Getting the oldest datas from a table that are older than a 100 days - sql

Question

I've been struggling with the following problem:
EXPLAINING
I have a table called part_subhourly_data that holds production data for a part (For the purpose of the problem, no need to know what a part is).
I need to archive the any data older than a 100 days. But since there's a lot of data (they arrive each 5 or 10 minutes) and we have more than 1000 parts, I need to do it the 5 oldest days each time.
This is the schema of my table:
part_subhourly_data
id INTERGER,
part_id INTEGER,
produced_at TIMESTAMP
data HSTORE
So basically I need to get all data that is in this table, where produced_at is prior to 100 days ago and limit that to the first 5 days, per part.
Example:
Part 1 has data from 15 Aug 2016 until 12 Dec 2016
Part 2 has data from 1st Sep 2016 until 12 Dec 2016
100 days ago would be 3 Sep 2016.
For Part 1 I would take data from 15 Aug 2016 until 19 Aug 2016 (5 days).
For Part 2 I would take data from 1st Sep 2016 until 3 Sep 2016 (3 days because of the 100 days old condition).
WHAT HAVE I TRIED
Well, I'm using rails on this, but a SQL solution is welcome as well. For now, What I'm doing is to grab the oldest data with:
SELECT "part_subhourly_data"."part_id", MIN(produced_at) produced_at
FROM "part_subhourly_data"
WHERE (produced_at < (NOW() - INTERVAL '100 days'))
GROUP BY "part_subhourly_data"."part_id"
And then I loop Over each part_id and grab the data based on the MIN(produced_at). It works, but it doesn't seems ideal. I'm sure that there is some SQL magic to make it simpler, and quicker, without having to loop over each part.
Any idea?

Answer 1

Take all records where produced_at is prior to 100 days ago.
dense rank the records per part_id ordered by produced_at::date in ascending order.
The records with the oldest date will get 1, the records with the next oldest date will get 2 etc.
select part_id,produced_at
from (select part_id,produced_at
,dense_rank () over (partition by part_id order by produced_at::date) as dr
from part_subhourly_data
where produced_at < now() - interval '100 days'
) p
where dr <= 5
;