PostgreSQL - “DISTINCT ON” and “GROUP BY” syntax

Question

I realized that a database query was returning unexpected results do to my improper use of \"DISTINCT ON\" and \"GROUP BY\"

I\'m hoping someone can set me straight on th

Answer 1

To be able to select all columns and not only object_id and MAX(event_timestamp), you can use DISTINCT ON

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

If you want the results ordered by event_timestamp DESC and not by object_id, you need to include it in a derived table or a CTE:

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 ;

Alternatively, you can use window functions, like ROW_NUMBER():

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 ;

or aggregate MAX() with OVER:

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 ;

Answer 2

It is probably not the best way of dealing with this but you can try using window function:

SELECT DISTINCT object_id, MAX(event_timestamp)
OVER (PARTITION BY object_id)  
FROM test_select ORDER BY max DESC;

From the other hand it works as well:

SELECT object_id, MAX(event_timestamp) as max_event_timestamp
FROM test_select 
GROUP BY object_id 
ORDER BY max_event_timestamp DESC;