README

Find Random
==========

The plugin provides a (somewhat) scalable way to select random rows from a table.  It runs
significantly faster than asking most databases to "ORDER BY RAND()".  Behind the scenes it
grabs all ids that match the query conditions, then selects a random subset of these for which 
it then asks the database for the full records. The initial query is done outside of ActiveRecord
to avoid needless object instantiation.

If only one result is requested a different strategy is used.  First a query is done to determine 
the number of results, then a single result is selected using a random offset.  This removes the
memory inefficiency of the method used when selecting multiple results.

Usage
----

  # select 5 random items that match the :conditions  
  @items = Item.random(5, :conditions => ["col1 = ? or col2 = ?", val1, val2])  

  # select any 5 random items  
  @items = Item.random(5)  


Limitations
-----------

Currently this plugin only supports joins if a single result is requested. Additionally the 
current implementation for selecting multiple random results is somewhat memory inefficient 
and probably won't perform well on tables with greater than 100,000 rows. Suggestions
for improvement welcome.


Benchmark
---------

Comparing this method against ORDER BY RAND() in MySQL on a table with 30,000 rows:

  $ ./script/performance/benchmarker 1 'Person.random(8)' 'Person.find(:all, :order => "RAND()", :limit => 8)'  
            user     system      total        real  
            #1      0.030000   0.000000   0.030000 (  0.149027)  
            #2      0.000000   0.000000   0.000000 (  5.516728)  

Contact
-------

* Ben Tucker <ben@greenriver.org>

Patches/suggestions very much welcome.