Group repeating attribute

Question

Basically, I have a table messageswith a field user_idthat identifies the user who created the message.

When I show a conversation (message set) between two users, I want to be able to group messages user_id, but in a complicated way:

Let's say there are messages (sorted by created_at desc):

  id: 1, user_id: 1
  id: 2, user_id: 1
  id: 3, user_id: 2
  id: 4, user_id: 2
  id: 5, user_id: 1

I want to get 3 groups of messages in the following order: [1,2], [3,4], [5]

It should group * user_id * until it sees another, and then group it.

I am using PostgreSQL and will be happy to use something specific, no matter what gives the best performance.

Answer 1

SQL

@Igor pure-SQL .
:

3 : [1,2], [3,4], [5]

, ORDER BY min(id):

SELECT array_agg(id) AS ids
FROM (
   SELECT id
         ,user_id
         ,row_number() OVER (ORDER BY id) -
          row_number() OVER (PARTITION BY user_id ORDER BY id) AS grp
   FROM   messages
   ORDER  BY id) t   -- for ordered arrays in result
GROUP  BY grp, user_id
ORDER  BY min(id);

SQL Fiddle.

. :

PL/pgSQL

PostgreSQL - , , .

SQL , . , , , , plpgsql , ORDER BY:

CREATE OR REPLACE FUNCTION f_msg_groups()
  RETURNS TABLE (ids int[]) AS
$func$
DECLARE
   _id    int;
   _uid   int;
   _id0   int;                         -- id of last row
   _uid0  int;                         -- user_id of last row
BEGIN
   FOR _id, _uid IN
       SELECT id, user_id FROM messages ORDER BY id
   LOOP
       IF _uid <> _uid0 THEN
          RETURN QUERY VALUES (ids);   -- output row (never happens after 1 row)
          ids := ARRAY[_id];           -- start new array
       ELSE
          ids := ids || _id;           -- add to array
       END IF;

       _id0  := _id;
       _uid0 := _uid;                  -- remember last row
   END LOOP;

   RETURN QUERY VALUES (ids);          -- output last iteration
END
$func$ LANGUAGE plpgsql;

:

SELECT * FROM f_msg_groups();

EXPLAIN ANALYZE 60k ( , , ):

SQL:
: 1009,549
Pl/PGSQL:
: 336,971

:

• GROUP BY
• BY ,
• /

Answer 2

- :

SELECT user_id, array_agg(id)
FROM (
SELECT id, 
       user_id, 
       row_number() OVER (ORDER BY created_at)-
       row_number() OVER (PARTITION BY user_id ORDER BY created_at) conv_id
FROM table1 ) t
GROUP BY user_id, conv_id;

:

row_number() OVER (ORDER BY created_at)-
row_number() OVER (PARTITION BY user_id ORDER BY created_at) conv_id

( conv_id user_id, user_id, conv_id )

SQLFiddle .

: row_number(), OVER (PARTITION BY ... ORDER BY ...)

Answer 3

The offer GROUP BYwill hide the answer in two entries: one with user_id1 and one with user_id2 regardless of the offer ORDER BY, so I recommend that you send onlyORDER BY created_at

prev_id = -1
messages.each do |m|
 if ! m.user_id == prev_id do 
    prev_id = m.user_id
    #do whatever you want with a new message group
 end
end

Answer 4

You can use chunk :

Message = Struct.new :id, :user_id

messages = []
messages << Message.new(1, 1)
messages << Message.new(2, 1)
messages << Message.new(3, 2)
messages << Message.new(4, 2)
messages << Message.new(5, 1)

messages.chunk(&:user_id).each do |user_id, records| 
  p "#{user_id} - #{records.inspect}" 
end

Output:

"1 - [#<struct Message id=1, user_id=1>, #<struct Message id=2, user_id=1>]"
"2 - [#<struct Message id=3, user_id=2>, #<struct Message id=4, user_id=2>]"
"1 - [#<struct Message id=5, user_id=1>]"