Q55359348, PostgreSQL

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with raw_data( user_name, question_id, answer, answer_date ) as
        (
         select  'user001',1,2, '2019-02-04' union all
         select  'user001',2,1, '2019-02-04' union all
         select  'user001',3,2, '2019-02-05' union all
         select  'user001',4,2, '2019-02-05' union all
         select  'user001',5,2, '2019-02-09' union all
         select  'user002',1,2, '2019-01-09' union all
         select  'user002',2,2, '2019-01-10' union all
         select  'user002',3,1, '2019-02-01' union all
         select  'user002',4,2, '2019-02-01' union all
         select  'user002',5,1, '2019-02-01' union all
         select  'user002',1,2, '2019-03-11' union all
         select  'user002',2,2, '2019-03-11' union all
         select  'user002',3,1, '2019-03-12' union all
         select  'user002',4,1, '2019-03-13' union all
         select  'user002',5,1, '2019-03-14'
        )    
       select user_name, sum(answer) as sum, min(answer_date) as start_date
         from 
         (
           select row_number() over (partition by question_id order by user_name, answer_date) as rn,
                  t.*
             from raw_data t
         ) t
        group by user_name, rn
        order by rn


absolute service time: 0,48 sec