ruby on rails - Postgree double group by repeating attribute -


i have table columns: id, user_id, message_id, message_type; example:

  id: 1, user_id: 1, message_id: 4, message_type: 'warning'   id: 2, user_id: 1, message_id: 5, message_type: 'warning'   id: 3, user_id: 1, message_id: 6, message_type: 'warning'   id: 4, user_id: 2, message_id: 4, message_type: 'error'   id: 5, user_id: 2, message_id: 1, message_type: 'exception'   id: 6, user_id: 1, message_id: 2, message_type: 'exception'   id: 7, user_id: 1, message_id: 3, message_type: 'exception'   id: 8, user_id: 2, message_id: 4, message_type: 'exception'

i want grouping result news in social networks. on columns user_id , message_type, while message_type repeating. , need limit 20 order id desc. example:

  id: 8, user_id: 2, message_id: 4, message_type: 'exception'   id: {6,7} user_id: 1, message_id: {2,3}, message_type: 'exception'   id: 5, user_id: 2, message_id: 1, message_type: 'exception'   id: 4, user_id: 2, message_id: 4, message_type: 'error'   id: {1, 2, 3}, user_id: 1, message_id: {4, 5, 6}, message_type: 'warning'

how best performance?

i found 1 way:

  1. with window function lead() find moment when changed dict (user, message type)
  2. with window function sum() set sequnce number each new dict
  3. group sequence , select need:

checking:

create table test (   id serial primary key,   user_id integer,   message_id integer,   message_type varchar );  insert test (user_id, message_id, message_type)  values  (1, 4, 'warning'), (1, 5, 'warning'), (1, 6, 'warning'), (2, 4, 'error'), (2, 1, 'exception'), (1, 2, 'exception'), (1, 3, 'exception'), (2, 4, 'exception') ;  select      array_agg(grouped.id) record_ids,     grouped.user_id,     array_agg(grouped.message_id) message_ids,     grouped.message_type (     select changed.*,          sum(changed.changed) on (order changed.id desc) group_n     (         select tt.*,             case when lag((user_id, message_type)) on (order tt.id desc) distinct (user_id, message_type) 1 else 0 end changed         test tt     ) changed     order id desc ) grouped group grouped.group_n, grouped.user_id, grouped.message_type order grouped.group_n ;

result:

record_ids | user_id | message_ids | message_type  ------------+---------+-------------+--------------  {8}        |       2 | {4}         | exception  {7,6}      |       1 | {3,2}       | exception  {5}        |       2 | {1}         | exception  {4}        |       2 | {4}         | error  {3,2,1}    |       1 | {6,5,4}     | warning (5 rows)

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