Question

I have 2 tables that manages the time spent on doing various things:

@times(id, time_in_minutes)
@times_intervals(id, times_id, time_in_minutes, start, end)

Then the @times might relate to different things:

@tasks(id, description)
@products(id, description, serial_number, year)

What is the best practice in order to reuse the same @times and @times_intervals for @task and @products?

I would think about:

@times(+task_id, +product_id) 
// add task_id and product_id to the original @times table

But if I do so, when I’d join the @times table with @task and @products table would be slower as should choice between the 2 (task_id or product_id). When task_id is not null join on the @tasks and viceversa.

(I’m using MySQL6)

Thanks a lot

Answer 1

I would drop the time_in_minutes column from the times table. This information is redundant if it is just the sum of the detail and is a premature optimization.

I would add a product_time table containing product_id, times_id and a task_time table containing task_id, time_id

Then to get the total time with a product:

SELECT *
FROM product p
INNER JOIN product_time pt
    ON pt.product_id = p.id
INNER JOIN (
    SELECT times_id, SUM(time_in_minutes) as time_in_minutes
    FROM times_intervals
    GROUP BY times_id
) AS t
    ON t.times_id = pt.times_id

Typically to make this perform, you would have a non-clustered covering index for times_intervals with columns times_id and time_in_minutes – note that the times table is simply a data-less header table at this point and the only purpose it to group the times_intervals and it’s only necessary because you have this very similar arrangement for tasks.

If there were not two (or more) entities using the times_intervals, you might simply put product_id in the times_intervals and treat it as your header/master id.