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Editorial Team
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Asked: 2026-06-15T00:28:21+00:00

I have the following query in rails which uses PostgreSQL 9 as the DB.

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I have the following query in rails which uses PostgreSQL 9 as the DB.

@user_random = User.where(:id => User.where(:pubic_profile_visible => true).order('random()').limit(1).select(:id).collect(&:id)).first

Which then on the DB Server:

Explain plan

Query plan  Limit  (cost=48655.53..48655.53 rows=1 width=4)
Query plan    ->  Sort  (cost=48655.53..50880.35 rows=889930 width=4)
Query plan          Sort Key: (random())
Query plan          ->  Seq Scan on users  (cost=0.00..44205.88 rows=889930 width=4)
Query plan                Filter: (pubic_profile_visible AND (deleted_at IS NULL))

Any suggestions on why this is causing a bottleneck? Thank you

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1 Answer

  1. Editorial Team

    Like @wildplasser says this query is doing exactly what you asked it to do.

    It can’t pick the lowest random() for your “LIMIT 1” without fetching all 800,000 rows, generating a random number and then sorting them.

    Try googling “postgresql select random row” and you’ll get half a dozen useful stackoverflow questions and also a link to an old blog post on depesz.com which covers the various options:

    http://www.depesz.com/2007/09/16/my-thoughts-on-getting-random-row/

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