Hey,
2 comments from me:
1 - using :include one-level-deep when you are fetching *one*
toplevel object *and* you are not issuing :conditions on
the :included tables is *always* (well, I've never found an
exception) slower.
x = Foo.find(4678. :include => [:incoming_messages, :outgoing_messages])
is slower than:
x = Foo.find(4678)
x.incoming_messages
x.outgoing_messages
If your reaction is to say "but it's always faster with eager
loading" then I urge you to *measure* it and get back to me if you
find that my measurements are wrong.
2 - I've got a plugin that improves your situation where you are
fetching multiple toplevel objects *and* you don't have
any :conditions that relate to the associations you're pulling in.
foos = Foo.find(:all, :hydrate =>
[:incoming_messages, :outgoing_messages])
It will split that find() into 3 queries, wiring up the relation
targets. I've chosen the explicit :hydrate option to give the user
more control over the strategy for pulling in associations and it
works just fine with Rick O's scope plugin too.
I've been sitting on the plugin since railsconf *last* year and never
released it because I had so many doubts about whether the strategy
was solid enough. I've used it enough times that I'm confident it
works. I'll be releasing it for public consumption in the next
couple of weeks.
Trev
On 7-Aug-07, at 1:55 PM, Jeremy Evans wrote:
>
> On 8/7/07, Frederick Cheung <[EMAIL PROTECTED]> wrote:
>> Executive Summary:
>> ================
>>
>> I've recently come across some performances problems with eager
>> loading multiple has_many relationships (and to a lesser extent a
>> single has_many with many objects in the collection) and had some
>> thoughts.
>>
>> The case I came across involved some models like so:
>>
>> class Question < ActiveRecord::Base
>> has_many :incoming_messages
>> has_many :outgoing_messages
>> end
>>
>> class IncomingMessage < ActiveRecord::Base
>> belongs_to :question
>> end
>>
>> class OutgoingMessage < ActiveRecord::Base
>> belongs_to :question
>> end
>>
>> In various parts of the app we load a question (or multiple ones)
>> with :include => [:incoming_messages, :outgoing_messages]
>>
>> Typically a question has a small number of incoming and outgoing
>> messages (often only 1 or 2) and this all works absolutely fine.
>> However at some point we ended up with a question with many incoming
>> and outgoing_messages. Our servers (quite literally) ground to a halt
>> whenever loading that question with the aforementioned includes, so I
>> had a look under the hood.
>>
>> The underlying thing is that in this case Question.find(1, :include
>> => [:incoming_messages, :outgoing_messages]) returns quite a few rows
>> and so even fairly small things add up very quickly
>>
>> I've put together some changes that improve the situation, along with
>> some numbers
>>
>> Numbers:
>> ===========
>>
>> In my benchmarks I've used 2 instances of Question: one with 150
>> incoming and 80 outgoing (big question) and one with 225 incoming and
>> 120 outgoing (huge question) (ie 50% more of each, so total row count
>> goes up by 2.25)
>
> The main issue you are running into is that Rails' SQL queries for
> multiple included has_many associations return the cartesian product
> of the has_many_associations. Ideally, the best way to handle this is
> to send two or three separate SQL statements. You'd have one
> statement for each association, and then combine them together. The
> most efficient way is probably n+1 queries where n is the number of
> has_many associations, with one query to get the information on the
> main object, and one query for each has_many association, that only
> includes the association information and the main object's id (in
> order to associate it). That would shorten the number of rows
> returned for the queries you mention from 12,000 to 231 and from 27000
> to 346. It's more complex than the current implementation, but it
> will preform much better. I'm not volunteering to implement it,
> though. :)
>
> As a workaround, how about:
>
> question = Question.find_by_id(big_question.id, :include
> => :incoming_messages)
> question.instance_variable_set('@outgoing_messages',
> Question.find_by_id(big_question.id, :include =>
> :outgoing_messages).outgoing_messages)
>
> Also, note that for a single object, you are probably better off using
> lazy loading has_many associations (eager loading belongs_to
> associations is fine). Eager loading has_many associations should
> only be done if you are getting multiple objects at once (i.e. find
> :all).
>
> Jeremy
>
>
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