On 5 March 2014 00:32, John Merlino <stoici...@aol.com> wrote:
> Initially, I was using this query:
>
>
> report_ids = []
> if units.size > 0
> units.map(&:id).uniq.each do |id|
> report_id = Report.select(:id).where(unit_id: id).order("time
> desc").first
> unless report_id.nil?
> report_ids << report_id
> end
> end
> end
> reports = Report.where(id: report_ids).order("longitude desc")
>
> It looks sloppy, and it took around 7 or 8 seconds to load all the data.
>
> So I wanted to create a cleaner, quicker query, so I tried this:
>
> reports = Report.find_by_sql(["SELECT DISTINCT ON (unit_id) r.*
> FROM reports r
> WHERE r.unit_id IN (?)
> ORDER BY unit_id, time DESC",
> units.map(&:id)])
>
> But this is running incredibly slow. In fact, after 15 minutes, I just
> cancelled the process.
>
>
> Any idea why the second query is taking markedly longer than the first?
No, but have you got indexes on unit_id and time?
Colin
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