will_paginate_postgresql_count.rb

# add this file as config/initializers/will_paginate_postgresql_count.rb
# in a Rails application

module WillPaginate
  module Finder
    module ClassMethods

      # add '1=1' to paginate conditions as marker such that the select from the pg_class
      # is used to approximate simple rows count, e.g.
      # Foo.paginate(:page => params[:page], :conditions => "1=1")
      def wp_count_with_postgresql(options, args, finder, &block)
        if options[:conditions] == ["1=1"] || options[:conditions] == "1=1"
          # counting rows in PostgreSQL is slow so use the pg_class table for 
          # approximate rows count on simple selects
          # http://wiki.postgresql.org/wiki/Slow_Counting
          # http://www.varlena.com/GeneralBits/120.php
          ActiveRecord::Base.count_by_sql "SELECT (reltuples)::integer FROM pg_class r WHERE relkind = 'r' AND relname = '#{self.table_name}'"
        else
          wp_count_without_postgresql(options, args, finder, &block)
        end

      end

      alias_method_chain :wp_count, :postgresql
    end
  end
end

this works for me in Rails 3 and will_paginate 3.0.pre2

module WillPaginate
  module Finders
    module ActiveRecord

      # add '1=1' to paginate conditions as marker such that the select from the pg_class
      # is used to approximate simple rows count, e.g.
      # Foo.paginate(:page => params[:page], :conditions => "1=1")
      def wp_count_with_postgresql(options, args, finder, &block)
        if options[:conditions] == ["1=1"] || options[:conditions] == "1=1"
          # counting rows in PostgreSQL is slow so use the pg_class table for
          # approximate rows count on simple selects
          # http://wiki.postgresql.org/wiki/Slow_Counting
          # http://www.varlena.com/GeneralBits/120.php
          ::ActiveRecord::Base.connection.execute("SELECT (reltuples)::integer FROM pg_class r WHERE relkind = 'r' AND relname = '#{self.table_name}'").first['reltuples'].to_i
        else
          wp_count_without_postgresql(options, args, finder, &block)
        end

      end
      self.send('public', :wp_count)
      alias_method_chain :wp_count, :postgresql
    end
  end
end