static_cast<Int64>(offset) - static_cast<Int64>(pos) + static_cast<Int64>(rows));
size_t length = std::min(
static_cast<Int64>(limit), std::min(
static_cast<Int64>(pos) - static_cast<Int64>(offset),
static_cast<Int64>(limit) + static_cast<Int64>(offset) - static_cast<Int64>(pos) + static_cast<Int64>(rows)));
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for (size_t i = 0; i < res.columns(); ++i)
res.safeGetByPosition(i).column = res.safeGetByPosition(i).column->cut(start, length);
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return res;
}
}