Re: Last event per user

[Luís Roberto Weck <luisroberto@xxxxxxxxxxxxxxx>]

On Mon, Aug 12, 2019 at 5:03 PM Luís Roberto Weck <luisroberto@xxxxxxxxxxxxxxx> wrote:

If you modify last_user_event_2 to select user and event info in the view, and just put there where clause directly on the view which is not joined to anything, instead of on the "extra copy" of the users table like you were showing previously, I would expect that the performance should be excellent.

But I need user_id and user_group to be outside of the view definition. user_id and user_group are dynamic values, as in, I need to call this query multiple times for different user_ids and user_groups .

I don't follow. Perhaps there is something within the limitations of the ORM layer that I am not expecting. If you have this view-

"last_user_event_2"

SELECT u.*, e.*

   FROM users u

        JOIN LATERAL (SELECT *

                        FROM events

                       WHERE user_id = u.user_id

                         AND user_group = u.user_group 

                       ORDER BY timestamp_inc DESC

                       LIMIT 1 ) e ON TRUE

And you execute a query like this-

SELECT * FROM last_user_event_2 e WHERE  user_id = 1272897 and user_group = 19117;

Then I would expect very good performance.

You're right, thanks! I just had to do a little adjustment on the lateral join. Since both users and events have user_id and user_group, PostgreSQL complains that I can't have more than one column with the same name. I fixed it by changing the LATERAL condition from "ON TRUE" to "USING (user_id,user_group)" (which I didn't even knew I could do).