Added MySQL-friendly version of SQL code for migration.

db/migrate/20111111025358_counter_cache_on_post_reshares.rb

@@ -4,16 +4,29 @@ class CounterCacheOnPostReshares < ActiveRecord::Migration
   def self.up
     add_column :posts, :reshares_count, :integer, :default => 0
 
-    execute %{
-      UPDATE posts
-      SET reshares_count = (
-        SELECT COUNT(*)
-        FROM posts p2
-        WHERE
-          p2.type = 'Reshare'
-          AND p2.root_guid = posts.guid
-      )
-    }
+    if postgres?
+      execute %{
+        UPDATE posts
+        SET reshares_count = (
+          SELECT COUNT(*)
+          FROM posts p2
+          WHERE
+            p2.type = 'Reshare'
+            AND p2.root_guid = posts.guid
+        )
+      }
+    else # mysql
+      execute "CREATE TEMPORARY TABLE posts_reshared SELECT * FROM posts WHERE type = 'Reshare'"
+      execute %{
+        UPDATE posts p1
+        SET reshares_count = (
+          SELECT COUNT(*)
+          FROM posts_reshared p2
+          WHERE p2.root_guid = p1.guid
+        )
+      }
+    end
+
   end
 
   def self.down