CODEWITHSUNDEEP
ruby-on-railsdatabaseruby-on-rails-3rails-activerecord
Yujun Wu
Yujun Wu

Reputation: 3012

query time in Rails

I have users table and polls table. Assume the tables are quite big, which query would be more efficient in Rails:

User.find(user_id).polls.where('category = ?',category)
Poll.where('user_id = ? AND category = ?',user_id,category)

You may assume a user can't have too many polls(maybe in average 100 polls). And user_id and category are both indexed

Upvotes: 1

Views: 156

Answers (1)

mu is too short
mu is too short

Reputation: 434615

The first one is equivalent to this:

user  = User.find(user_id)
polls = user.polls.where(:category => category)

The User.find will issue a

select * from users where id = ?

and then the user.polls.where(...) part will do a:

select * from polls where user_id = ? and category = ?

Oddly enough, that's exactly what the plain Poll.where(...) will do.

So the second should be quicker since it doesn't do the select ... from users. OTOH, if you already have the user then user.polls.where(...) would probably make more sense.

Upvotes: 1

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