Optimize GROUP BY query to retrieve latest row per user

Question

I have the following log table for user messages (simplified form) in Postgres 9.2:

CREATE TABLE log (
    log_date DATE,
    user_id  INTEGER,
    payload           


        

Answer 1

Perhaps a different index on the table would help. Try this one: log(user_id, log_date). I am not positive that Postgres will make optimal use with distinct on.

So, I would stick with that index and try this version:

select *
from log l
where not exists (select 1
                  from log l2
                  where l2.user_id = l.user_id and
                        l2.log_date <= :mydate and
                        l2.log_date > l.log_date
                 );

This should replace the sorting/grouping with index look ups. It might be faster.