make updating the hierarchy way faster on mysql

Author: joevandyk

lib/closure_tree/hierarchy_maintenance.rb

@@ -95,14 +95,13 @@ def delete_hierarchy_references
  # See http://dev.mysql.com/doc/refman/5.0/en/subquery-errors.html
  # Also: PostgreSQL doesn't support INNER JOIN on DELETE, so we can't use that.
  _ct.connection.execute <<-SQL.strip_heredoc
- DELETE FROM #{_ct.quoted_hierarchy_table_name}
- WHERE descendant_id IN (
- SELECT DISTINCT descendant_id
- FROM (SELECT descendant_id
- FROM #{_ct.quoted_hierarchy_table_name}
- WHERE ancestor_id = #{_ct.quote(id)}
- OR descendant_id = #{_ct.quote(id)}
- ) #{ _ct.t_alias_keyword } x )
+ DELETE #{_ct.quoted_hierarchy_table_name} FROM #{_ct.quoted_hierarchy_table_name}
+ JOIN (
+ SELECT descendant_id FROM #{_ct.quoted_hierarchy_table_name}
+ WHERE ancestor_id = #{_ct.quote(id)}
+ OR descendant_id = #{_ct.quote(id)}
+ ) #{ _ct.t_alias_keyword } x
+ ON x.descendant_id = #{_ct.quoted_hierarchy_table_name}.descendant_id
  SQL
  end
  end