Optimize ActiveRecord::Base#exists? to use #select_all instead of #find. Closes #10605 [jamesh, fcheung, protocool]

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@8531 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

activerecord/CHANGELOG

@@ -1,5 +1,7 @@
 *SVN*
 
+* Optimize ActiveRecord::Base#exists? to use #select_all instead of #find.  Closes #10605 [jamesh, fcheung, protocool]
+
 * Don't unnecessarily load has_many associations in after_update callbacks.  Closes #6822 [stopdropandrew, canadaduane]
 
 * Eager belongs_to :include infers the foreign key from the association name rather than the class name.  #10517 [Jonathan Viney]

activerecord/lib/active_record/base.rb

@@ -548,8 +548,14 @@ module ActiveRecord #:nodoc:
       #   Person.exists?(:name => "David")
       #   Person.exists?(['name LIKE ?', "%#{query}%"])
       def exists?(id_or_conditions)
-        !find(:first, :select => "#{quoted_table_name}.#{primary_key}",
-              :conditions => expand_id_conditions(id_or_conditions)).nil?
+        connection.select_all(
+          construct_finder_sql(
+            :select     => "#{quoted_table_name}.#{primary_key}", 
+            :conditions => expand_id_conditions(id_or_conditions), 
+            :limit      => 1
+          ), 
+          "#{name} Exists"
+        ).size > 0
       end
 
       # Creates an object (or multiple objects) and saves it to the database, if validations pass.