PostgreSQL-DISTINCTONおよびGROUPBY構文

object_idだけでなくすべての列を選択できるようにするため およびMAX(event_timestamp) 、DISTINCT ONを使用できます

SELECT DISTINCT ON (object_id) 
    object_id, event_timestamp    ---, more columns
FROM test_select 
ORDER BY object_id, event_timestamp DESC ;

event_timestamp DESCで並べ替えた結果が必要な場合 object_idではありません 、派生テーブルまたはCTEに含める必要があります：

SELECT *
FROM 
  ( SELECT DISTINCT ON (object_id) 
        object_id, event_timestamp    ---, more columns
    FROM test_select 
    ORDER BY object_id, event_timestamp DESC 
  ) AS t
ORDER BY event_timestamp DESC ;

または、ROW_NUMBER()などのウィンドウ関数を使用することもできます。 ：

WITH cte AS
  ( SELECT ROW_NUMBER() OVER (PARTITION BY object_id 
                              ORDER BY event_timestamp DESC) 
             AS rn, 
           object_id, event_timestamp    ---, more columns
    FROM test_select 
  )
SELECT object_id, event_timestamp    ---, more columns
FROM cte
WHERE rn = 1
ORDER BY event_timestamp DESC ;

または集計MAX() OVERで ：

WITH cte AS
  ( SELECT MAX(event_timestamp) OVER (PARTITION BY object_id) 
             AS max_event_timestamp, 
           object_id, event_timestamp    ---, more columns
    FROM test_select 
  )
SELECT object_id, event_timestamp    ---, more columns
FROM cte
WHERE event_timestamp = max_event_timestamp
ORDER BY event_timestamp DESC ;