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paginate very slow on large queries with :group

#255

In finder.rb, wp_count() passes off to count() in order to figure out how many total results exist. When :group is specified, count() uses execute_grouped_calculation() in ActiveRecord::Calculations, which calls ActiveSupport::OrderedHash[]= . ActiveSupport::OrderedHash[]= calls assoc() on the its internal Array, which compares the value passed to assoc() to the first value of each array within its internal Array. As a result, for my application, which has a total of 2,895 matching rows, a total of 4,189,065 == comparisons are made in the course of determining the number of rows.

This is obviously very slow. Is there be a faster way to calculate the number of rows for queries with :group?

Reported by Michael Ragalie · December 10th, 2008 @ 03:37 AM

State: resolved
Milestone: none
Assigned to: Mislav Mislav

Activity

  1. Mislav
    Mislav
    • State changed from new to open

    That's rather unfortunate.

    Does this help?

    
Model.paginate :page => 1, :per_page => 10,
  :group => 'foo', :count => { :group => nil }

    That will cause the count to be performed like the :group option wasn't even there.

    December 11th, 2008 @ 11:45 PM

  2. Michael Ragalie
    Michael Ragalie

    That worked like a charm. All I did was use

    
Model.paginate :page => 1, :per_page => 10, :group => 'foo.id', :count => {:group => nil, :select => "DISTINCT foo.id"}

    and got the expected behavior.

    Thanks for your help!

    December 12th, 2008 @ 12:57 AM

  3. Mislav
    Mislav
    • State changed from open to resolved

    No prob ;)

    December 12th, 2008 @ 02:22 AM

  4. Sebastian
    Sebastian

    I*m sorry. But I think this is absolutly wrong, and will return wrong results.

    When you group something by a specific value, and then count without the group option as described above you'll get the count of all ungrouped results.

    I don't know about a better workaround to speed things up.

    February 6th, 2009 @ 12:12 AM

  5. Mislav
    Mislav

    You may be right.

    But still, little can be done from will_paginate. Maybe you guys should open a Rails ticket? On the other hand, the internal implementation of OrderedHash was rewritten and now the problem may be gone (better check to be sure)

    February 6th, 2009 @ 01:32 AM

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