```sql
SELECT count()
FROM 
(
    SELECT number
    FROM numbers_mt(100000000)
    ORDER BY number DESC
    LIMIT 100
    UNION ALL
    SELECT number
    FROM numbers_mt(100000000)
    ORDER BY number DESC
    LIMIT 100
    UNION ALL
    SELECT number
    FROM numbers_mt(100000000)
    ORDER BY number DESC
    LIMIT 100
)


SET experimental_use_processors = 0;
-- 1 rows in set. Elapsed: 1.581 sec. Processed 300.00 million rows, 2.40 GB (189.78 million rows/s., 1.52 GB/s.) 

SET experimental_use_processors = 1;
-- 1 rows in set. Elapsed: 3.569 sec. Processed 300.00 million rows, 2.40 GB (84.06 million rows/s., 672.50 MB/s.) 
```

Similar for MergeTree:

```sql
create table test engine=MergeTree order by tuple() as select * from numbers(30000000);

select count() from (select number from test group by number order by number limit 10 union all select number from test group by number order by number limit 10);

set experimental_use_processors=0;
-- Elapsed: 2.664 sec

set experimental_use_processors=1;
-- Elapsed: 3.656 sec
```
{
    if (collected_processors && collected_processors_)
        throw Exception("Cannot set collected processors to QueryPipeline because "
                        "another one object was already created for current pipeline." , ErrorCodes::LOGICAL_ERROR);
    std::swap(collected_processors, collected_processors_);
    return collected_processors_;
}
QueryPipelineProcessorsCollector::QueryPipelineProcessorsCollector(QueryPipeline & pipeline_, IQueryPlanStep * step_)
    : pipeline(pipeline_), step(step_)
{
    pipeline.processors.setCollectedProcessors(&processors);
}
QueryPipelineProcessorsCollector::~QueryPipelineProcessorsCollector()
{
    pipeline.processors.setCollectedProcessors(nullptr);
}
Processors QueryPipelineProcessorsCollector::detachProcessors(size_t group)
{
    for (auto & processor : processors)
        processor->setQueryPlanStep(step, group);
    Processors res;
    res.swap(processors);
    return res;
}
}