```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
```
#include <Processors/QueryPlan/UnionStep.h>
#include <Processors/QueryPipeline.h>
#include <Processors/Sources/NullSource.h>
#include <Interpreters/Context.h>
namespace DB
{
UnionStep::UnionStep(DataStreams input_streams_, Block result_header, size_t max_threads_)
    : header(std::move(result_header))
    , max_threads(max_threads_)
{
    input_streams = std::move(input_streams_);
    if (input_streams.size() == 1)
        output_stream = input_streams.front();
    else
        output_stream = DataStream{.header = header};
}
QueryPipelinePtr UnionStep::updatePipeline(QueryPipelines pipelines)