Query optimization for index page - ruby-on-rails

Question

I have a System and a Report model. System has_many reports and Report belongs_to system. Each daily report consists of 175 records per system.
I need a query on my system#index page which should list all systems filtered on most recent report creation. This was my first attempt.
#systems = System.joins('LEFT JOIN reports ON reports.system_id = systems.id').group('systems.id').order('MAX(reports.created_at) ASC')
This lists systems with a report (System Load (2.1ms)) but sorted by system_id not by report created_at.
Second attempt
#systems = System.joins(:reports).where("reports.created_at = (SELECT MAX(created_at) FROM reports p group by system_id having p.system_id = reports.system_id)").order('reports.created_at DESC')
This query does the job, but is really slow ( System Load (546.2ms)), despite having an index on report.created_at.
Third attempt
#systems = System.joins(:reports).where("reports.id = (SELECT MAX(id) FROM reports p group by system_id having p.system_id = reports.system_id)").order('reports.id DESC')
Also does the job, slightly faster than the second attempt (System Load (468.3ms)) but still not fast enough.
Any tips?
EDIT 03032017
I did the numbers on a small test dataset
old query
SELECT s.* FROM systems s
JOIN reports r ON r.system_id = s.id
WHERE r.created_at = (
SELECT MAX(created_at)
FROM reports p
group by p.system_id
having p.system_id = r.system_id)
ORDER BY r.id DESC
Time: 622.683 ms
Philip Couling solution (clean, returns only systems with reports)
SELECT systems.*
FROM systems
JOIN (
SELECT reports.system_id
, MAX(reports.created_at) created
FROM reports
GROUP BY reports.system_id
) AS r_date ON systems.id = r_date.system_id
ORDER BY r_date.created;
Time: 1.434 ms
BookofGreg solution (will give me all systems, report or no report)
select systems.* from systems order by updated_at;
Time: 0.253 ms
I couldn't get systemjack's solution to work.
Fastest solution: bookofgreg
Cleanest solution: philip couling
Thanks for your input.

Answer 1

An index on (reports.system_id, reports.created_at) might make this work efficiently:
#systems = System.joins(:reports).where("reports.created_at = (SELECT MAX(created_at) FROM reports p where p.system_id = reports.system_id) system_id)").order('reports.created_at DESC')
Alternativly...
Your second piece of code:
System.joins(:reports).where("reports.id = (SELECT MAX(id) FROM reports p group by system_id having p.system_id = reports.system_id)").order('reports.id DESC')
expands to:
SELECT system.*
JOIN reports ON system.id = reports.system_id
WHERE reports.created_at = (
SELECT MAX(created_at)
FROM reports p
group by p.system_id
having p.system_id = reports.system_id)
)
ORDER BY reports.id DESC
Notice how it has to look at reports twice. Also, because you include p.system_id = reports.system_id) the nested query will be called once per system record.
Ideally you want to get a list of system_ids and report dates:
So...
SELECT reports.system_id
, MAX(reports.created_at) created
FROM reports
GROUP BY reports.system_id
And then join to that:
SELECT systems.*
FROM systems
JOIN (
SELECT reports.system_id
, MAX(reports.created_at) created
FROM reports
GROUP BY reports.system_id
) AS r_date ON systems.id = r_date.systems_id
ORDER BY r_date.created

Answer 2

One possible solution, if you do not need the report data on the page, is have Report after_save -> { self.system.touch } # in Report when it is updated. This will cause the System's updated_at to take on the time the Report was updated.
This will mean you can just sort the System by their updated at without joining at all.
This solution assumes that there is no other way to update System. If there is then you can specify a time cache column which you can use to order on like after_save -> { self.system.touch(:report_cached_updated_at) }
http://api.rubyonrails.org/classes/ActiveRecord/Persistence.html#method-i-touch

Answer 3

A window function might perform well for you. Not sure how to implement this in rails, but the query to get the latest report for each system could look like:
select * from (
select s.*, r.sytem_id, r.created_at,
row_number() OVER (PARTITION BY s.id ORDER BY r.created_at desc) AS row
from systems s
left join reports r on r.system_id = s.id
) where (row = 1 OR r.system_id is null)
The check for null is there because you have a left join in your example so you must want systems even if there is no report.
or simpler (but not as sure syntax is right):
select *
from systems s
left join reports r on r.system_id = s.id
having (r.system_id is null
OR row_number() OVER (PARTITION BY s.id ORDER BY r.created_at desc) = 1)