Question

I need to get certain tracked data, including a couple of max and mins. I already have a horrible way to do it below, but as you know this is REALLY slow because i’m using subqueries and the table has a couple of thousands of rows. Specifically I need to get a different MAX values per row, depending on the type_dev of the current row. with 24000 records it becomes unusable, and I think is even slower when I try with MAX clauses instead.

SELECT dt.some_data, dt.date_visit, dt.url, 
      (SELECT date_visit FROM device_tracker 
       WHERE type_dev = dt.type_dev ORDER BY date_visit DESC LIMIT 1) last_visit,
      (SELECT date_visit FROM device_tracker 
       WHERE type_dev = dt.type_dev and url = dt.url ORDER BY date_visit DESC LIMIT 1) last_visit_to_this_url 
   FROM device_tracker dt WHERE some_where_clauses;

Please note that I don’t need global max values (it could be really easy), but for example in this case I’m getting the max date of every single type_dev (every row can be a different type or not) and also the max date of a certain url regarding each row.

I Need the same with the minimun dates.

For sure there are a lot of nicer ways to do it. Could anyone throw some light on it?

Answer 1

SELECT  *
FROM    (
        SELECT  type_dev, MAX(date_visit) AS maxt, MIN(date_visit) AS mint
        FROM    device_tracker
        GROUP BY
                type_dev
        ) t
JOIN    (
        SELECT  type_dev, url, MAX(date_visit) AS maxtu, MIN(date_visit) AS mintu
        FROM    device_tracker
        GROUP BY
                type_dev, url
        ) tu
ON      tu.type_dev = t.type_dev
JOIN    device_tracker dt
ON      dt.type_dev = tu.type_dev
        AND dt.url = tu.url

Create a composite index on (type_dev, url, date_visit) for this to work fast.