Pick a random record for a LARGE record set using arel / rails

Question

I need to pick a random record for a VERY large ActiveRecord set.

What's the best way to do this? I have something like this, but it still takes a long time.

Model.select('id').where("id = ? AND attr = ?", self.id, false).offset(rand(size)).limit(1)

Thank you

http://stackoverflow.com/a/4329447/125816 – Sergio Tulentsev Feb 01 '14 at 16:49 — Sergio Tulentsev, Feb 01 '14 at 16:49

score 0 · Answer 1 · answered Feb 01 '14 at 17:52

The usual techniques for this rely on generating a series of random numbers that represent the possible id's of records, and pulling back the relevant row.

You generally need to read the maximum (and maybe minimum) id's, and generate a bunch of random numbers between them. You generally can't guarantee that every id will exist, hence generating quite a few (or looping with a new random number each time) until you find one for which a row exists.

score -1 · Answer 2 · answered Feb 01 '14 at 16:39

-1

Model.select('id').where("id = ? AND attr = ?", self.id, false).order("RANDOM()").first

Seems OK, but I feel like there is a better way.

answered Feb 01 '14 at 16:39

neofetter

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Pick a random record for a LARGE record set using arel / rails

2 Answers2