```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
```
bool isCompleted() { return initialized() && streams.empty(); }
    
    
    
    enum class StreamType
    {
        Main = 0, 
        Totals,  
        Extremes, 
    };
    using ProcessorGetter = std::function<ProcessorPtr(const Block & header)>;
    using ProcessorGetterWithStreamKind = std::function<ProcessorPtr(const Block & header, StreamType stream_type)>;
    
    void addSimpleTransform(const ProcessorGetter & getter);
    void addSimpleTransform(const ProcessorGetterWithStreamKind & getter);
    
    
    void addPipe(Processors pipe);
    
    void addTotalsHavingTransform(ProcessorPtr transform);
    
    void addExtremesTransform();
    
    void addCreatingSetsTransform(ProcessorPtr transform);
    
    void setOutputFormat(ProcessorPtr output);
    
    IOutputFormat * getOutputFormat() const { return output_format; }