Rails / Postgresql: Strange behaviour of order, limit and offset

Question

I totally don't get what's going on here - what am I doing wrong?

irb(main):022:0> Toy.order('rating DESC').all.map(&:id)
  Toy Load (2.0ms)  SELECT "toys".* FROM "toys" ORDER BY rating DESC
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99]

irb(main):029:0> Toy.order('rating DESC').limit(8).offset(0).all.map(&:id)
  Toy Load (0.8ms)  SELECT "toys".* FROM "toys" ORDER BY rating DESC LIMIT 8 OFFSET 0
=> [2, 3, 4, 5, 6, 7, 8, 1]

irb(main):029:0> Toy.order('rating DESC').limit(8).offset(8).all.map(&:id)
  Toy Load (0.8ms)  SELECT "toys".* FROM "toys" ORDER BY rating DESC LIMIT 8 OFFSET 8
=> [10, 11, 12, 13, 14, 15, 16, 1]

irb(main):029:0> Toy.order('rating DESC').limit(8).offset(16).all.map(&:id)
  Toy Load (0.8ms)  SELECT "toys".* FROM "toys" ORDER BY rating DESC LIMIT 8 OFFSET 16
=> [18, 19, 20, 21, 22, 23, 24, 1]

If I don't scope limit and offset with order everything is fine. As soon as I add ordering, the first object always appears in the end no matter what offset I set. Any idea why this is happening? Thanks!

Have you tried SELECT id FROM toys ORDER BY rating DESC LIMIT 8 OFFSET 0 in the psql PostgreSQL shell? — mu is too short
– mu is too short, Commented Nov 30, 2011 at 5:43

TomH · Accepted Answer · 2011-11-30 08:51:08Z

2

At a guess rating is not a unique key, so your sort is not stable and may return rows in a different order each time it is used. Try adding id to the sort as a secondary key to make sure that the sort order is well defined, like this:

Toy.order('rating DESC, id ASC').limit(8).offset(8).map(&:id)

Note also that you shouldn't need the .all as you can apply map directly to an AREL object without forcing it to be realised first.

answered Nov 30, 2011 at 8:51

TomH

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1 Comment

user1053134 Over a year ago

Thanks - that's exactly it. I need to remind myself using unique key on paging next time!

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